Gibbs energy and phase equilibria

Question 1

What is the change in Gibbs energy of reaction at equilibrium?

Answer 1

0

Question 2

What does ΔrxnG = 0 mean?

Answer 2

There is no difference between final and initial state at equilibrium

Question 3

Which phase of water has the lowest value of Gibbs energy?

Answer 3

Steam

Question 4

Which phase of water has the highest value of Gibbs energy?

Answer 4

Ice

Question 5

What is the entropy like in ice, water and steam?

Answer 5

Least entropy in ice, then water and the most entropy in steam

Question 6

What equation shows the changes in Gibbs energy with the small changes in temperature?

Answer 6

(ΔG)pni = -SΔT

Question 7

What equation for changes in G can you use if ΔS does not change with temperature?

Answer 7

ΔG2 = ΔG1 - ΔS (T2-T1)

Question 8

Why is evaporation of liquids favoured as the temperature rises?

Answer 8

The entropy of vaporization is always positive and large

Question 9

What equation shows how Gibbs energy varies with small changes in pressure?

Answer 9

(ΔG)tn = VΔP

Question 10

What values are fixed when you look at changes in pressure?

Answer 10

Temperature and moles

Question 11

How do you work out ΔG for reactions?

Answer 11

G = G(products) - G(reactants)

Question 12

What equation is used to work out changes in Gibbs energy when both pressure and volume change?

Answer 12

(dΔG)tn = ΔVdp

Question 13

What equation do you use when ΔV does not change with p?

Answer 13

ΔG2 - ΔG1 = ΔV (p2 - p1)

Question 14

When are the products favoured?

Answer 14

As the pressure rises if the volume of the reaction is negative

Question 15

When are the reactants favoured?

Answer 15

If the volume of the reaction is positive

Question 16

What is the critical point of a phase diagram?

Answer 16

The highest temperature at which the liquid phase exists

Question 17

What happens beyond the critical point?

Answer 17

There is no liquid / gas boundary

Question 18

What is the pressure at the critical point called?

Answer 18

Critical pressure

Pc

Question 19

What is the temperature at the critical point called?

Answer 19

Critical temperature

Tc

Question 20

What is the Clapeyron equation?

Answer 20

dp / dT = ΔS / ΔV

Question 21

What is the triple point?

Answer 21

When solid, liquid and gas phases are all in equilibrium

Question 22

What is most likely the triple point?

Answer 22

The lowest temperature at which the liquid phase is stable

Question 23

What is Gibbs phase rule?

Answer 23

F = C - P + 2

Question 24

What do the components of the Gibbs phase rule mean?

Answer 24

F = the number of degrees of freedom 
C = The number of components 
P = The number of phases

Question 25

What are intensive variables?

Answer 25

p, T xi

Independent of the system size

Question 26

What is a phase?

Answer 26

Uniform state of the system

Question 27

What is the value of C for a pure substance?

Answer 27

1

Question 28

What is the value of F for a pure substance?

Answer 28

F = 3 - P