Thermodynamics

Question 1

Define entropy, S

Answer 1

• measure of disorder in system
• more disordered a system, the larger its entropy

Question 2

Define entropy change, ∆S

Answer 2

• ∆S oni depend on entropy of initial & final state
• ∆S = Sfinal - Sinitial (Unit: J mol-1 K-1)
• ∆S > 0 => final state more disordered
• ∆S < 0 => final state less disordered

Question 3

What factors affect entropy of chemical system?

Answer 3

• change in amt gas particles
• mix particles
• change temp
• change phase

Question 4

What happens to entropy with increase in number of gaseous particles in a reaction?

Answer 4

increase in no of gas particle:
- gas expand occupy whole vessel, so more ways for particles to arrange themselves
- increase disorder
- ∆S > 0 (entropy increase)

Question 5

What happens to entropy with mixing of gas, liquid particles and dissolving solid in liquid solvent?

Answer 5

1. When gas mix:
   - more ways for particles arrange themselves
   - increase disorder
   - ∆S > 0 (entropy increase)
2. When liquid mix: (oni for miscible liquids)
   - liquids of similar polarities (ie compatible intermolecular force attract n) eg benzene, hexane, mix tgt towards increased disorder
   - ∆ S >0 (entropy increase)
   *miscible in each other cause more way arranging particles in system
3. When solid dissolve in liquid solvent (eg NaCl into water)
   (solid NaCl hv ions in highly ordered crystalline state)
   - on dissociat n, ion bcome mobile, so more disordered state of ions
   - BUT, H2O molecule r arranged ard each ion, so more ordered state of H2O molecules
   - overall effect is increased disorder; entropy increase (∆S > 0)
   * higher charge ion hv more influence on arrangemt H2O molecule

Question 6

What happens to entropy when temperature increases?

Answer 6

increase in temp:
- broadens Maxwell-Boltzmann energy distribut n
- more ways to arrange energy quanta in particles
- increase in disorder (∆S > 0)

*DON’T say that increase in temp increase Ek of particles such that more random movement increase entropy (incomplete reason)

Question 7

What happens to entropy when there is change in phase?

Answer 7

• entropy of solid < liquid «_space;gas
  1. solid to liquid (melt)
• highly ordered, regular arrangemt in solid destroyed
• particles in liquid more randomly arranged than in solid
• increase in disorder & entropy (∆S >0)

2. Liquid to gas (evaporate/boil) OR solid to gas (sublimat n)
   - order in solid/liquid destroyed
   - particles in gas state most disordered as most randomly arranged
   - vv large increase in disorder & entropy (∆S»_space; 0)

Question 8

Define spontaneous reaction

Answer 8

• rxn occurring w/o any external intervent n (ie no heat needed, etc.)
• irreversible rxn, cnt return to og state

Question 9

Give Gibbs free energy change of reaction formula, ∆G

Answer 9

∆G = ∆H - T∆S

where
∆G is Gibbs free energy change
∆H is enthalpy change
∆S is entropy change
T is temp (in K)

Question 10

What is the relationship between ∆G and spontaneity of reaction?

Answer 10

1. ∆G < 0 (-ve value)
   - rxn spontaneous, feasible in forward rxn
2. ∆G = 0
   - system at eqm
3. ∆G > 0 (+ve value)
   - rxn not spontaneous, not feasible in forward rxn

Question 11

Give STANDARD Gibbs free energy change equation, ∆Gϴ

Answer 11

∆Gϴ = ∆Hϴ - T∆Sϴ

Units:
∆Gϴ: kJ mol-1 OR J mol-1
∆Hϴ: kJ mol-1 OR J mol-1
∆Sϴ: usually J mol-1 K-1
T: K
*Thus, ensure convert all units to the same base unit

Question 12

What to note about -T∆Sϴ term in Standard Gibbs free energy change equation?

Answer 12

∆Gϴ is temp dependent
- ∆Hϴ & ∆Sϴ change relatively little w temp, may b assumed temp independent (assumpt n –> MUST state)

Question 13

Name some limitations of use of ∆Gϴ to predict spontaneity of reaction

Answer 13

• spontaneity is whether rxn hv tendency proceed forward vs back, unrelated to rate of rxn fast enough to b observed
• ∆Gϴ indicate spontaneity at standard condit n, assume all gas species hv partial Pa 1 bar, all aq species hv conc 1 mol dm-3; when these condit n not met, ∆G is used
• some rxn hv vv large Ea barrier, so rxn vv slow no noticeable change, even if rxn spontaneous

Question 14

Based on Gibbs equation, what observations can be made for different values of ∆H, ∆S and -T∆S?

Answer 14

1. -ve ∆H, +ve ∆S so -ve -T∆S
   - exo, increase in entropy
   - ∆G always -ve
   - rxn spontaneous at all temp
2. -ve ∆H, -ve ∆S so +ve -T∆S
   - exo, decrease in entropy
   - ∆G -ve as long as mag ∆H > mag T∆S
   - rxn spontaneous oni at low temp
3. +ve ∆H, +ve ∆S so -ve -T∆S
   - endo, increase in entropy
   - ∆G -ve as long as mag ∆H < mag T∆S
   - rxn spontaneous oni at high temp
4. +ve ∆H, -ve ∆S so +ve -T∆S
   - endo, decrease entropy
   - ∆G always +ve
   - rxn non-spontaneous at all temp

Question 15

How is ∆G related to ∆Gϴ?

Answer 15

∆G = ∆Gϴ + RTlnQ

where
Q is rxn quotient (not eqm const; b4 reach eqm)

Question 16

How does ∆G affect extent of forward or backward reaction?

Answer 16

1. ∆G < 0 (forward rxn spontaneous)
   - forward rxn occur at faster rate than backward rxn
2. ∆G > 0 (backward rxn spontaneous)
   - backward rxn occur at faster rate than forward rxn
3. ∆G = 0 (system in dynamic eqm)
   - both forward, backward rxn occur at same rate

Question 17

Explain sign of ∆Gϴ

Answer 17

• provide info on if rxt or pdt dominant at eqm
• at eqm, where ∆G = 0, Q = Kc (eqm const), we get following eqn:
  ∆Gϴ = -RTlnKc
  =>
• if ∆Gϴ < 0, Kc >1 so eqm pos n lie toward right (more pdt than rxt at eqm)
• if ∆Gϴ > 0, Kc < 1 so eqm pos n lie toward left (more rxt than pdt at eqm)