Learning Objectives

  • Apply Le Chatelier’s principle to predict the effect of changes in conditions on the position of equilibrium
  • Understand how changing conditions affect the value of the equilibrium constant and the position of equilibrium

Le Chatelier’s principle

When a system at equilibrium is subjected to a change, it will respond in such a way as to minimize the effect of the change.

The Effect of Temperature

In an endothermic reaction

A(g) ⇔ B(g)          △H > 0

If the temperature is increased,

A(g) +        heat    ⇔   B(g) 

Effect:                            +      

Reaction:    –                 –                +       (Reaction shifts to the right side)

In an exothermic reaction

X(g) ⇔ Y(g)          △H < 0

If the temperature is increased,

X(g) ⇔      Y(g)  +   heat

Effect:                                            +      

Reaction:   +                –               –        (Reaction shifts to the left side)

“The only factor that changes the value of Kc is TEMPERATURE!!!”

Predict the effect of increasing temperature on the position of equilibrium in the following systems:

  1. NO(g) + NO2(g) ⇔ N2O3(g)                         ΔH = −40 kJ mol−1
  2. CH4(g) + H2O(g) ⇔ CO(g) + 3H2(g)             ΔH = +206 kJ mol−1
  3. CO(g) + 2H2(g) ⇔ CH3OH(g)                          ΔH = −90 kJ mol−1

Predict the effect of increasing temperature on the position of equilibrium in the following systems:

  1. NO(g) + NO2(g) ⇔ N2O3(g)                         ΔH = −40 kJ mol−1
  2. CH4(g) + H2O(g) ⇔ CO(g) + 3H2(g)             ΔH = +206 kJ mol−1
  3. CO(g) + 2H2(g) ⇔ CH3OH(g)                          ΔH = −90 kJ mol−1

1. left, 2. right, 3. left

The Effect of Concentration

In general, if the concentration of one of the species in an equilibrium mixture is increased, the position of equilibrium shifts to the opposite side to reduce the concentration of this species.

For example: The formation reaction of ammonia.

N2(g) + 3H2(g) ⇔ 2NH3(g)                    

If some amount of H2 gas is added to the container with a constant volume at constant temperature, system shift to the right to decrease the amount of H2.

By the way, following concentration versus time graph is obtained:

After H2 is added, the concentrations of N2 and H2 decrease, while the concentration of NH3 rises as the rate of the forward reaction increases and the equilibrium shifts to the right.

Predict the effect of the following changes on the position of equilibrium:

1. Removing the CO2 from the equilibrium:

CaCO3(s) ⇔ CaO(s) + CO2(g)

2. Adding acid (H+) to the system:

NH4+(aq) ⇔ H+(aq) + NH3(aq)

3. Adding sodium hydroxide to the system:

CH3COOH(aq) ⇔ CH3COO(aq) + H+(aq)

Predict the effect of the following changes on the position of equilibrium:

1. Removing the CO2 from the equilibrium:

CaCO3(s) ⇔ CaO(s) + CO2(g)

2. Adding acid (H+) to the system:

NH4+(aq) ⇔ H+(aq) + NH3(aq)

3. Adding sodium hydroxide to the system:

CH3COOH(aq) ⇔ CH3COO(aq) + H+(aq)

(1. right, 2. left, 3. right)

The Effect of Pressure

For gaseous reactions, in general pressure is changed by changing volume of container. Pressure and volume are inversely proportional.

For example; 2NO2(g) ⇔ N2O4(g)

Predict the effect of increasing pressure on the position of equilibrium in the following systems:

  1. CH4(g) + 2H2O(g) ⇔ CO2(g) + 4H2(g)
  2. N2O5(g) + NO(g) ⇔ 3NO2(g)
  3. NO(g) + NO2(g) ⇔ N2O3(g)

Predict the effect of increasing pressure on the position of equilibrium in the following systems:

  1. CH4(g) + 2H2O(g) ⇔ CO2(g) + 4H2(g)
  2. N2O5(g) + NO(g) ⇔ 3NO2(g)
  3. NO(g) + NO2(g) ⇔ N2O3(g)

(1. left, 2. left, 3. right)

Catalyst and Equilibrium

A catalyst,

  • increases the rate of forward and reverse reactions equally,
  • has no effect on the position of equilibrium or the value of the equilibrium constant,
  • only reduces the time taken to reach equilibrium.

1. The following reaction was allowed to reach equilibrium at 761 K.

H2(g) + I2(g) ⇌ 2HI(g)                 ΔHθ < 0

Outline the effect, if any, of each of the following changes on the position of equilibrium, giving a reason in each case.

2. Explain the effect of stated changes in conditions on positions of equilibrium and the value of the equilibrium constant, K.

1.

2.