# 7.1.5 Le Chatelier's Principle

### Le Chatelier's Principle

#### Position of the equilibrium

• The position of the equilibrium refers to the relative amounts of products and reactants in an equilibrium mixture.
• When the position of equilibrium shifts to the left, it means the concentration of reactants increases
• When the position of equilibrium shifts to the right, it means the concentration of products increases

#### Le Chatelier’s principle

• Le Chatelier’s principle says that if a change is made to a system at dynamic equilibrium, the position of the equilibrium moves to minimise this change
• The principle is used to predict changes to the position of equilibrium when there are changes in temperature, pressure or concentration

#### Worked Example

A. Using the reaction below:

CH3COOH (l) + C2H5OH (l) ⇌ CH3COOC2H5 (l) + H2O (l)

Explain what happens to the position of equilibrium when:

2. Some C2H5OH(l)  is removed

B. Use the reaction below:

Ce4+ (aq) + Fe2+ (aq) ⇌ Ce3+ (aq) + Fe3+ (aq)

Explain what happens to the position of equilibrium when

3. Water is added to the equilibrium mixture

• The position of the equilibrium moves to the left and more ethanoic acid and ethanol are formed.
• The reaction moves in this direction to oppose the effect of added ethyl ethanoate, so the ethyl ethanoate decreases in concentration.

• The position of the equilibrium moves to the left and more ethanoic acid and ethanol are formed.
• The reaction moves in this direction to oppose the removal of ethanol so more ethanol (and ethanoic acid) are formed from ethyl ethanoate and water.

• There is no effect as the water dilutes all the ions equally so there is no change in the ratio of reactants to products.

#### Effects of pressure

• Changes in pressure only affect reactions where the reactants or products are gases

Effects of Pressure Table #### Worked Example

Predict the effect of increasing the pressure on the following reactions:

1. N2O4 (g)  ⇌  2NO2 (g)
2. CaCO3 (s)  ⇌  CaO (s) + CO2(g)

Predict the effect of decreasing the pressure on the following reaction:

3.  2NO2 (g)  ⇌  2NO (g)  + O2 (g)

• The equilibrium shifts to the left as there are fewer gas molecules on the left.
• This causes a decrease in pressure.

• The equilibrium shifts to the left as there are no gas molecules on the left but there is CO2 on the right.
• This causes a decrease in pressure.

• The equilibrium shifts to the right as there is a greater number of gas molecules on the right.
• This causes an increase in pressure.

#### Worked Example

Using the reaction below:

H2 (g)  + CO2 (g)  ⇌  H2O (g) +  CO (g)          ∆H = + 41.2 kJ mol-1

1. Predict the effect of increasing the temperature on this reaction

Using the reaction below:

Ag2CO3 (s)  ⇌  Ag2O (s) + CO2(g)

2. Increasing the temperature increases the amount of CO2(g) at constant pressure. Is this reaction exothermic or endothermic?

• The reaction will absorb the excess heat and since the forward reaction is endothermic, the equilibrium will shift to the right.

• The reaction will absorb the excess heat and since this causes a shift of the equilibrium towards the right (as more CO2(g) is formed) this means that the reaction is endothermic (because endothermic reactions favour the products).

#### Effects of catalysts

• A catalyst is a substance that increases the rate of a chemical reaction (they increase the rate of the forward and reverse reaction equally)
• Catalysts only cause a reaction to reach its equilibrium faster
• Catalysts therefore have no effect on the position of the equilibrium once this is reached

#### Exam Tip

You are not required to quote Le Chatelier’s Principle in an exam, but you must know how to apply it to systems in equilibrium

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