Practice: Write the equilibrium expression for the following reaction.
4 NH3 (g) + 3 O2 (g) ⇌ 2 N2 (g) + 6 H2O (l)
Under an equilibrium state the rates of the reverse and forward reaction are equal.
The Equilibrium State
Concept #1: Most chemical reactions do not go to completion and instead reach a state of chemical equilibrium.
Equilibrium is reached when the rates of the forward and reverse reaction are equal.
The magnitude of K determines which direction is favored at equilibrium. If K is greater than 1 then products are favored over reactants.
If K is less than 1 then reactants are favored over products.
If K is equal to 1 then both reactants and products are favored.
Equilibrium State Calculations
Example #1: For the following chemical reaction N2 (g) + O2 (g) 2 NO (g), Kc = 3.7 x 10-5, kf = 2.5 x 10-3 and kr = 67.57. Addition of a catalyst increases the forward rate constant 1.8 x 10-1. What is the new reverse rate constant after the addition of the catalyst?
Example #2: Consider the following reactions at 25°C:
Reaction Kc
2 NO (g) ⇌ N2 (g) + O2 (g) 1 × 1030
2 H2O (g) ⇌ 2 H2 (g) + O2 (g) 5 × 10−82
2 CO (g) + O2 (g) ⇌ 2 CO2 (g) 3 × 1091
Which compound is most likely to dissociate and give O2 (g) at 25°C?
a) CO2 b) NO c) CO d) H2O
Example #3: When reaction 1 and 2 below are added together, the result is reaction 3.
1) H2O (l) + HNO2 (aq) ⇌ H3O+ (aq) + NO2‒ (aq) K1 = 4.50 × 10‒4
2) H3O+ (aq) + OH‒ (aq) ⇌ 2 H2O (l) K2 = 1.00 × 1014
3) HNO2 (aq) + OH‒ (aq) ⇌ NO2‒ (aq) + H2O (l) K3 = ?
Find the equilibrium constant, K3.
A) 4.50 × 10‒18 B) 2.22 × 1017 C) 4.50 × 1010 D) 1.00 × 1014
Example #4: What is the equilibrium constant for the reaction
NH3 (aq) + H3O+ (aq) ⇌ NH4+ (aq) + H2O (l)
Given the following information:
NH3 (aq) + H2O (l) ⇌ NH4+ (aq) + OH– (aq) Kc = 1.8 × 10‒5
4 H2O (l) ⇌ 2 OH‒ (aq) + 2 H3O+ (aq) Kc = 1 × 10‒28
a) 1.8 × 109 b) 4.5 × 107 c) 9.0 × 1010 d) 1.8 × 10-5 e) 2.7 × 106
