Chemistry 102(001) Fall 2012

18-1CHEM 102, Spring 2012 LA TECH

CTH 328 10:00-11:15 am

Instructor: Dr. Upali Siriwardane

e-mail: [email protected]

Office: CTH 311 Phone 257-4941

Office Hours: M,W 8:00-9:00 & 11:00-12:00 am; Tu, Th, F 8:00 - 10:00am..

Exams: 10:00-11:15 am, CTH 328.

September 27, 2012 (Test 1): Chapter 13

October 18, 2012 (Test 2): Chapter 14 &15

November 13, 2012 (Test 3): Chapter 16 &18

Optional Comprehensive Final Exam: November 15, 2012 :

Chapters 13, 14, 15, 16, 17, and 18

Chemistry 102(001) Fall 2012


Review of Chapter 6. Energy and Chemical Reactions 6.1 The Nature of Energy 6.2 Conservation of Energy 6.3 Heat Capacity 6.4 Energy and Enthalpy 6.5 Thermochemical Equations 6.6 Enthalpy change for chemical Rections 6.7 Where does the Energy come from? 6.8 Measuring Enthalpy Changes: Calorimetry 6.9 Hess's Law 6.10 Standard Enthalpy of Formation 6.11 Chemical Fuels for Home and Industry 6.12 Food Fuels for Our Bodies


Chapter 18. Thermodynamics: Directionality of Chemical Reactions

18.1 Reactant-Favored and Product-Favored Processes

18.2 Probability and Chemical Reactions18.3 Measuring Dispersal or Disorder: Entropy18.4 Calculating Entropy Changes18.5 Entropy and the Second Law of

Thermodynamics18.6 Gibbs Free Energy18.7 Gibbs Free Energy Changes and Equilibrium Constants18.8 Gibbs Free Energy, Maximum Work, and

Energy Resources18.9 Gibbs Free Energy and Biological Systems18.10 Conservation of Gibbs Free Energy18.11 Thermodynamic and Kinetic Stability


What forms of energy are found in the Universe?mechanical thermalelectrical nuclearmass: E = mc2

others yet to discover


What is 1st Law of Thermodynamics

Eenergy is conserved in the Universe

All forms of energy are inter-convertible and conserved

Energy is neither created nor destroyed.


What exactly is DH?Heat measured at constant pressure qp Chemical reactions exposed to atmosphere

and are held at a constant pressure. Volume of materials or gases produced can

change. Volume expansion work = -PDV DU = qp + w; DU = qp -PDV qp = DU + PDV; w = -PDV DH = DU + PDV; qp = DH(enthalpy )


What is the internal energy change (DU) of a system? DU is part of energy associated with changes in atoms, molecules and subatomic particles

Etotal = Eke + E pe + DU DU = heat (q) + w (work) DU = q + w DU = q -P DV; w =- P DV


Heat measured at constant volume qv

Chemical reactions take place inside a closed chamber like a bomb

calorimeter.

Volume of materials or gases produced can not change. ie: work = -PDV=

0

DU = qv + w

qv = DU + o; w = 0

DU = qv = DU(internal energy )

How is Internal Energy, DU measured?


EnthalpyHeat changes at constant pressure

during chemical reactionsThermochemical equation. eg.

H2 (g) + O2 (g) ---> 2H2O(l) DH =- 256 kJ; DH is called the enthalpy of reaction.if DH is + reaction is called endothermicif DH is - reaction is called exothermic


The thermodynamic property related to randomness is ENTROPY, S.

Product-favored processes: final state is more DISORDERED or RANDOM than the original.

Spontaneity is related to an increase in randomness.

Reaction of K with water

Entropy, S


Physical Process” S[H2O(l)] > S[H2O(s)] at 0° C.


Standard Molar Entropy Values


Chemical Thermodynamicsspontaneous reaction – reaction which

proceed without external assistance once started

chemical thermodynamics helps predict which reactions are spontaneous


Will the rearrangement of a system decrease its energy?

If yes, system is favored to react — a product-favored system.

Most product-favored reactions are exothermic.Often referred to as spontaneous reactions.“Spontaneous” does not imply anything about time for

reaction to occur. Kinetic factors are more important for certain reactions.

Thermodynamics


Thermodynamics Standard States The thermodynamic standard state of a substance is

its most stable pure form under standard pressure (1 atm) and at some specific

temperature (25 ºC or 298 K)

superscript circle is used to denote a thermodynamic quantity that is under standard state conditions:

ΔH = ΔH°ΔS = ΔS°ΔG = ΔG°


1) Give the definitions of the following:a) Enthalpy (H):

b) Enthalpy change of a thermo-chemical reaction (DH):

c) Entropy of a substance (S): d) Entropy change of a chemical reaction(DS): e) Thermodynamic Standard State(0):


Laws of Thermodynamics

Zeroth: Thermal equilibrium and temperature

First : The total energy of the universe is constant

Second : The total entropy (S) of the universe is always increasing

Third : The entropy(S) of a pure, perfectly formed crystalline substance at absolute

zero is zero


2) Give the definitions of the following:

a) Zeroth Law of thermodynamics:

b) First Law of thermodynamics:

c) Second Law of thermodynamics:

d) Third Law of thermodynamics:


Why is it necessary to divide Universe into System and SurroundingUniverse = System +

Surrounding

system surroundings

universe


Types of Systems

Isolated system

no mass or energy exchange

Closed system

only energy exchange

Open system

both mass and energy exchange


Universe = System + Surrounding

Why is it necessary to divide Universe into System and Surrounding


3) Why we need to divide universe into surroundings and system for thermodynamic calculations?

Give the signs of the DH (heat) and DS (disorder) and DG ( free energy) when system lose or gain them.

Loss

Gain

DH (heat)

DS (disorder)

DG ( free energy)


Second Law of ThermodynamicsIn the universe the ENTROPY cannot decrease for

any spontaneous processThe entropy of the universe strives for a

maximumin any spontaneous process, the entropy of the

universe increasesfor product-favored processDSuniverse = ( Ssys + Ssurr) > 0 DSuniv = entropy of the UniverseDSsys = entropy of the SystemDSsurr = entropy of the SurroundingDSuniv = DSsys + DSsurr


Entropy of the UniverseDSuniv = DSsys + DSsurr

Dsuniv DSsys DSsurr

+ + ++ +(DSsys>DSsurr) - + - + (DSsurr>DSsys)


4) Explain the ways that DS of the universe, DSuniv could be +.

DSuniv = DSsys + DSsurr

+

+

+


Entropy and Dissolving


5) Assign a sign to the entropy change for the following systems.

a) mixing aqueous solutions of NaCl and KNO3 together:

b) spreading grass seed on a lawn: c) raking and bagging leaves in the fall: d) shuffling a deck of cards:

e) raking and burning leaves in the fall:


Expansion of a GasThe positional

probability is higher when particles are dispersed over a larger volume

Matter tends to expand unless it is restricted


Gas Expansion and Probability


Entropies of Solid, Liquidand Gas Phases

S (gases) > S (liquids) > S (solids)


6) Taking following examples explain how disorder is related to a measuring positional probability) or dispersion among the allowed energy states?

a) Expansion of gases: Two gas molecules trapped in two vessels with a tube with a stop cock.


6) Taking following examples explain how disorder is related to a measuring positional probability) or dispersion among the allowed energy states.

b) Distribution of Kinetic energy at 0, 25 and 100°C for O2


Entropy and Molecular Structure


Entropy, S

Entropies of ionic solids depend on coulombic attractions.

So

(J/K•mol)

MgO 26.9

NaF 51.5


Qualitative Guidelines for Entropy Changes

Entropies of gases higher than liquids higher than solids

Entropies are higher for more complex structures than simpler structures

Entropies of ionic solids are inversely related to the strength of ionic forces

Entropy increases when making solutions of pure solids or pure liquids in a liquid solvent

Entropy decrease when making solutions of gases in a liquid


Entropy of a Solution of a Gas


7) Arrange following in the order of increasing entropy?• a) C(s) (diamond)

• b) C(s) (graphite)

• c) O2(g)

• d) CO2(g)

• e) CO(g)

• f) Hg(l)


Entropy Change

Entropy (DS) normally increase (+) for the following

changes:i) Solid ---> liquid (melting) +ii) Liquid ---> gas +iii) Solid ----> gas most +iv) Increase in temperature +v) Increasing in pressure(constant volume, and

temperature) +vi) Increase in volume +


Qualitative prediction of DS of Chemical Reactions Look for (l) or (s) --> (g) If all are gases: calculate DnDn = Sn (gaseous prod.) - S n(gaseous reac.)N2 (g) + 3 H2 (g) --------> 2 NH3 (g) Dn = 2 - 4 = -2If Dn is - DS is negative (decrease in S)If Dn is + DS is positive (increase in S)


Predict DS!

2 C2H6(g) + 7 O2(g)--> 4 CO2(g) + 6H2O(g)

2 CO(g) + O2(g)-->2 CO2(g)

HCl(g) + NH3(g)-->NH4Cl(s)

H2(g) + Br2(l) --> 2 HBr(g)


8) Taking following physical and chemical changes qualitatively predict the sign of DS.

a) 2H2O (g) ------> 2 H2O (l) b) 2H2O (g) ------> 2 H2 (g) + O2 (g) c) N2 (g) + 3 H2 (g) ------> 2 NH3 (g)


Entropy Changes for Phase Changes

For a phase change, DSSYS = qSYS/T

(q = heat transferred)Boiling Water

H2O (liq) H2O(g)DH = q = +40,700 J/mol

mol•J/K 109+ = K 373.15

J/mol 40,700 = Tq = SD


9) How is entropy related to the heat and temperature?


Phase TransitionsHeat of Fusionenergy associated with phase transition solid-to-

liquid or liquid-to-solidDGfusion = 0 = DHfusion - T DSfusion

0 = DHfusion - T DSfusion

DHfusion = T DSfusion

Heat of Vaporizationenergy associated with phase transition gas-to-

liquid or liquid-to-gasDHvaporization = T DSvaporization


10) The normal boiling point of benzene is 80.1°C and heat of evaporation (∆H°vap)is 30.7 kJ/mol. Calculate the ∆Ssurr (in J/K mol) for the evaporation of benzene.


Can calc. that DHo

rxn = DHosystem = -571.7 kJ

2 H2(g) + O2(g) 2 H2O(liq)DSo

sys = -326.9 J/KEntropy Changes in the Surroundings

TH-

= T

q = systemsurrgssurroundin

DD oS

K 298.15J/kJ) kJ)(1000 (-571.7 - = gssurroundin

oSD

= +1917 J/K

2nd Law of Thermodynamics


2 H2(g) + O2(g) 2 H2O(liq)DSo

sys = -326.9 J/KDSo

surr = +1917 J/KDSo

uni = +1590. J/KThe entropy of the universe is increasing, so

the reaction is product-favored.

2nd Law of Thermodynamics


Gibbs Free Energy, GDSuniv = DSsurr + DSsys

Multiply through by (-T)-TDSuniv = DHsys - TDSsys

-TDSuniv = DGsystem

Under standard conditions —

DGo = DHo - TDSo

D S univ = -D H sys

T + D S sys


Gibbs Free Energy, G DGo = DHo - T DSo

Gibbs free energy change = difference between the enthalpy of a system and

the product of its absolute temperature and entropy

predictor of spontaneity Total energy change for system -

energy lost in disordering the system


11) Define the following:a) Gibbs Free Energy (G): b) Gibbs Free Energy change for a reaction (DG):

c) How is DGsys is related to DSuni and temperature?


The sign of DG indicates whether a reaction will occur spontaneously.

+ Not spontaneous

0 At equilibrium

- Spontaneous

The fact that the effect of DS will vary as a function of temperature is important.

This can result in changing the sign of DG.

Free energy, DG


The sign of DG indicates whether a reaction will occur spontaneously.

Therefore Ecell value have to be + (positive) for spontaneous redox reaction

DG = -nFEcell

n = number of electrons transferred

F = Faraday constant ((96500 C/mol)

Ecell = E½(cathode)- E½(anode)

DG and Ecell


How do you calculate DG at different T and P

DG = DGo + RT ln Q Q = reaction quotientat equilibrium DG = 00 = DGo + RT ln K DGo = - RT ln KIf you know DGo you could calculate K or

vice versa.


11) Define the following: d) How you decided from the sign of DG whether and

chemical reaction is? i) Spontaneous ii) Never take place iii) Equilibrium e) How is Gibbs Free Energy change (DG°) related to Ecell:

f) How is non standard (DG) related to (DG°) and Q (reaction quotient)


11) Define the following:g) How is standard (DG°) related to Keq (equilibrium constant)?


Gibbs Free Energy, G

DGo = DHo - TDSo

DHo DSo DGo Reactionexo(-) increase(+) - Prod-favoredendo(+) decrease(-) + React-

favoredexo(-) decrease(-) ? T dependentendo(+) increase(+) ? T

dependent


12) Predict the DGsys changes for different signs of DHsys and DSsys at low/high temperatures for the equation:

DGsys = DHsys - TDSsys

DGsys DHsys + DTDSsys

a)

b)

c)

d)

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Chemistry 102(001) Fall 2012