Chemistry 102(001) Fall 2012
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Transcript of 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
18-2CHEM 102, Spring 2012 LA TECH
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
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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
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What forms of energy are found in the Universe?mechanical thermalelectrical nuclearmass: E = mc2
others yet to discover
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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.
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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 )
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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
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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?
18-9CHEM 102, Spring 2012 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
18-10CHEM 102, Spring 2012 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
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Physical Process” S[H2O(l)] > S[H2O(s)] at 0° C.
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Standard Molar Entropy Values
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Chemical Thermodynamicsspontaneous reaction – reaction which
proceed without external assistance once started
chemical thermodynamics helps predict which reactions are spontaneous
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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
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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°
18-16CHEM 102, Spring 2012 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):
18-17CHEM 102, Spring 2012 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
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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:
18-19CHEM 102, Spring 2012 LA TECH
Why is it necessary to divide Universe into System and SurroundingUniverse = System +
Surrounding
system surroundings
universe
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Types of Systems
Isolated system
no mass or energy exchange
Closed system
only energy exchange
Open system
both mass and energy exchange
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Universe = System + Surrounding
Why is it necessary to divide Universe into System and Surrounding
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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)
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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
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Entropy of the UniverseDSuniv = DSsys + DSsurr
Dsuniv DSsys DSsurr
+ + ++ +(DSsys>DSsurr) - + - + (DSsurr>DSsys)
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4) Explain the ways that DS of the universe, DSuniv could be +.
DSuniv = DSsys + DSsurr
+
+
+
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Entropy and Dissolving
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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:
18-28CHEM 102, Spring 2012 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
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Gas Expansion and Probability
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Entropies of Solid, Liquidand Gas Phases
S (gases) > S (liquids) > S (solids)
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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.
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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
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Entropy and Molecular Structure
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Entropy, S
Entropies of ionic solids depend on coulombic attractions.
So
(J/K•mol)
MgO 26.9
NaF 51.5
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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
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Entropy of a Solution of a Gas
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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)
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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 +
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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)
18-40CHEM 102, Spring 2012 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)
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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)
18-42CHEM 102, Spring 2012 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
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9) How is entropy related to the heat and temperature?
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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
18-45CHEM 102, Spring 2012 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.
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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
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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
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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
18-49CHEM 102, Spring 2012 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
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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?
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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
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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
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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.
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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)
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11) Define the following:g) How is standard (DG°) related to Keq (equilibrium constant)?
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Gibbs Free Energy, G
DGo = DHo - TDSo
DHo DSo DGo Reactionexo(-) increase(+) - Prod-favoredendo(+) decrease(-) + React-
favoredexo(-) decrease(-) ? T dependentendo(+) increase(+) ? T
dependent
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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)