Dynamic Equilibrium

Key Stage 4

Dynamic Equilibrium is when a reversible reaction has been going long enough that the forward reaction and the back reaction happen at the same rate causing a constant quantity of reactants and products in the reaction mixture.

During dynamic equilibrium the amount of reactants and products is constant. However, the reactions have not stopped taking place.

When the reaction mixture is initially added to a reaction vessel the rate of the forward reaction is high. As the reaction progresses the concentration of original reactants decreases, slowing rate of reaction. Meanwhile the concentration of products increases, which increases the rate of the back reaction. Once the two rates of reaction are equal then the reaction mixture is in dynamic equilibrium.

The position of the dynamic equilibrium (the ratio of reactants to products) can be affected by the temperature and by the pressure.

An increase in temperature will increase the rate of an endothermic reaction as it requires energy from the environment to take place.

This means for an endothermic forward reaction a higher temperature will result in more product being formed. However, for an exothermic forward reaction less product will be formed as the back reaction will be endothermic and would increase its rate with higher temperature.

For reactions involving a gas as either one of the reactants or products changing the pressure will affect the position of dynamic equilibrium.

The number of moles of gas in the reactants and products will determine which direction of reaction is favoured by higher pressure.

If the forward reaction produces more moles of gas than the back reaction then high pressure will reduce the amount of products formed.

If the forward reaction produces fewer moles of gas than the back reaction then high pressure will increase the amount of products formed.

Hydrogen + Nitrogen ⇌ Ammonia

\( 3H_2(g) + N_2(g) ⇌ 2NH_3(g) \)

In this reversible reaction there are more moles of gas in the reactants than the products so higher pressure will favour the forward reaction and the equilibrium position will have a greater concentration of products than reactants.

Ammonium Chloride ⇌ Ammonia + Hydrochloric Acid

\( NH_4Cl(s) ⇌ NH_3(g) + HCl(g) \)

In this reversible reaction there are more moles of gas in the products than the reactant so higher pressure will favour the back reaction and the equilibrium position will have a greater concentration of reactant than products.