17-1CHEM 102, Fall 2014 LA TECH
Instructor: Dr. Upali Siriwardane
e-mail: [email protected]
Office: CTH 311
Phone 257-4941
Office Hours: M,W 8:00-9:30 & 11:00-12:30 am;
Tu,Th,F 8:00 - 10:00 am. or by appointment.
Test Dates: 10:00-11:15 am., CTH 328
Chemistry 102(02) Fall 2014
September 23, 2014 (Test 1): Chapter 13
October 16, 2014 (Test 2): Chapter 14 &15
November 11, 2014 (Test 3) Chapter 16 &17
November 13, 2014 (Make-up test) comprehensive:
Chapters 13-17
17-2CHEM 102, Fall 2014 LA TECH
Chapter 6. Thermochemistry
6.1 Chemical Hand Warmers 2316.2 The Nature of Energy: Key Definitions 2326.3 The First Law of Thermodynamics: There Is No Free Lunch 2346.4 Quantifying Heat and Work 2406.5 Measuring for Chemical Reactions: Constant-Volume Calorimetry2466.6 Enthalpy: The Heat Evolved in a Chemical Reaction at Constant
Pressure 249
6.7 Constant-Pressure Calorimetry: Measuring 2536.8 Relationships Involving
2556.9 Determining Enthalpies of Reaction from Standard Enthalpies of Formation
2576.1 0 Energy Use and the Environment 263
17-3CHEM 102, Fall 2014 LA TECH
Chapter 17. Free Energy and Thermodynamics17.1 Nature’s Heat Tax: You Can’t Win and You Can’t Break Even 769
17.2 Spontaneous and Nonspontaneous Processes 771
17.3 Entropy and the Second Law of Thermodynamics 773
17.4 Heat Transfer and Changes in the Entropy of the Surroundings 780
17.5 Gibbs Free Energy 784
17.6 Entropy Changes in Chemical Reactions: Calculating 788
17.7 Free Energy Changes in Chemical Reactions: Calculating 792
17.8 Free Energy Changes for Nonstandard States: The Relationship
between and 798
17.9 Free Energy and Equilibrium: Relating to the Equilibrium Constant
(K)
17-4CHEM 102, Fall 2014 LA TECH
What forms of energy are found in the Universe?mechanical thermalelectrical Lightnuclearmass: E = mc2
others yet to discover
17-5CHEM 102, Fall 2014 LA TECH
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.
17-6CHEM 102, Fall 2014 LA TECH
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.
17-7CHEM 102, Fall 2014 LA TECH
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
17-8CHEM 102, Fall 2014 LA TECH
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?
17-9CHEM 102, Fall 2014 LA TECH
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
17-10CHEM 102, Fall 2014 LA TECH
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
17-11CHEM 102, Fall 2014 LA TECH
Physical Process” S[H2O(l)] > S[H2O(s)] at 0° C.
17-12CHEM 102, Fall 2014 LA TECH
DGo = DHo - T DSo
1. If DH is negative it helps product to be favored2. If DS is positive it helps product to be favored 3. If DG is negative reaction is product favored Gibbs free energy change = difference between the enthalpy of a system and the product of its absolute temperature and entropypredictor of spontaneity Total energy change of the system -
energy lost in disordering the system
Gibbs Free Energy, G
17-13CHEM 102, Fall 2014 LA TECH
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°
17-14CHEM 102, Fall 2014 LA TECH
Hydrogen ΔHof
(kJ/mol)ΔGo
f (kJ/mol) So (J/mol K)
H2 (g) 0 0 130.7H (g) 218.0 203.2 114.7
H2O (l) -285.8 -237.1 69.9H2O (g) -241.8 -228.6 188.8H2O2 (l) -187.8 -120.4 109.6
Standard Thermodynamic Data
ΔHo
f - Standard Enthalpy of Formation
ΔGo
f - Standard Free Energy of Formation
So
- Standard Free Energy of Formation
17-15CHEM 102, Fall 2014 LA TECH
Standard Molar Entropy Values
17-16CHEM 102, Fall 2014 LA TECH
Chemical Thermodynamicsspontaneous reaction – reaction which
proceed without external assistance once started
chemical thermodynamics helps predict which reactions are spontaneous
17-17CHEM 102, Fall 2014 LA TECH
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
17-18CHEM 102, Fall 2014 LA TECH
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):
17-19CHEM 102, Fall 2014 LA TECH
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
17-20CHEM 102, Fall 2014 LA TECH
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:
17-21CHEM 102, Fall 2014 LA TECH
Why is it necessary to divide Universe into System and SurroundingUniverse = System + Surrounding
system surroundings
universe
Boundary?
17-22CHEM 102, Fall 2014 LA TECH
Types of Systems
Isolated system
no mass or energy exchange
Closed system
only energy exchange
Open system
both mass and energy exchange
17-23CHEM 102, Fall 2014 LA TECH
Universe = System + Surrounding
Why is it necessary to divide Universe into System and Surrounding
17-24CHEM 102, Fall 2014 LA TECH
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)
17-25CHEM 102, Fall 2014 LA TECH
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 processDSuniv = DSsys + DSsurr
DSuniverse = ( Ssys + Ssurr) > 0 DSuniv = entropy of the UniverseDSsys = entropy of the SystemDSsurr = entropy of the Surrounding
universe
system surroundings
17-26CHEM 102, Fall 2014 LA TECH
Entropy of the UniverseVarious ways Dsuniv could become +
DSuniv = DSsys + DSsurr
Dsuniv DSsys DSsurr
+ + ++ +(DSsys>DSsurr) - + + (DSsurr>DSsys)
17-27CHEM 102, Fall 2014 LA TECH
4) Explain the ways that DS of the universe, DSuniv could be +.
DSuniv = DSsys + DSsurr
+
+
+
17-28CHEM 102, Fall 2014 LA TECH
Entropy and Dissolving
17-29CHEM 102, Fall 2014 LA TECH
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:
17-30CHEM 102, Fall 2014 LA TECH
Expansion of a GasThe positional
probability is higher when particles are dispersed over a larger volume
Matter tends to expand unless it is restricted
17-31CHEM 102, Fall 2014 LA TECH
Gas Expansion and Probability
17-32CHEM 102, Fall 2014 LA TECH
Entropies of Solid, Liquidand Gas Phases
S (gases) > S (liquids) > S (solids)
17-33CHEM 102, Fall 2014 LA TECH
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.
17-34CHEM 102, Fall 2014 LA TECH
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
17-35CHEM 102, Fall 2014 LA TECH
Entropy and Molecular Structure
17-36CHEM 102, Fall 2014 LA TECH
Entropy, S
Entropies of ionic solids depend on coulombic attractions.
So
(J/K•mol)
MgO 26.9
NaF 51.5
17-37CHEM 102, Fall 2014 LA TECH
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
17-38CHEM 102, Fall 2014 LA TECH
Entropy of a Solution of a Gas
17-39CHEM 102, Fall 2014 LA TECH
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)
17-40CHEM 102, Fall 2014 LA TECH
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 +
17-41CHEM 102, Fall 2014 LA TECH
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 react.)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)
17-42CHEM 102, Fall 2014 LA TECH
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)
17-43CHEM 102, Fall 2014 LA TECH
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)
17-44CHEM 102, Fall 2014 LA TECH
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
17-45CHEM 102, Fall 2014 LA TECH
9) How is entropy related to the heat and temperature?
17-46CHEM 102, Fall 2014 LA TECH
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
17-47CHEM 102, Fall 2014 LA TECH
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.
17-48CHEM 102, Fall 2014 LA TECH
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
17-49CHEM 102, Fall 2014 LA TECH
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
17-50CHEM 102, Fall 2014 LA TECH
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
17-51CHEM 102, Fall 2014 LA TECH
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
17-52CHEM 102, Fall 2014 LA TECH
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?
17-53CHEM 102, Fall 2014 LA TECH
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
17-54CHEM 102, Fall 2014 LA TECH
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
17-55CHEM 102, Fall 2014 LA TECH
How do you calculate DG at different T and PDG = 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.
Nerst Equation, since DG = -nFEcell
17-56CHEM 102, Fall 2014 LA TECH
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)
17-57CHEM 102, Fall 2014 LA TECH
11) Define the following:g) How is standard (DG°) related to Keq (equilibrium constant)?
17-58CHEM 102, Fall 2014 LA TECH
Gibbs Free Energy, G
DGo = DHo - TDSo
DHo DSo DGo Reactionexo(-) increase(+) - Prod-favoredendo(+) decrease(-) + React-
favoredexo(-) decrease(-) ? T dependentendo(+) increase(+) ? T
dependent
17-59CHEM 102, Fall 2014 LA TECH
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)