Topic 10: Equilibrium

Question 1

How does Keq change when stoichiometry changes?

Answer 1

• If times the stoichiometry with n, then Keq → Keq^n
• If reverse the reaction, Keq → 1/Keq

Question 2

When 2 coupled reactions combine, how to calculate the overall Keq from each equation’s Keq?

Answer 2

Product of the Keq of step reactions
Keq = K1*K2

Question 3

What is the Keq of solids and pure liquids and explain?

Answer 3

Solids and pure liquids have Keq=1
- Solids have constant composition, so the concentration of composition (i.e density) is always 1.
- Pure liquids: the concentration of a liquid in itself is 1

Question 4

What is Gibbs free energy, how is it calculated and what does it say about spontaneity of reactions?

Answer 4

Gibbs free energy tells the spontaneity of a reaction.

∆G = ∆H - T∆S

∆G < 0 then the reaction is spontaneous

Question 5

Explain the temperature dependence of ∆G in terms of values of ∆H and ∆S

Answer 5

• ∆H<0 and ∆S>0: always spontaneous
• ∆H<0 and ∆S<0: spontaneous at low T
• ∆H>0 and ∆S>0: spontaneous at high T
• ∆H>0 and ∆S<0: never spontaneous

Question 6

Explain how equilibrium changes when temperature, concentration and volume change

Answer 6

• Temperature:
  
  • For exothermic reactions, adding T shifts reaction to the left and favor reactants.
  • For endothermic reactions, adding T shifts reaction to the right and favor products.
• Concentration: change in concentration → change Q value → reaction shift to make Q = K
  
  • If concentration increases, the reaction will shift to consume it.
  • If concentration decreases, the reaction will shift to produce more of it.
• Pressure:
  
  • When volume is compressed and pressure increases, the reaction will shift to make fewer molecules present.
  • When volume is expanded and pressure decreases, the reaction will shift to make more molecules.

Question 7

Describe the relationship between ∆G and Keq at standard conditions when ∆G=0 and Q=Keq

Answer 7

lnKeq = -∆G°/RT where
∆G° = ∆H° - T∆S°