Chapter 2: 2.4 Free Energy

Question 1

When is a process in an isolated system spontaneous?

Answer 1

If ΔS sys > 0

Question 2

When a process occurs at constant pressure and tempearture…

Answer 2

The heat change associated with the surroundings is given by ΔH surr

Question 3

How can entropy change of the surroundings be calculated?

Answer 3

ΔS surr = ΔH surr / T

Question 4

State the relation between:

ΔH surr and ΔH sys
(in terms of heat flow)

Answer 4

ΔH surr = -ΔH sys

Question 5

What does the entropy change of the surroundings equal in terms of ΔH sys?

Answer 5

ΔS surr = -ΔH sys / T

Question 6

State the formula for Gibbs free energy

Answer 6

G = H -TS

Question 7

State the proof for:

The formula for Gibbs Free Energy

Answer 7

1. Substituting ΔS surr = -ΔH sys/T into the formula ΔS sys + ΔS surr > 0
2. Multiply both sides by -T
3. Result: ΔH - TΔS < 0

Question 8

Define:

Gibbs Free Energy

Answer 8

The amount of energy that is available (or “free”) to enable spontaneous change to occur at constant temperature and pressure

Question 9

True or False:

Gibbs Free Energy is a state function

Answer 9

True

Question 10

State the formula for:

Free energy change

Answer 10

ΔG = ΣG (products) - ΣG (reactants)
OR
ΔG = ΔH - TΔS

Question 11

State the spontaneity of the process if:

1. ΔG < 0
2. ΔG = 0
3. ΔG > 0

Answer 11

1. The reaction will proceed spontaneously as written
2. The system is at equilibrium (number of moles of reactans and products do not change over time)
3. The reaction will not proceed spontaneously as written (the reverse process is spontaneous)

Question 12

True or False:

ΔG < 0 means the reaction favours the reactants

Answer 12

False, ΔG < 0 means that the reaction favours the formation of the products

Question 13

What does the reaction favour when ΔG > 0?

Answer 13

The reaction favours the reactants, with little or no products formed

Question 14

Determine the sign of ΔG and what drives the reaction when:

• -ΔH
• +ΔS

Answer 14

-ΔG
* Enthalpically and entropically driven

Question 15

Determine the sign of ΔG and what drives the reaction when:

• +ΔH
• -ΔS

Answer 15

+ΔG
* Reaction not spontaneous

Question 16

Determine the sign of ΔG and what drives the reaction when:

• -ΔH
• -ΔS

Answer 16

+ΔG Or -ΔG
* Depends on temperature
* Enthalpically driven if spontaneous

Question 17

Determine the sign of ΔG and what drives the reaction when:

• +ΔH
• +ΔS

Answer 17

+ΔG Or -ΔG
* Depends on temperature
* Entropically driven, if spontaneous

Question 18

What is the sign and magnitude of ΔG affected by?

Answer 18

• Temperature
• Pressure
• Concentrations of reactants
• Concentrations of products

Question 19

List:

Standard conditions

Answer 19

• 298.15 K
• 1 atm pressure
• 1 M concentration

Question 20

How is free energy change under standard conditions denoted? State the formula

Answer 20

ΔG^o
* ΔG^o = ΔH^o - TΔS^o

Question 21

True or False:

The absolute free energy of a substance can be determined

Answer 21

False

Question 22

Define and give the notation of:

Standard Free Energy of Formation

Answer 22

• The standard free energy change that occurs when 1 mol of a compound is made from elements in their standard states
• ΔG^o f

Question 23

What is the ΔG^o f of any element in its standard state?

Answer 23

Zero

Question 24

State the formula for:

ΔG^o

Answer 24

ΔG^o = ΣΔG^o f (products) - ΣΔG^o f (reactants)