Chemical Equilibrium — Problem Types
Type 1
Given:
● All equilibrium [ ]'s
● Calculate K_eq
Strategy: i) Write K_eq expression, substitute and solve ii) K_eq value has NO units
Example
At equilibrium, [H₂] = 0.46 mol/L, [I₂] = 0.39 mol/L and [HI] = 3 mol/L. What is the value of the
equilibrium constant if temperature is constant?
Reaction: H₂(g) + I₂(g) ⇌ 2HI(g)
K_eq expression:
K_eq = [HI]² / [H₂][I₂] = (3)² / (0.46)(0.39)
K_eq = 50.2
Type 2
Given: i) All initial [ ] or # of moles ii) One equilibrium [ ] or # of moles
Calculate: i) All equilibrium [ ]'s ii) K_eq
Strategy: i) Use ICE method to calculate equilibrium [ ]'s / amounts ii) Use K_eq expression to
calculate K_eq
Example
In a 10.0 L flask, initially there are 0.4 mol of PCl₅. At equilibrium, there are 0.250 mol of Cl₂.
Calculate all the equilibrium [ ]'s and the K_eq value.
Reaction: PCl₅(g) ⇌ PCl₃(g) + Cl₂(g)
Type 1
Given:
● All equilibrium [ ]'s
● Calculate K_eq
Strategy: i) Write K_eq expression, substitute and solve ii) K_eq value has NO units
Example
At equilibrium, [H₂] = 0.46 mol/L, [I₂] = 0.39 mol/L and [HI] = 3 mol/L. What is the value of the
equilibrium constant if temperature is constant?
Reaction: H₂(g) + I₂(g) ⇌ 2HI(g)
K_eq expression:
K_eq = [HI]² / [H₂][I₂] = (3)² / (0.46)(0.39)
K_eq = 50.2
Type 2
Given: i) All initial [ ] or # of moles ii) One equilibrium [ ] or # of moles
Calculate: i) All equilibrium [ ]'s ii) K_eq
Strategy: i) Use ICE method to calculate equilibrium [ ]'s / amounts ii) Use K_eq expression to
calculate K_eq
Example
In a 10.0 L flask, initially there are 0.4 mol of PCl₅. At equilibrium, there are 0.250 mol of Cl₂.
Calculate all the equilibrium [ ]'s and the K_eq value.
Reaction: PCl₅(g) ⇌ PCl₃(g) + Cl₂(g)
