Week 9

Question 1

Entropy change

Answer 1

Explain spontaneous processes like the flow of heat from high temp to low temp.
* ∆S = q/T

Question 2

Direction of heat flow

Answer 2

• Heat flowing out: q is negative
• Heat flowing in: q is positive

ΔStotal > 0 to ensure that q must be > 0

Question 3

The 2nd law of thermodynamics

Answer 3

For any process to occur spontaneously, the total entropy must increase, i.e.
* ΔStotal = ΔSsys + ΔSsurround > 0

Question 4

Trends in entropy

Answer 4

• Entropy increases in a material when the temperature increases.
• Entropy increases when molecules go from a dense state (liquids and solids) to the gas state.
• Entropy decreases when an ideal gas is compressed at constant temperature

Question 5

Entropy is zero at absolute zero

Answer 5

True,
A material at T = 0 K is defined as having an entropy of zero.
3rd law of thermodynamics

Question 6

Phase transition

Answer 6

jumps in entropy

Question 7

Entropy at standard state

Answer 7

Standard entropies (S^0 298) are the entropy change from T = 0 K (where S = 0) to T = 298 K;
* Solids have relatively low S^0 298
* Liquids have higher S^0 298 and
* Gases much higher still

Question 8

Heat lost (or gained) by the system is gained (or lost) by the surroundings

Answer 8

qsurrounds = -qsys
qsurrounds = -ΔHsys

Question 9

∆G

Answer 9

• ΔGsys < 0
• ΔGsys = -TΔStotal

Question 10

On the spontaneity of exothermic reactions

Answer 10

(ΔH is negative), most (or all) of the entropy increase comes from ΔSsurroundings due to the flow of heat into the surroundings from the reaction system

Question 11

On the spontaneity of endothermic reactions

Answer 11

(ΔH is positive), all of the entropy increase comes from the ΔSsystem. This entropy increase might come from a number of sources
* The transformation of solids into liquids, the increase in entropy of the solvent around solutes, etc.

Question 12

Many chemical species exist in a state of “equilibrium”,

Answer 12

where a reverse reaction competes with the forward reaction.
⇌

Question 13

Equilibrium constant, Keq

Answer 13

It is the ratio Keq and not the individual concentrations that defines equilibrium
* A+B ⇌ C+D
* then Keq = (CxD)/(AxB)

Question 14

Which compounds are favoured at equilibrium?

Answer 14

• Small Keq : equilibrium favours reactants
• Large Keq : equilibrium favours products

Question 15

Reversible reactions

Answer 15

Reactions that have Keq that do not strongly favour reactants or products can be reversed by changing the conditions

Question 16

The reaction quotient, Q

Answer 16

Calculated in exactly the same way as Keq, except using current concentrations rather than equilibrium concentration

Question 17

The equilibrium constant must be associated with a specific chemical process and a specific stoichiometry

Answer 17

If you multiply the each component in an equation by n, you change
* Keq→ (Keq)^n

If you reverse the chemical equation, you change
* Keq→ 1/Keq

Question 18

Different kinds of Keq

Answer 18

Ka = acid dissociation equilibrium constant;
Kb = base diss. eq. constant;
Kw = water diss. eq. constant;
Kstab = eq. constant for stability of a metal complex;
Ksp = solubility product

Question 19

The concentrations of a pure phase of fixed volume…

Answer 19

(liquid or solid) are omitted from the equilibrium expression

Question 20

Koverall =

Answer 20

K1 x K2
* i.e. when you ADD chemical equations, you MULTIPLY the K’s