Addition or Removal of a Product or Reactant

Addition or removal of a product or reactant:

• If the concentration of a reactant is increased, the reaction shifts towards products to reduce it.
• If the concentration of a product is increased, the reaction shifts towards reactants to reduce it.

For example, if the reaction below is at equilibrium, there is a mixture of all three compounds.

S(s) + O₂ ↔ SO₂(g)

• If additional O₂(g) is added, the reaction shifts to the right to reduce its concentration. More product is produced.
• If additional SO₂(g) is added, the reaction shifts to the left to reduce its concentration. More reactant is produced.

Note that "concentration" is only meaningful for gases (number of moles of the gas per unit volume of the container) or solutions (number of moles of the solute per litre of solution). A solid has a constant concentration (its density) - if more solid is added, then the number of moles and the volume are both increased to the same extent and the concentration remains unchanged. A pure liquid similarly has a constant concentration (its density) - if more liquid is added, then the number of moles and the volume are both increased to the same extent and the concentration remains unchanged. Solids and pure liquids are not included in the reaction quotient or the equilibrium constant and changing the amount of them does not affect the position of equilibrium.

• If additional S(s) is added, the reaction does not shift.

Changing the Pressure in Gaseous Reactions

Changing the pressure in gaseous reactions:

This change be achieved in two ways:

Changing the volume of the reaction container

Changing the volume of the reaction container: If the volume of the container is changed, the reaction shifts to try to reduce the pressure change:

• If the number of moles of gas does not change during the reaction, changing the pressure by changing the container volume has no effect on the reaction.
  
  
• If the number of moles of gas increases in the reaction, increasing the volume of the container favours the production of more product to fill the volume.
• If the number of moles of gas increases in the reaction, decreasing the volume of the container favours the production of less product.
• If the number of moles of gas decreases in the reaction, increasing the volume of the container favours the production of more reactants to fill the volume.
• If the number of moles of gas decreases in the reaction, decreasing the volume of the container favours the production of more product.

For example, in the reaction below, 1 mol of reactant gas produces 1 mol of product gas. Changing the pressure by changing the container volume has no effect on the equilibrium.

S(s) + O₂ ↔ SO₂(g)

In the reaction below, however, 2 mol of reactant gas produces 1 mol of product gas. If the pressure is increased by decreasing the volume, the reaction shifts to try to reduce the pressure. It can do this by producing more product. The reaction shifts to the right.

PCl₃(g) + Cl₂(g) ↔ PCl₅(g)

Adding an inert gas

Adding an inert gas:

• Adding an inert gas has no effect on the equilibrium.

For example, consider the reaction below at equilibrium:

S(s) + O₂ ↔ SO₂(g)

Adding some inert He(g) to the reaction container increases the pressure inside it. However, it does not change the amount of O₂(g) or SO₂(g) present and it does not change the volume of the container. The concentrations (and partial pressures) are not changed. The reaction does not shift.

Changing the temperature

Changing the temperature:

If the temperature changes, the reaction shifts to try to minimize the temperature change

• If the reaction is exothermic, increasing the temperture shifts the reaction towards reactants.
• If the reaction is exothermic, decreasing the temperture shifts the reaction towards products.
  
  
• If the reaction is endothermic, increasing the temperture shifts the reaction towards products.
• If the reaction is endothermic, decreasing the temperture shifts the reaction towards reactants.

For example, the reaction below is exothermic. This means production of products leads to an increase in temperature.

S(s) + O₂ ↔ SO₂(g)     ΔH < 0

If the temperature is increased, the reaction shifts to the left to reduce it (more S and SO₂). If the temperature is decreased, the reaction shifts to the right to increase it (more SO₃).

As this reaction is exothermic, the reverse reaction, shown below is endothermic.

SO₂(g) ↔ S(s) + O₂     ΔH > 0

If the temperature is increased, the reaction shifts to the right to reduce it (more S and O₂). If the temperature is decreased, the reaction shifts to the left to increase it (more SO₃).

Addition of a catalyst

Addition of a catalyst:

Catalysts alter the rates of reactions without being used up. A catalyst reduces the time taken to get to equilibrium but:

• A catalyst has no effect on the position of equilibrium.

Self test quizzes

Test your understanding using the quiz below.