-
Atom
ic Structure
Radioactivity
History of discovery
BecquerelD
iscovery of radioactivity
Curie
Extraction of radium and
polonium
Rutherford
Study the nature of,
andradiation
Nature of radioactivity
particlefast m
oving helium nuclei
5% of speed of light
particlevery fast m
oving electron
up to 99% speed of light
radiationhigh energy EM
wave
100% speed of light
Origin of radioactivity
particle
replusion among proton
binding force between proton
and neutron
lowering of p:n ratio
particle
instablility of neutron when
isolated
increasing of p:n ration
radiation
Nature state of atom
s afterthe big bang / nucelusform
ation
re-orientation of the spinningproton
Nuclear Reaction
Natural decay
Meaning of H
alf-life
Constant Half-life
First order reaction
Independent of temp.
Artificial transmutation
Use of Radioactive isotope
Leak Detection
Geiger-M
uller counter
Bio-tracerI-131 (thyroid disease)
P-32 (metabolism
of plant)
RadiotherapyI-131 (thyroid cancer)
Co-60 / Cs-137 (tumor)
Carbon-14 dating
Nuclear pow
erN
uclear bomb
Mass of an atom
Mass spectrom
eter
Construction
Vaporization chamber
Ionization chamber
Accelerating plates
Deflecting m
agnetic field
Detector
Recorder
Vacuum Pum
p
Working principle - response to
m/e ratio
To determine the ionization
energy of an atom
Interpretation of mass
spectrum
Determ
ination of isotopic mass
Determ
ination of atomic m
ass
Determ
ination of molecular m
ass
Determ
ination of molecular structure
Relative isotopic mass
The relative mass of a particular isotope in C-12 scale w
hereC-12 isotope is defined to have exactly 12 units of m
ass.
Relative atomic m
ass
The weighted average of the relative isotopic m
asses for a particularelem
ent in C-12 scale according to their natural abundance
Relative molecular m
ass
The relative mass of a m
olecule where that the value is the
same as the sum
of relative atomic m
ass of all atoms present
in the molecule.
Developm
ent of atomic
model
Dem
ocritusIdea of atom
introduced
No experim
ental evidence
Dalton
Atom is the sm
allest unit ofm
atter and is not breakable
With experim
ental evidencefrom
the study of gas
Goldstein
Invention of cathode ray tube
Thomson
Behaviour of cathode ray inm
agnetic and electric fieldN
egatively charged
fast moving particle
Invention of the word "electron".
Raisin pudding model of atom
Atom is no longer unbreakable
RutherfordG
old foil scattering experiment
The idea of presence of tiny nucleus
Chadwick
Discovery of neutron
Study of radioactivityThe idea of nucleus is breakable
1. Atom
ic Structure.m
map - 2005/5/23 -
-
Mole Concept
How
much
is onem
ole ?
How
many ?
Definition
Avogadro constant(Experim
entalm
easurement required)
How
heavy ?
Molar
mass
Carbon-12 Scale
How
large ?
Molar volum
e
Molar
volume
of a gas
r.t.p.
s.t.p.
Avogadro's law
Charges
Relationship between
Current I, Amount of charge
Q and tim
e t i.e Q=It
Faraday'sLaw
s ofelectrolysis
1st Lawm
Q
2nd Lawm
1/n
Charges carried by1 m
ole of electronFaraday constant
1 F = 96500 C
Behaviour of Ideal gas
Boyle's Law P
1/V
Charles' Law V
T
Avogadro's Law V
N
Ideal gas Law/ EquationPV = nRT
Determ
ination of m
olecular mass
of a volatile liquid
Principle
Volatile liquid vaporizes easily by heating
Assuming the vapor behaves ideally
No interaction am
ong particles
Individual particle occupies no volume
Molecular m
ass is numerically the
same as m
olar mass
Conditions a real gas behavesvirtually ideal
High tem
perature
particles arem
oving fastinteraction am
ong particles is negligible
Low pressure
particles arefar apart
space occupied by individual particle is negligible
Setup of apparatus
Measurem
ent of PBarom
eter in the laboratory
Measurem
ent of V
Increase in volume of
the gas syringePrecaution
Take reading only when the
reading is steady
Some liquid m
ay havenot vaporized
The vapor may be still
expanding
Measurem
ent of m
Difference in m
ass ofthe hypoderm
ic syringebefore and afterinjection
Precaution
Hold the hypoderm
icsyringe horizontal
The liquid may leak out.
Volatile liquid usuallyhas low
surface tension.
Hold the end of the
syringe only
Body warm
th may heat up
the liquid and expel it fromthe syringe
Insert the needle fullyinto the gas syringe
Inject the liquid into thedeeper part, usually hotter,of the gas
syringe toensure better vaporization.
Measurem
ent of T
Therm
ometer
(final reading)Precaution
Take reading only when the
reading is steadyThe system
may have not
reach thermo-equilibrium
yet.
Operation
Allow the system
to reach a thermo-equilibrium
Take all initial readings
Take final reading when all readings are steady
Dalton's Law
of partialpressure
Total pressure of a gaseousm
ixture is the same of the sum
of the partial pressure of all ofthe com
ponent present
Definition of partial pressure - the pressure
of a component in the gaseous m
ixture ifthat com
ponent occupies the same volum
eby itself alone.
mole fraction (percentage by num
ber)
2. Mole C
oncept.mm
ap - 21/5/2005 -
-
Chemical Equations
and Stoichiom
etry
Formulae
Formulae of
Compounds
Empirical form
ulaSim
plest whole num
ber ratio
Molecular form
ulaActual no. of atom
s
Structural formula
Connectivity of atoms
Determ
inationof Form
ulae
Empirical form
ulaBy Com
bustion Analysis
From Com
position by mass
Molecular form
ula
From em
pirical formula and
m
olecular mass
Water of crystallization
Chemical Equations
Word Equation
Chemical Equation
Stoichiometry
Stoichiometric coefficients
Calculation based on equationsReacting m
asses
Volumes of gases
Volum
etricA
nalysis
Titration
Standard solutionA solution w
ith accurately known concentration
Standardization
Process of determination of concentration of a
non-standard solution
Primary standard
A substance which can be used to prepare a
standard solution directly
criteria
known com
position
stable composition
readily soluble
preferably with higher
m
olecular mass
Equivalence point
The point at which the 2 reagents just react w
ith
each other completely.
End pointThe point w
hen the end of titration is detected
Acid-base
titration
Using indicator
Choice of indicator
strong acid vs strong base
methyl orange or
phenolphthalein
strong acid vs weak base
methyl orange
weak acid vs strong base
phenolphthalein
Without indicator
pH m
eter (titration curve)
thermom
etric titration
conductometric titration
Redox titration
Iodometric titration
iodine / potassium iodide
starch solution
sodium thiosulphate
Determ
ination of concentration of
chlorine bleach
KMnO
4 vs FeSO4
KMnO
4 vs H2C2O
4.2H2O
Constant heating is required
Back titrationExam
ple
Determ
ination of percentage by mass of calcium
carbonate in chalk
Analysis of aspirin
Used if the reaction of direct titration is not
instantaneous
/ if no suitable indicator for direct titration
methods for titration of
weak acid vs w
eak alkali
3. Chem
ical Equations and S
toichiometry.m
map - 8/1/2005 -
-
The ElectronicStructure of A
toms
Different m
odels ofatom
Bohr's model
Atomic Em
ission spectrum of
hydrogen
Setup of the apparatus
gas discharge tube
slit
prism or diffraction grating
photographic film
Characteristic of the hydrogenspectrum
Num
erous lines
Different series
Unevenly spaced lines in each
series
Convergence limits at the
higher frequency end of eachseries
Continuum
Interpretation of the hydrogenspectrum
Line spectrum
Introduction of concept ofquantum
Planck's equation
Process of excitation andrelaxation
ground state
excited state
Num
erous lines
Presence of numerous energy
levels (shells)
principalquantum
number
Multiple transitions take place at the sam
e time
Unevenly spaced lines
Unevenly spaced energy levels
Convergence limit
and continuumIonization of atom
Uniqueness of atom
ic emission
spectraAs a m
ean of identification
Flame test
Atomic em
ission spectroscopy
Discovery of helium
Identification of element at
remote star
Red shift
Neon light w
ith differentcolours
Ionization energies
Definition
Successive ionization energiesEvidence of shell
1st ionization energies acrossperiods
Evidence of subshell
Features of Bohr's model
Electrons are orbiting around the nucleus in orbits with w
ith different energy
The allowed energy of the electron in an atom
is quantumized
Able to explain the emission spectrum
of hydrogen spectrum but fail to explain the em
ission spectrum of those
multi-electron system
.
Wave-m
echanical model /
Orbital m
odel
Wave-particle duality of electron
Heisenberg's uncertainty principle
Quantum
numbers
By mathem
atical modeling
Principal (n)
n = 1,2,3 ...
principal energy
size
shell no.
Subsidiary (l)
l = 0,1,2...(n-1)
shape of orbital / subshell
energy in multi-electron
system
Magnetic (m
)m
= -l,...,0,...+l
spatial orientation of orbital
Spin (s)s = +
or -
spin of electron
Pauli Exclusion principle
Orbital / Electron cloud representation
Radial probability distribution
Nodal surface
Nodal plane
Background knowledge
Electromagnetic Spectrum
Continuous Spectrum
Line Spectrum
4. The Electronic S
tructure of Atom
s.mm
ap - 19/1/2005 -
-
Electronicconfigurations andthe Periodic table
Building up of electronicconfiguration on paper
Aufbau PrincipleElectrons occupy the orbital w
ith the lowest energy first
Order of energy of different orbitals
Pauli exclusion principle
All electrons behave different in an atom i.e. no 2
electrons in an atom have the sam
e set of quantum no.
Each orbital can accomm
odate2 electrons w
ith opposite spin
Hund's Rule (Rule of M
aximum
Multiplicity)
Electrons will occupy degenerate orbitals singly
with parallel spin before pairing up occurs.
Irregularity
Energy of 4s orbital is only a littlebit low
er than that of 3d orbital
The difference gets even smaller w
hen3d orbitals are being filled up
3d electrons shield 4s electron from the nucleus
and make 4s electrons m
ore energetic
Exception
Chromium
half-filled 3d and 4s orbital
no electron pair-up in 3d and 4sless repulsion
more evenly distributed electron cloud
less repulsion
Copperfull-filled 3d and half-filled 4s orbital
more evenly distributed electron cloud
less repulsion
Representation ofelectronic configuration
by notation
by electron in boxes
use of abbreviationN
oble gas cores e.g. [He], [N
e] etc.
Completely filled quantum
shell e.g. K, L, M
Variation of Successiveionization energies
Successive ionization energy is always
much higher than its predecessor
the p to e- ratio is increasing
the repulsion/shielding among electron is getting less
more difficult to rem
ove electron from a positive particle
Plot of Successive ionizationenergies of an atom
In log scale
the increases in successive ionization energies are toolarge to
be represented by linear scale
appear in tiersProof of the presence of shells
Plot of successive ionizationenergies of different atom
s
shift upwards
the p to e- ratio is increasing
shift to the right
the electronic configuration of an ion is the same as
the electron configuration of the preceding atom
Atom
ic radius
Covalent radius
including metallic radius
half of the distance between 2 chem
icallybonded nuclei of the sam
e element
Depending
on
Attraction with the nucleus
Nuclear charge
Repulsionam
ongelectrons
Relative position of theouterm
ost e- (Electronic configurationof the atom
)
Primary
shielding effect
repulsion with
the inner e-
usually stronger
Secondary shielding effect
repulsion with the
e- in the same shell
usually weaker
General trend
contract across a period
the increase in nuclear charge outweighs the increase in
secondary shielding effect.
expand down a group
the increase in primary shielding effect outw
eighs theincrease in nuclear charge
van der Waals' radius
half of the distance between 2 nuclei of the sam
e element w
hichare not chem
ically bonded together in a crystal lattice
Periodic Tables-block, p-block, d-block and f-block elem
ents
Variation of FirstIonization energyacross the first 20elem
ents
Definition
Energy required torem
ove 1 mole of
electrons from 1
mole of gaseous
atom
Value reflects the netattraction betw
een the outerm
ost electronand the nucleus
Gets m
ore endothermic w
henincrease in attraction w
ith e-> increase in repulsion am
ongelectrons, and vice versa
Energy required to move the outerm
ost electron to infinityand escape from
the attraction of the nucleus
Variation
Depending
on
Effective nuclearcharge (nuclearcharge -
shieldingeffect)experienced bythe outerm
ost e-
Attraction with the nucleus
Nuclear charge
Repulsionam
ongelectrons
Relativeposition of theouterm
ost e- (Electronicconfiguration ofthe atom
)
Primary
shielding effect
repulsion with
the inner e-
usually stronger
Secondary shielding effect
repulsion with the
e- in the same shell
usually weaker
Position of the outermost e- (Size of atom
)
General
pattern
Main trend
Getting m
ore endothermic
across a period
the increase in nuclear chargeoutw
eighs the increase insecondary shielding effect
Getting less endotherm
ic down
a group
the increase in primary
shielding effect outweighs the
increase in nuclear charge
Showing 2-3-3
pattern acrossperiod 2 and 3
Imply presence of subshell
Imply pairing up of electrons in an orbital
Unexpected drops
at B and Al(or peaks at Be and M
g)
Electron goes into a more
energetic p-subshell(or full filled s-subsell in Beand M
g - more evenly
distributed charges)
Unexpected drops
at O and S
(or peaks at N and P)
Electrons starts pairing up in Oand S - extra repulsion (or
halffilled p-subshell in N
and P - m
ore evenly distributed charges)
5. Electronic configurations and the P
eriodic table.mm
ap - 6/1/2005 -
-
Energetics SignificanceThe link betw
een the microscopic interaction am
ongparticles and m
acroscopic observable change.
EnthalpyandInternalenergy
Enthalpy changeD
efinition : heat change of a system at constant pressure
Internal energy change
Definition : heat change of a system
at constantvolum
e
Relationship between Enthalpy
change and Internal energychange
Enthalpy change = Internal energy change +W
ork done on the atmosphere
Exothermic and
Endothermic process
Exothermic
process
Heat flow
s out of the system because of the
increase in temperature of the system
Potential energy among the particles is converted
to heat energy when there is an overall
strengthening of attractions among the particles.
Bond formation > Bond breaking
Endothermic
process
Heat flow
s into the system as the tem
peratureof the system
decreases
Heat energy is converted to potential energy
when there is an overall w
eakening of attractionsam
ong the particles.
Bond formation < Bond breaking
Hess's Law
The total enthalpy change of a chemical reaction is
independent
of the route by which the reaction takes place
Another version of law of
conservation of energy
Energy cannot be created ordestroyed
Energy cycle & energy level diagram
Born-Haber cycle as energy cycle used
to determine the heat of form
ation
Need of H
ess's Law
Some changes has no direct reaction at all
The direct reaction may be too vigorous
and too dangerous to be carried out
The extent (% of conversion) ofreaction cannot be
controlled.
Formation of side products
Calculation
Prerequisite
Balanced equation according to definitions
Calculation involving heat capacity
Calculation of mole
Patience and careful manipulation
Some com
mon references in
constructing energy cycles
1. Constituent elements
2. Combustion products
3. Constituent atoms
unit and sign of the answer
+ and - sign must be included
Unit, usually in kJm
ol-1 must
be included
Standard enthalpychanges
Standard conditions298K
1 atm
Standard state
pure, if it is a liquid or solid
most stable form
at 298K, 1 atm
1M if it is an solution
Need of standard state and
condition
Presence of allotrope andpolym
orph
Different form
s of the same elem
ent /com
pound in the same physical state
e.g. graphite and diamond,
ozone and diatomic oxygen
Enthalpy change as Heat flow
(q) of a system at constant pressure returning
the system to the original tem
perature under a specific condition.
Examples and definitions
Heat of N
eutralizationForm
ation of 1 mole of w
ater
Neutralization of strong/w
eak acid / alkali
Heat of Solution
A solution with specific concentration /
an infinitely dilute solution is prepared.
Heat of Form
ation
1 mole of com
pound is formed from
its constituentelem
ents in their standard states
Heat of Com
bustion
Complete com
bustion of 1 mole of a specific
substance in excess oxygen
Heat of H
ydration
Formation of 1 m
ole of hydrated crystalfrom
anhydrous solid
Experimental determ
ination
bomb calorim
etry todeterm
ine the internalenergy change
Experimental setup
conversion to enthalpy changeby calculation
Philips Harris calorim
eter todeterm
ine heat of combustion
Experimental setup
simple calorim
etry todeterm
ine the enthaplychange
Experimental setup
Polystyrene cup + cover
Insulating layer
thermom
eter
stirrer
Com
mon
assumptions
Specific heat capacity ofsolution is the sam
e as that ofw
ater
The rate of heat loss isuniform
/ No heat loss to the
surrounding.
The heat capacity of some
components in the system
canbe ignored.
The specific heat capacities ofsom
e components are only
approximation.
Heat evolved w
ill be the same
as heat flow in the original
definition of enthalpy change
6. Energetics.m
map - 17/1/2005 -
-
Ionic Bonding
Energeticsofform
ation
Born Haber Cycle
Heat of form
ationform
ation of 1 mole of com
pound
Atomization energy of elem
ent
formation of 1 m
ole of gaseousatom
s
always positive
Ionization enthalpy of atom
Removal of 1 m
ole of electronsfrom
1 mole of gaseous atom
s/ ions
always positive
Electron Affinity of atom
Addition of 1 mole of electrons
to 1 mole of gaseous atom
s /ions
negative : attraction between the incom
ingelectron and the nucleus > the repulsion betw
eenthe incom
ing electron and the existing electrons
positive : attraction between the incom
ingelectron and the nucleus < the repulsionbetw
een the incoming electron and the existing
electrons
Lattice energy
Formation of 1 m
ole of ioniccom
pound from its constituent
gaseous ions
always negative
a measure of strength of ionic
bond
depending on charge densityof ions involved
depending on the packingefficiency i.e. crystalarrangem
ent
Stoichiometry
ofioniccom
pounds
Limitation of using stable
electronic configuration toexplain
Positive metal ion + free electron are actually m
oreenergetic (less stable) than a gaseous m
etal atom
Indeed, the stability of an ionic compound is m
ainlyarising from
the formation of ionic bond am
ongoppositely charged ions.
Nature prefers the
stoichiometry w
iththe m
ost exothermic
heat of formation
i.e. highest stability com
paring with the
constituent elements
Construction of Born Harber cycle for
the naturally occurring ioniccom
pound e.g MgCl2
Construction of BornH
aber Cycle for thehypothetical ioniccom
pounds e.g. M
gCl and MgCl3
Hypothetical lattice energy
calculated from the ionic m
odel
Assum
ptionsofionicm
odel
The latticearrangem
ent of thehypothetical com
poundis know
n
All ions are perfect spheres
Cations and anions are just incontact w
ith each other
The bond is purely ionic
The value of heat of formation is m
ainly determined
by the values of ionization energies and lattice energy
Ioniccrystals
Different
arrangements
of lattice points
face centered cubic(f.c.c.) structure
e.g. NaCl
C.N. = 6 for both ions
simple cubic structure
e.g. CsClC.N
. = 8 for both ions
Actual arrangement is
depending on
Stoichiometry of the
compound
ionic radius ratio
Some term
s
Coordination number (C.N
.) -no. of nearest neighbour
Determ
ination of empirical
formula
Unit Cell
Definition
1. the smallest unit containing
all the fundamentals of a crystal
without repetition.
2. the smallest unit of a crystal
from w
hich the original crystalstructure can be recreated byrepetition
in 3-dim
ensional space.
Determ
ination of no. ofdifferent ions in an unit cell
Determ
ination ofem
pirical formula
Lattice point
The position of a formula unit
of the compound considered.
ionic radius
Definition - D
istance from the centre of an ion to
the point of zero electron density between 2
adjacent ions.
determined by X-ray
diffractionElectron density m
ap
Value
depending on
no. of atoms
present
polyatomic ion
is bigger thansim
ple ion
p to e- ratio
In general, negativeion is bigger thanpositive ion
Size of isoelectronicion
As the no. of electrons andelectronic configurations are
thesam
e, the size is only dependingon the num
ber of protons present.
Balance between
non-directionalelectrostatic attractionand repulsion
Attraction among oppositely
charged ions
Repulsion among sim
ilarlycharged ions
Potential well diagram
7. Ionic bonding.mm
ap - 19/1/2005 -
-
Covalent bonding
Balance between
electrostatic attractionand repulsion
Attraction between nucleus,
bonding electron and anothernucleus
directional attraction
Replusion among the electrons
Replusion between the nuclei
Potential well diagram
Formation of
covalent bond
Overlapping of atom
ic orbital
Constructive interference ofatom
ic orbitals
Increase in electron densityalong the inter-nuclear axis
Pairing up ofunpaired electrons(bonding electrons)
Promotion of paired electrons
to low lying energy orbital
(e.g. 2p comparing w
ith 2s, 3dcom
paring with 3p) to yield
more unpaired electrons for
bond formation
Formation of
more bonds
More overall
exothermic change
High overall stability
Dative covalent bond
a covalent bond in which the
bonding electrons are suppliedby one atom
only
Overlapping of a lone pair w
itha vacant orbital
e.g. NH
4+, H3O
+ NH
3BF3, Al2Cl6
Draw
ing of Lewis
structure
Formal charge
# of p - # of e- associated with
an atom assum
ing that bondinge- are equally shared
Oxidation num
ber
# of p - # of e- associated with
an atom assum
ing that allbonding e- are belonging tothe m
ore electronegative atom
Cases obeying octet rule
Period 2 elements never
excess octet
3s orbital has much higher
energy than 2p orbital
Energetically unfavorable
Can be lessthan octet
e.g. BF3
an electron acceptor with a
vacant p-orbital for dativebond form
ation
Cases not obeyingoctet rule
Expansion of octet
Availability of low lying energy
orbital (e.g. 3d orbitalcom
paring with 3p orbital)
e.g. PCl5, SO42-
Giant Covalent Structure
Diam
ond
tetrahedrally arranged
strong C-C single bond only
high hardness, high melting point, electrical insulator,
excellent heat conductor
Graphite
arrange in hexagonal layers
strong C-C bond, bond order 1 1/3, within layer
weak van der W
aals force between layers
low hardness, very high m
elting point, conducting along the layer, slipperybetw
een layers
Quartz (SiO
2)high hardness, high m
elting point
Shape of molecules and
polyatomic ions
Valence shell electron pairrepulsion theory
Electron centres : lone pair,single bond, double bond,triple
bond
Electrons centres tend toseparate from
each other asfar as possible to m
inimize
mutual repulsion
Electrostatic repulsion is verysensitive to distance
repulsion between lone
pair-lone pair > lone pair-bondpair > bond pair-bond
pair
lone pair always occupies
equatorial position in a fiveelectron centres system
Description of shapes
linear, angular (V-shaped or bent), trigonal planar,
tetrahedral, trigonalpyram
idal, trigonal bipyramidal, seesaw
, T-shaped, octehedral, squareplanar
In the describing of the shape of molecule, the presence of lone
pair is
not included.
Strength of Covalentbond
In the range of 150-1000 kJmol-1
Bond Dissociation Energy
D(X-Y)
Energy required to break a particular bond
(Average) Bondenthalpy/energy E(X-Y)
The average energy required to break a type of bond
Estimation using enthalpy change of atom
ization
Using bond energy to estim
ate the enthalpy change of a reaction
Factors affecting bondstrength
Bond orderno. of bonding electrons / 2
Bond length
The longer the bond length, the weaker w
ill be thebond(Prim
ary) shielding effect affects the nuclearattraction experienced
by the bonding electrons
Atomic size
(covalent radius)
Estimation of bond length
Breakup of additivity ofcovalent radius
Polarity of bond
Resonance
bond polarity
covalent bond with certain ionic character is found to be
stronger than purely covalent or purely ionic bond
repulsion among the lone pairs
on adjacent atoms
e.g. weak F-F, O
-O and N
-Nbond
8. Covalent bonding.m
map - 2005/5/21 -
-
Intermediate Type ofBonding
Starting frompure ionic bond
Assum
ptionsof sim
pleionicm
odel
The lattice arrangement of the com
pound is known, if it is hypothetical
All ions are perfect spheres
Cations and anions are just in contact with each other
and clearly separated from each other
The bond is purely ionic. i.e. no sharing of electron or
incomplete transfer of electron
For all real ionic com
pounds
The lattice energycalculated based onthe ionic m
odel isnever the sam
e asthe experim
entalvalue
A more agreed value im
pliesa situation sim
ilar to theassum
ptions of ionic model -
pure ionic bonde.g. N
aCl (almost purely ionic)
A very differentvalue im
plies failureof the ionic m
odel -w
ith strongcovalent character
The ions are notperfectlyspherical
Mutual repulsion
of electron cloudin an ionic lattice
Polarization ofanion
Ions may not
clearly separatedfrom
each other
Certain covalentcharacter
Polarization ofanion
Anion is always
polarized bythe cation
Polarization - distortion of electron cloudof anion under the
influence of cation
e.g. AgCl, AgBr, AgI, ZnS
Polarizabilityof an anion
ease of distortion ofelectron cloud
inversely proportionalto the nuclearattraction on theelectron
cloud
p to e- ratio
electronicconfiguration
primary
shieldingeffect
secondaryshieldingeffect
Polarizing power of a cation
directly proportional to thecharge density of a cation
Electronegativity
Tendency of an atom to
attract the bonding electron ina stable m
olecule
Increase across a period andup a group
Not applicable to noble gases
Depending on the effective
nuclear charge experienced bythe bonding electron
nuclear charge
shielding effect
Pauling's scale
ionization energy
reflects the attraction on theouterm
ost electron in an atom
electron affinity
reflects the attraction on theincom
ing electron to be addedto an atom
Fluorine has the highest assigned value : 4.0
The percentage of ionic character of a covalent bond is
depending onthe difference in electronegativity of the 2 elem
ents bonded together
Starting frompure covalentbond
Pure covalent bond
The bonding electronsare shared equally
The bond isnon-polar
e.g. Cl2
Polarcovalentbond
The bonding electronsare not shared equally
atoms have different
electronegativitye.g. H
Cl
(Permanent)
Dipole
mom
ent
Definition - product of quantity of charge (q)
and distance of charge separation (d)
The dipole mom
ent along a bond is pointing fromthe less electronegativity
atom
to the more
electronegativity atom
Lone pair
Contribute a dipole mom
entpointing aw
ay from the atom
Polarityof am
olecule
Depending on the vector
sum of all dipole
mom
entsG
eometry of the m
olecule
magnitude of individual
dipole mom
ent
Non-polar
molecule
A molecule w
ith non-polarbond only
A molecule w
ith all dipolem
oments canceling out each
other
Do not /
weakly
attracted byan electricfield
Non-polar m
oleculesare also attracted byan electric field as aw
eak temporary dipole
mom
ent will be
induced onto them by
the electric field
Polar molecule (D
ipole)
A molecule w
ith non-zeronet dipole m
oment
May not be able to tell the
actual direction of the netdipole m
oment
Attracted by an electric field
9. Intermeidate type of bonding.m
map - 26/1/2005 -
-
Metallic bonding
Nature of m
etallic bond
Sea of electrons model
lattice of positive ions
imm
ersed in a sea ofdelocalized electrons
Electrostatic attraction among positive
ions and sea of delocalized electronsN
on-directional in nature
metallic radius = covalent radius
uniqueproperties ofm
etal
high conductivity
agitation of delocalizedelectrons
high ductility andm
alleability
delocalized electrons capable to reform the
metallic bond even if the positive ions are shifted
metallic luster
delocalized electrons getexcited and give out luster
Strength of metallic
bond
In the range of 100-500 kJmol-1
Attraction on the delocalizedelectrons
Charge density of the positiveion e.g. N
a+ > K+ > Rb+
Availability of delocalizedelectron
no. of valence electrons e.g.Al > M
g > Na
high charge density of the positive ion limited the
availability of the valence electrons e.g. Hg
Big difference between m
.p.and b.p.
In molten state, m
ost metallic
bond are not broken
Metallic crystal (G
iantm
etallic structure)
close-packed structure (74%)
hexagonal close packing(h.c.p.)
C.N. = 12
abab... pattern
(face centred) cubic closepacking (f.c.c. or c.c.p.)
C.N. = 12
abcabc... pattern
tetrahedral holes andoctahedral holes
open-structure (68%)body centered cubic (b.c.c.)
C.N. = 8
Determ
ination of no. of atoms
in an unit cell10. M
etallic bonding.mm
ap - 22/2/2005 -
-
Intermolecular
Forces
Type of dipole (polarm
olecule)
permanent dipole
permanent separation of centre of positive charge and centre of
negative charge
possessed by all polar molecules (dipoles)
instantaneous/temporary
dipole
the fluctuation of electron cloud createsan instantaneous
dipole
possessed by all kinds of molecules including polar m
olecules
induced dipoleinduced by a perm
anent dipole
induced by instantaneous dipole
van der Waals forces
EvidenceReal gas equation
Real gas doesnot obey idealgas lawperfectly
assumption of
ideal gas law
individual particle occupies novolum
e
no interaction (attraction andrepulsion) am
ong particles
van der Waals' constants
a - attraction among particles
b - volume of individual particle
Condensation of solid and liquid at low tem
p.
Origin
dipole-dipole interaction
also called permanent
dipole-permanent dipole
interaction
attractions among opposite
ends of permanent dipole
dipole-induced dipoleinteraction
also called permanent dipole
induced dipole interaction
a permanent dipole induce a
temporary dipole onto another
polar/non-polar molecule
the opposite ends attractedeach other
instantaneous dipole-induceddipole interaction
other names
dispersion force
London force
temporary dipole-tem
porarydipole interaction
induced dipole-induced dipoleinteraction
an instantaneous dipole inducea tem
porary dipole on anotherm
olecule
the opposite end of differenttem
porary dipoles attract eachother
Strength (1/10 ofcovalent bond)5-50 kJm
ol-1
Polarm
olecules
with dipole-dipole
interaction, dipole-induceddipole interaction
andinstantaneousdipole-induced dipoleinteraction
stronger overallstrength form
olecules with
comparable size
higher b.p. and m.p.
Non-polar
molecules
with instantaneous
dipole-induced dipoleinteraction only
weaker overall strength
for molecules w
ithcom
parable sizelow
er b.p. and m.p.
depending on thepolarizability ofthe m
olecule
increases with increasing
molecular size / surface area
e.g butane vs 2-methylpropane
branched isomer
of hydrocarbonhas low
er boilingpoint
polarizability of constituent atoms
e.g. telfon/poly(tetrafluoroethene)
van der Waals' radius
half of the internuclear distance between 2 atom
sof the sam
e element w
hich are not chemically
bonded together in a crystal lattice
Molecular crystal
iodine
face-centred cubic structure(f.c.c)
dry ice
face-centred cubic structure(f.c.c)
Hydrogen bond
An exceptionally strong directional dipole-dipole
interaction
Attraction between a
naked proton and a lonepair
hydrogen atom bonded onto a very
electronegative atom / entity
H on F, O
and N
including H on CH
Cl3lone pair on a very electronegative atomlone pair on F, O
and N only
Strength (1/10 of covalent bond)5-50 kJm
ol-1
overall strength depends on
difference in electronegatvitiy
average no. of hydrogen bondper m
olecule
2 per H2O
molecule, 1 per H
F,N
H3, M
eOH
molecule
hydrogen bonded molecules also
possesses van der Waals' forces
Hydrogen bonded m
olecule has higher boiling point thatordinary polar m
olecule with com
parable size.
e.g. HF, H
2O, N
H3, M
eOH
comparing w
ith otherm
olecules with com
parable size
Estimation of strength of hydrogen bond
Experimental
H-bond form
ationM
ixing trichoromethane and ethyl ethanoate
H-bond breaking
Mixing ethanol and hexane
Period 2 hydrides comparing w
ith hydrides of period 3 to 5
intermolecular and
intramolecular hydrogen
bond
intermolecular
formed betw
een molecules
(usually) gives higher b.p.
(usually) gives higher solubilityin w
ater
formation of new
intermolecular forces >
breaking of old intermolecular
forcesa substance w
ould be soluble
ability to form hydrogen bond w
ith water
overall polarity of the molecule
(actually more im
portant thanability to form
H-bond)
e.g. trans-butenedioic acid
e.g. alcohols
intramolecular
formed w
ithin a molecule
prevents the formation of interm
olecular hydrogen bond(usually) gives a low
er b.p.
Examples
dimerization of carboxylic
acids in vapour state /non-aqueous solvent
forming bond is better than
not forming bond
forming strong bond is better
than forming w
eak bond
unique properties of water
highest density at 4 C
Low density of ice
water expands upon freezing
open structure of icediam
ond like structure
six mem
bered rings of O linked
by O-H
O linkage
Variation of density with
temperature (K.E.)
from below
0 C to 0 C
thermal expansion of solid
structuredensity drops
at 0 Cm
elting of ice
rapid collapsing of open structuredensity increases sharply
from 0 C to 4 C
continuous collapsingof open structure
density increases continuously
still possesses certain hexagonal network
from 4 C to 100 C
thermal expansion of liquid w
aterdensity drops
formation of secondary
structure in protein and DN
A
Proteinsingle helical structure
hydrogen bond between N
-H and C=O
group among different am
idegroups along the polypeptide chain
DN
Adouble helical structure
hydrogen bond between
heterocyclic bases on 2separate strands
cytosine - guanine pair3 H
-bonds
thymine - adenine pair
2 H-bonds
11. Intermolecular Forces.m
map - 2005/5/23 -
-
Structure andproperties of
materials M
olecular structure
Simple m
olecular
e.g. H2O
, NO
2, S8, P4
under 100 atoms
fat and oil - marginal cases, w
ith just over 100 atoms
weaker interm
olecular forces
usually gases, liquids or low m
elting solids
soft and weak
solubility depending on the polarity of the solute and solventIn
general, like dissolves like
Macrom
olecular
e.g. starch, protein, plastic
over thousands of atoms
stronger intermolecular forces
usually solid
non-conductingno delocalized electrons
Giant structure
Giant covalent structure
e.g. diamond, quartz, graphite
strong covalent bond
hard
high density
high melting point
insoluble in any solventinterm
olecular forces never stronger than covalent bond
usually non-conductor ofelectricity
bonding electrons in most covalent bonds are not delocalized
usually poor conductor of heat
Exception - diamond is an
excellent conductor of heatthe rigid structure transm
its thevibration of the C atom
s efficiently
Speical case : Graphite
brittlew
eak van der Waals' forces betw
een layers
slipperyflat hexagonal layers
weak van der W
aals' forces between layers
conductingpresence of delocalized electrons along the hexagonal
layers
high melting, even higher than diam
ondstrong C-C bond, bond order 1 1/3
lower density than diam
ondlarge space betw
een layersw
eak van der Waals' forces
between layers
Giant ionic structure
strong ionic bond
high m.p. and b.p.
hard
high density
brittleslight dislocation brings ions of sim
ilar charge togetherrepulsion cleaves the crystal
presence of ionsthe ions are not free to m
ove in solid stateonly conducts in m
olten or aqueous state
If soluble, only soluble in aqueous solventdipole - ion
interaction
increase in disorderness
Giant m
etallic structure
sometim
e, strong metallic bond
high m.p. and b.p.
hard
high density
Presence of sea of delocalized electronsm
alleable and ductile
high conductivity of heat and electricity
Soluble in mercury
12. Structures and properties of m
aterials.mm
ap - 2005/6/5 -
-
Rate of Chemical
Reactions Definition of Rate
Change in quantity per unit time
Quantity is usually expressed in m
oldm-3
Other possible units : m
ol, g For gases : Pa, m
mH
g, atm,
cm3, dm
3
Time is usually expressed in s
Unit of rate of reaction
Depending on the units of quantity and tim
e
Usually expressed in m
oldm-3s-1
Different rate
Average Rate
Instantaneous rate
Initial Rate(Instantaneous rate at t = 0)
Differential form
of rate equation / law
Measuring of
Reaction Rate
Constant time approach
(fix the time interval
and determine the
amount present)
By titrationQ
uenching of reaction before titration is done
by sudden cooling
by dilution with a large am
ount of water
by removing the catalyst
by removing one of the reactant
By measuring a physical quantity related to the
amount of a chem
ical
Measuring of volum
e of gas
Measuring of colour intensity
Transmittance and Absorbance
Need of a calibration curve
Measuring of m
ass of thereaction m
ixture
Constant amount approach
(fix the amount to be
reacted and determine
the time required to
complete the reaction)
Disproportionation of
Na2S2O
3 in acidic medium
constant amount of S ppt.
produced to mask the cross
Methanal Clock
experiment
constant amount of m
ethanal reacted / HSO
3- consumed
phenolphthaleinindicator
Iodine Clockexperim
entI- vs H
2O2
constant amount of I2 produced / S2O
32- consumed
starch indicator
Reactionbetw
een Br- andBrO
3- in acidicm
edium
constant amount of Br2 produced / phenol consum
ed
methyl red
indicator
Factors affectingReaction Rates
Concentration of reactants
Pressure
Temperature
Surface Area
Catalyst
Light
v
13. Rate of C
hemical R
eactions.mm
ap - 2005/6/5 -
-
Rate Equations andOrder of Reactions
Rate Equation / Law
Differential form
Ordinal form
Order of reactionOrder with respect to individual reactant
Overall order of the reaction
Rate constant k
Unit of rate constant is depending on the overall order ofthe
reaction
A value depending on temperature only
Different order withrespect to a reactant
Zeroth order
the rate is independent of theconcentration of the reactant
Usually occurring in catalyzed reactionwhere the availability of
the catalystsurface is the limiting factor of the rate
First order
the rate is directly proportional to theconcentration of the
reactant
Second order
the rate is directly proportional to thesquare of the
concentration of thereactant
Integratedform
Zeroth order reaction
half life
First orderreaction
half lifecarbon-14dating
The concentration of C-14 inatmosphere is assumed to
beconstant
The percentage of C-14 in a livingorganism is assumed to be
constant.
By comparing the percentage/betaemission of C-14 in an
archaeologicalsample and the percentage/beta emissionof C-14 in a
new sample, the age of thearchaeological sample can be
estimated.
The rate of decay is independent oftemperature as it doesn't
involve collisionamong particles
Second order reaction
half life
Determination ofrate equation / law
Initial Rate method From the initial rate with different initial
concentrations
By plottinggraph
If there is more than one reactant involved, keep the
concentrations of all reactantsother than the one being studied
virtually constant by keeping them in large excess
By trial anderror
By plottingconcentration againsttime Straight inclined line
zeroth order
By plotting rate againstconcentration
Horizontal line zeroth order
Straight inclined line first order
parabola second order
By plotting rate against [A] or [A]2 etc to see which will givea
straight inclined line passing through the origin
By plotting [A] or ln[A] or 1/[A] against time
Plotting log (rate) against log(concentration)
The slope will be the order of reaction withrespect to the
reactant
14. Rate Equations and Order of Reactions.mmap - 2005/1/31 -
-
The Effect ofTem
perature Changeand Catalyst onReaction Rate
Simple Collision
Theory (EffectiveCollision)
(A)Arrhenius Constant
(Z) CollisionFrequency
Vary only a little when the tem
perature change
(P) Orientation / geom
etrical/ probability factor
Depending on the
nature of the reaction
Energy larger thanactivation energy
Arrhenius Equation
Activation Energy
An energy barrier the reactants need to overcome for the
reaction to proceed.
Determ
ination ofactivation energy
By varying the temperature and m
easuring the rate of a reactionA plot of ln k versus 1/TA plot
of ln rate versus 1/T ifinitial rate m
ethod is usedA plot of ln 1/t versus 1/T if constantam
ount approach is adopted
Repeat a reaction at two
different temperature
In the reaction k can bereplaced by rate if initial ratem
ethod is usedk can be replaced by 1/t if constantam
ount approach is adopted
Energy profile
transition state (oractivated com
plex)
e.g. a pentavalent transitionstate
can never be isolated
Single stage reaction
Multi-stage Reaction
Reaction intermediate
e.g. carbocation
can be isolated at low tem
perature
rate-determining step
(r.d.s.)The step w
ith the highest activation energyThe step w
ith the lowest rate
Other exam
ples
Haber process
Fe(s)
Contact processPt(s) or V2O
5(s)hydrogenation of oilN
i(s), Pt(s) or Pd(s)
Gasification of naphtha in product of tow
n gasN
i(s) and NiO
Catalytic ConverterRh(s) and Pt(s)
Enzymes
zymase in the ferm
entation by yeast
Catalysis
Effect of catalyst
Providing analternative pathw
ayw
ith a differentactivation energy,usually low
er.
positive catalyste.g iron in H
arber processBy breaking up a single step into a m
ulti-stepsreaction w
ith lower activation energy
negative catalyste.g. H
+ in decomposition of H
2O2
homogeneous
catalysis
The catalyst has the same phase as the reactants and product
e.g.acid-catalysedesterification
heterogeneouscatalysis
The catalyst has a different phase as the reactants and
product
e.g. Catalyticdecom
positionof hydrogenperoxide bym
aganese(IV)oxide
chemsorption / adsorption on the surface of the catalysis
Maxw
ell-Boltzmann
distribution ofm
olecular speeds
The distribution istem
peraturedependent
The total area under the curve is independent of
temperature.
i.e. the total no. of particles present
15. The Effect of Tem
perature Change and C
atalyst on Reaction R
ate.mm
ap - 21/5/2005 -
-
Dynam
ic Equilibrium
Equilibrium
A state where no m
acroscopic
change is observed.
All chemical equilibria are
dynam
ic in nature. i.e.
forward rate = backw
ard rateExam
ples
Thermal dissociation of
calcium
carbonate
Esterification
Action of acid and alkali on
bromine w
ater
Action of acid and alkaline on
potassium dichrom
ate
Action of acid and alkali on
bismuth chloride
The amounts of reactant and
product are usually not equal
in an equilibrium
.
Equilibrium
position
Definition : a set of reactant and product concentration
Reaction Quotient (Q
)
Using of equilibrium
constant and reaction quotient
to predict the direction to which the equilibrium
position w
ill be shifted.
Q = K : the system
is at equilibrium
Q > K : the equilibrium
position will be shifted to the left
Q < K : the equilibrium
position will be shifted to the right
Le Chatelier's principle
The principle states that when a system
in equilibrium is
subjected to a change, the system
will response in a w
ay so that
the effect of the change imposed w
ill be minim
ized.
Effect of change in concentration
Effect of change
in pressure
by decreasing/increasing the volume
by adding one of the product/reactant
by adding an inert gas
this has no effect on
the equilibrium
position
Effect of change in
tem
perature
Use of change in
distribution of m
olecular
speed to explain the
effect of change in
temperature on the
equilibrium
position.
The rate of a reaction
with a higher
activation energy is
more sensitive to the
change in
tem
perature
Effect of catalyst
Catalyst has no effect on the equilibrium position.
It only shortens the tim
e required for a system to
reach a state of equilibrium
.
Catalyst does not change the relative stability of
the products comparing w
ith the reactants
Equilibrium
Law
At equilibrium and only at equilibrium
,
the order of reaction with respect to a
species w
ill be the same as the
stochiom
etric coefficient of the species
Equilibrium
constants
Examples
Generic equilibrium
constant, Kincluding all species
Equilibrium constant in term
s
of concentration, Kc
Equilibrium constant in term
s
of partial pressure, Kp
Unit of equilibrium
constant
Determ
ination of Equilibrium
constant
The equilibrium concentrations of all species are determ
ined by the
initial concentrations of them and the equilibrium
concentration of
any one of them.
Esterification
Assuming that the rate of reaction is extrem
ely slow
and quenching is not required before any titration is
done.
Formation of
[Fe(NCS)]2+(aq)
complex
Construction of a
calibration curve
using colorimeter
Preparation of solutions with
know
n [Fe(NCS)]2+(aq)
concentration using excess
Fe3+(aq) or N
CS-(aq)
Determ
ination of the equilibrium concentration of
[Fe(N
CS)]2+(aq) using colorimeter and the calibration
curve prepared
Reaction between
Fe2+(aq) and Ag+(aq)
Assuming that the rate of reaction is extrem
ely
slow and quenching is not required before any
titration is done
The titration is self-indicating if standard
KCNS(aq) is used in the determ
ination of the
concentration of Ag+(aq). The solution will turn
red at the end point w
ith the formation of
blood red [Fe(N
CS)]2+(aq) complex.
Significance of equilibrium
constant
The value shows the relative am
ounts of products and reactants when
the system
is at equilibrium i.e. extent of reaction at equilibrium
.
The value is depending on the
relative stability of the
products comparing w
ith the
reactants
The value is temperature
dependent.
Excluding those species having no
effect on the equilibrium position e.g.
solvent(species in large excess),
insoluble solid/liquid
16. Dynam
ic Equilibrium
.mm
ap - 6/4/2005 -
-
Acid Base
Equilibrium
I : Basic Concepts
Different
definitions
Arrhenius definition
acid : substance that produces
H+ in w
ater
base : substance that produces
OH
- ions in water
Bronsted Lowry
definition
acid : proton donor
base : proton acceptor
Lewis defintion
acid : electron acceptor
base : electron donor
An Arrhenius acid (base) must be a Bronsted Low
ry acid (base) and a
Bronsted Lowry acid m
ust be a Lewis acid (base) but not vice versa.
Auto-ionization
(dissociation) of w
ater
Water is a very poor conductor as it alw
ays possesses very small
am
ount of H3O
+(aq) and OH
-(aq) ions i.e. 1 pair of ions for every 556
million w
ater molecules
A neutral solution
has a pH
7
only at 25C.
pH scale
Originated from
French "pouvoir hydrogene" (power of hydrogen).
Every unit increase / decrease in pH scale m
eans a ten-fold decrease / increase
in the concentration of hydroxonium (H
3O+) ion
Measurem
ent of pH
using pH paper / universal indicator
Using pH
meter
Need of calibration using a
buffer solution before use
Acidity and
Basicity
constants
A more negative pKa (pKb) value m
eans a larger Ka (Kb)
value, thus a stronger acid (base).
Successive
ionization of polyprotic
(polybasic) acid
Ka1 is always m
uch
larger than Ka2
HA- is relatively m
ore stable
when com
paring with H
2A than
A2- when com
paring with H
A-
A2- has a much higher charge
density than H
A- and has a very
high potential energy i.e. more
unstable
It is more difficult to rem
ove a
positive proton from negative
H
A-(aq) than from electrically
neutral H
2A(aq)
Beside the small value of Ka2, the second step of the
ionization is also suppressed by the presence of H
3O+ from
the first step of dissociation w
hich will tend to shift the
equilibrium
position of the second step to the left. This is
known as com
mon ion effect (of H
3O+ in this case).
Eventually, the am
ount of H3O
+ from the second step of
ionization w
ill be very minim
al.
Determ
ination of acidity constant Ka by half-way neutralization
The value of Ka is only depending on temperature but the % of
dissociation of a
w
eak acid is depending on concentration as well.
Introduction of the concept of
conjugate acid-base pair
17. Acid B
ase Equilibrium
I.mm
ap - 6/4/2005 -
-
Acid-base
Equilibrium II :
Buffers andIndicators Buffer solution
Definition : A solution that resists change in pH
when
a small am
ount of acid or base is added to it.
Composition : A m
ixture of weak acid and w
eak basei.e. usually a w
eak acid (base) and its salt i.e. theconjugate base (acid).
In the calculation of the pH of a buffer solution, it is
assumed that the inter-conversion betw
een the acid(base) and its salt upon m
ixing is negligible.
The buffering capacity of a buffer solution is dependingon the
concentrations of the acid/base and its salt.
Acid-base
indicators
Acid-base indicator is usually a weak acid (base) w
hichcolour is different from
its conjugate base (acid).
Hum
an eyes are only capable to detect a colour changew
hen the concentration of one coloured substance is more
than 10 times of another coloured substance.
Double
indicatortitration
Add phenolphthalein to the mixture, then
do the first part of the titration (V1).Thereafter, add m
ethyl orange and do thesecond part of the titration (V2).
Examples
A mixture of N
a2CO3 (V1+V2)
and NaH
CO3 (V2)
A mixture of N
aOH
(V1) andN
a2CO3 (V1+V2)
A mixture of N
aOH
(V1)and N
aHCO
3 (V2)
Choosing ofindicator andpH
titrationCurves
strong acid vsstrong alkali
phenolphthaleinor m
ethyl orange
strong acid vsw
eak alkalim
ethyl orange
weak acid vs
strong alkaliphenolphthalein
weak acid vs
weak alkali
no suitableindicator
18. Acid-base E
quilibrium II.m
map - 21/5/2005 -
-
Redox Reactions
Definition
of redoxreaction
Oxidation
Addition of oxygen atom
Removal of hydrogen atom
Losing of electron
Increase in oxidation no.
Reduction
Removal of oxygen atom
Addition of hydrogen atom
Gaining of electron
Decrease in oxidation no.
Disproportionation
reaction
A redox reaction inw
hich an element
undergoesoxidation andreduction at thesam
e time.
Oxidation
no.
no. of proton - no. of electron associatedw
ith an atom assum
ing that all bondingelectrons are belonging to the m
oreelectronegative atom
Rules ofassigningoxidationnum
ber
the sum of the oxidation no. of
all atoms in a species equals
the overall charge carried bythe species.
BalancingRedoxEquations
By Combining
two half
equations
Oxidation
halfequation
Remem
ber thecom
mon strong
reducing agents
reducing agent isthe reagent w
hichis oxidized in aredox reaction
Reductionhalfequation
Remem
ber thecom
mon strong
oxidizing agents
oxidizing agent isthe reagentw
hich is reducedin a redoxreaction
By change inoxidation no.
The change in oxidation no. per atom is the sam
eas the no. of electron being lost or gained by thatparticular
atom
Figure out the no. of atom that w
ill be oxidized andthe no. of atom
that will be reduced by using the
change in O.N
.
Add H2O
to either side of the equation to get thenum
ber of O atom
balanced if necessary.
Add H+ to either side of the equation to get the
number of H
atom balanced if necessary.
Check the presence of H+ or O
H- on the both sides
to ensure it agrees with the acidity/alkalinity of
the medium
used.
Check them
edium of
reaction
For reactions in acidic medium
, there shouldbe no O
H- ion on the both sides of the
equation
For reactions in alkaline medium
, thereshould be no H
+ ion on both sides of theequation
19. Redox R
eactions.mm
ap - 2005/5/23 -
-
Electrochemical Cells
Electrode potential(of a half-cell)
Definition : The difference betw
een the charge on an electrode and thecharge in the solution in
w
hich the electrode is imm
ersed in.
Practically, it is not possible to determine the absolute
potential of a half cell. Therefore, for the
sake of comparison, the electrode potential of standard hydrogen
electrode is defined as 0 V.
The value is depending on the activity of the electrons in the
electrode
This value is also related to the nature, including
concentration, of theelectrolyte in w
hich the electrode is imm
ersed in.
By definition, the electrodepotential of an unknow
n half-cellis the e.m
.f. of anelectrochem
ical cell measured at
standard condition where a
standard hydrogen electrode(SH
E) is used as the left-handelectrode and the unknow
nelectrode is used as theright-hand electrode.
By definition, overall e.m.f of a cell = E(right) - E(left).
When hydrogen electrode is used as the left-hand electrode,
E(right, the unknown electrode) = overall e.m
.f. of the cell
The sign of the electrode potential of the unknown half cell
is the same as the polarity of the right hand electrode
(unknown half cell) in the cell diagram
.
Half
cell
Involvingm
etal
The metal concerned w
ill be usedas the electrode im
mersed in a
solution containing the salt of them
etal. e.g. Cu(s) | CuSO
4(aq)
In some cases, the solution w
ill be saturatedw
ith the salt and with som
e insoluble saltpresent. e.g. Ag(s)|AgCl(s)|Cl-(aq)
Not involving
metal
An inert electrode, e.g. Pt, is imm
ersedin a m
ixture of the reduced and oxidizedform
s of the reagent concerned. e.g. Fe3+(aq) /Fe2+(aq) ,
I2(aq)/I-(aq)
e.g. Pt(s) | Fe2+(aq) , Fe3+(aq)
e.g. C(graphite) | 2I-(aq) , I2(aq)
Standard hydrogen electrodePlatinized = covered w
ith platinum black
= covered with platinum
powder
Pt(s) | H2(g) | 2H
+(aq)
Electrochemical cells
The electrodes of the two half cells are connected
together by an external wire.
The electrolytes of the two
half cell are connectedtogether by using a salt bridge.
Only KN
O3 and N
H4Cl are used in the salt
bridge since the migration speed of the
cation and anion are similar and the voltage
reading will not be affected.
The salt bridge is used to provide cations andanions to balance
the negative and positivecharge cum
ulated in the two half cells during
the discharge of the cell.
In some electrochem
ical cell,a porous partition is used toseparate the tw
o electrolytesinstead of using a salt bridge.
The use of porous partition will create a
junction potential across the partition andthe voltage m
easured will be different from
the case if a salt bridge is used.
Measurem
ent of e.m.f of an
electrochemical cells.
To measure the e.m
.f. of a cell, the currentflow
ing through the external circuit should bekept as m
inimal as possible.
Use a voltm
eter with high im
pedance
Use a potentiom
eter
IUPA
C Celldiagram
A set of symbols are used to represent the set up of an
electrochem
ical cell.
Symbols
used
Depending on the em
phasis and interpretation of the actual setup, it may be
possible to construct more than one IU
PAC cell diagram for a given chem
ical cell.
For a given half-cell, no matter it is being used as a left-hand
or right-hand electrode, the reduced form
should be placed on the leftif the reduced form
and oxidized form are in the sam
e phase. e.g. Pt(s) | Fe2+(aq) , Fe3+(aq) and Fe2+(aq) ,
Fe3+(aq) | Pt(s)
Some
examples
Primary cell
e.g. Zinc-carbon cell
Secondary cell(rechargeablecell)
e.g. Lead acid accumulator
Fuel celle.g. H
ydrogen-oxygen fuel cell
Electrochemical Series
(ECS)
Electrochemical series is constructed by arranging the standard
electrode
potentials measured in an ascending order. i.e. the m
ost negative value first.
By convention, the reduction half equations are tabulatedin the
ECS. Therefore, standard electrode potential is alsoknow
n as standard reduction potential.
The species on the left of thetable are oxidizing agents.
The species on the right of thetable are reducing agents.
Those oxidizing agents at the bottom of the list are strong
oxidizing agents
while those reducing agents at the top of the list are strong
reducing agents.
Concentration andtem
peraturedependence ofelectrode potential
The values list in an ECS are all measured at standard
conditions
and all species involved are in their standard states.
Dependence on concentration
An increase in concentration of an oxidzing agent will shift
the
equilibrium position to the right and m
ake the value more positive.
An increase in concentration of a reducing agent will shift
the
equilibrium position to the left and m
ake the value more negative.
The value is only depending onconcentration (including partial
pressure incase of gas) and tem
perature, the size ofthe electrode has no effect on the
value.
Nernst Equation
(Not required in H
KAL syllabus)Q
: reaction quotient
Use of
electrochemical
series
Compare the strengths of oxidizing agents (or reducing
agents)
To predict the energeticfeasibility of a redoxreaction
A redox reaction with a
positive overall e.m.f.
will be energetically
feasible.
The feasibility of a reaction is also depending on thekinetic
stability (activation energy) of the system
which is
not related to the overall e.m.f. determ
ined.
e.g. disproportionation of Cu+(aq) in water
To predict the overall e.m.f.
of an electrochemical cell
To ensure the overall e.m.f. of the cell be positive,
E(cell) = E(cathode) - E(anode) = E(m
ore positive) - E(less positive)
20. Electrochem
ical Cells.m
map - 9/5/2005 -
-
Phase Equilibrium I :One-component system
PhaseDefinition : region with identical properties.Phase is not
the same as physical state. Usually, there are only 3 physical
states i.e. solid, liquid and gas. e.g. Diamond and graphite are
both in solidstate but they are two different phases of carbon.
PVT surface
Consider an ideal gas obeying ideal gas law PV=nRT, for a given
amount of gas, P is a function of V and T. i.e. P(V,T).
By plotting a graph usingP as the z-axis, V as thex-axis and T
as they-axis, a PVT surface ofan ideal gas can beconstructed. PVT
surface of of a
real substance e.g. water
As it isdifficultto study
a3-dimensionalsurface,theprojectionsalong thevolumeaxis
andthetemperatureaxis areusuallystudiedseparately.
The projection alongthe temperature axis(Isotherms)
Ideal gas
The isotherms of an ideal gasshows that no matter how lowthe
temperature is, the idealgas is still a gas and the volumeis always
inversely proportionalto pressure.
Real substance e.g. water Real gas doesn't obey Boyle's law at
low temperature.
Real gas starts condensing at a temp. lower than the critical
temp. of the substance.
Theprojectionalong thevolume axis(isochors)
Ideal gasThe isochors of an ideal gas shows that no matter how
low the temperature, the ideal gas is still a gas and has zero
volume at 0 K.
Real substance e.g. water
The diagram is known as phase diagram.critical temp : above this
temperature, gas cannot be compressed into liquid without
cooling.AB : sublimation curve - conditions that solid and vapour
can coexist at equilibrium.BC : vaporization / boiling curve -
conditions that water and vapour can coexist at equilibrium.BD :
fusion / melting curve - conditions that solid and liquid can
coexist at equilibrium.B : triple point - the conditions that
solid, liquid and vapor can coexist at equilibrium.BE :
supercooling curve - the condition that a liquid exists at a temp.
lower than its meltingpoint and the liquid is at a metastable
state. This is because freezing also requires activationenergy. If
a liquid is cooled very calmly, the liquid may become supercooled
withoutsolidification.
Real gas condenses at low temperature and becomes liquid and
solid.
Specialfeatures ofthe phasediagram ofwater
The fusion curve of water has a negative slope. This implies
that themelting point of water decreases with increasing pressure.
When a greatpressure is applied to ice, the open structure of ice
will collapse and theice will be turned to water.According to the
Le Chatelier's principle, when a pressure is applied to an
equilibriumsystem, the system will react in a way to minimize the
effect of the change imposed. Forice, it will turn to water which
has a smaller volume to lower the increase in pressure.
Specialfeatures ofthe phasediagram ofcarbondioxide
Like most substances, the fusion curve of carbon dioxide has a
positiveslope. This implies that the melting point of carbon
dioxide increaseswith increasing pressure.The triple point pressure
is higher than atmospheric pressure. This implies that dry ice
willsublime directly to gaseous carbon dioxide without going
through the liquid state.
Specialfeatures ofthe phasediagram ofcarbon
It is possible to convert graphite to diamond by applying
pressure tographite. However, the rate of conversion is very slow
as this involvesrearrangement of atoms in solid state.If graphite
is heated to molten state and allowed to crystallize under high
pressure,artificial diamond can be made. Moreover, the
crystallization is very fast comparing withthe natural process of
formation of diamond, the diamond crystal obtained will be smalland
may not be suitable for making jewelry.
Isotherms
Isochors
Isotherms
Phase diagram ofwater
21.Phase Equilibrium I . One-component system.mmap - 2005/5/9
-
-
Phase Equilibrium II :
Two-Com
ponent System
Difference betw
een1-com
ponent and2-com
ponent systems
In a 1-component system
, the properties of a substance is a function of P and T. This
requires a 3-dimensional space to represent.
In a 2-component system
, theproperties of a substance w
illbe depending on P, T and X(m
ole fraction) which w
illneed a 4-dim
ensional space torepresent.
Instead of studying the problem in a 4-dim
ensional space, the study will be lim
ited to the case in which a liquid and its
saturated vapour coexist in equilibrium.
For a 1-component system
, the (saturated) vapour pressure(P) of a substance is a
constant at a given tem
perature.
For a 2-component system
, the (saturated) vapourpressure at a given tem
perature will be a function of X
(mole fraction) of the com
ponent present.
Raoult's Law
The boiling point of an ideal solution varies linearly with the
com
position approximately.
Deviation from
Raoult's Law(non-ideal solutions)
Positivedeviation
Azeotropic mixture (or constant boiling m
ixture) - A mixture w
ith a constant composition
when it is being boiled continuously.
i.e. the composition of the liquid m
ixture = the composition of the vapour
The components cannot be
separated by fractionaldistillation com
pletely.
Extreme positive
deviation(2 im
miscible liquids)
The 2 components vaporize independently and give a total
vapour pressure higher than that of either component.
Negative
deviation
Extreme negative
deviation(a solution ofinvolatile solute)
The lowering of vapour pressure w
ill cause depression in melting point and elevation of boiling
point of the solvent.
22. Phase E
quilibrium II . Tw
o-Com
ponent System
.mm
ap - 19/5/2005 -
-
Phase Equilibrium III :
Three-Component
Systems
Partition of a solutebetw
een 2 phases
Solvent Extraction(betw
een 2 imm
isciblesolvents)
Partition of a solute between 2 im
miscible solvents
At equilibrium, the rates of diffusion of the solute betw
een two
imm
iscible solvents in opposite directions are the same.
Partition Law - At a
given temp. and at a
state of equilibrium,
the ratio ofconcentrations of asolute in tw
oim
miscible solvents is
constant.
As the unit of concentrationw
ill be canceled out in thecalculation, the unit can beexpressed
in any unit andpartition coefficient w
ill haveno unit eventually.
Most com
mon organic solvent are lighter than w
ater except chloroform(CH
Cl3), tetrachloromethane(CCl4) and
1,1,1-trichloroethane
If the density of the solvents are not known, w
hen we say solute A
is partitioned between solvent B and C, it usually m
eans
The law is only application to the case if
the solute has the same m
olecularstructure in the tw
o solvents.
e.g. The partition law is not applicable to the case w
here ethanoicacid is partitioned betw
een hexane and water as ethanoic acid exists
as dimm
er in hexane (non-polar solvent) and as monom
er in water
(polar solvent)
It would be m
ore effective to carry out a solvent extraction in successive
extractions, a small portion at a tim
e, for a given amount of solvent.
Paper Chromatography
(between a stationery phase
and a mobile phase)
Partition of a solution between a
stationery phase and a mobile phase
Stationery phasee.g. the m
icroscopic water film
on the surface of the chromatography paper
e.g. silica gel on an aluminium
foil
Mobile phase
e.g. different kind of solvent or mixture of solvents
At a given temperature, w
ith agiven chrom
atography paperand a given solvent, the solutebeing analyzed
ischaracterized by its Rf value
Analysis of amino acids - A m
ixture of colourless amino acids can be separated by paper
chromatography. The positions of am
ino acids spot in the chromatograph can be detected by
spraying
with ninhydrin (a developer), w
hich reacts with am
ino acids to yield highly coloured products (usuallypurple in
colour).
23. Phase E
quilibrium III . Three-C
omponent S
ystems.m
map - 2005/5/23 -
