The effect of changing concentration (HT only): If the concentration of a reactant is increased, more products will be formed until equilibrium is reached again.

The idea

At dynamic equilibrium, the forward and reverse reaction rates are equal. Increasing the concentration of a reactant gives more reactant particles per unit volume, so the forward reaction rate increases while the reverse rate is momentarily unchanged. The forward reaction therefore dominates, forming more products. As products accumulate, the reverse reaction speeds up until the two rates are equal again and a new equilibrium is reached. For example, in N₂(g) + 3H₂(g) ⇌ 2NH₃(g), adding more N₂ increases the forward rate, so more NH₃ forms until equilibrium is re-established. The new equilibrium mixture contains more product than before, but not all the added reactant is converted. This concept is assessed at Higher Tier only.

What earns the marks

• States that increasing the concentration of a reactant increases the rate of the forward reaction.
• Explains that the reverse reaction rate is initially unchanged, so the forward reaction is faster than the reverse.
• Concludes that more products are formed until the rates become equal again and equilibrium is re-established.
• Recognises that the new equilibrium mixture contains a greater amount of product than the original equilibrium mixture.
• Applies the explanation to a given reversible reaction, identifying which substance is the added reactant and which substances are products.
• Uses the idea that the system shifts to reduce the effect of the added reactant.

Common mistakes and how to put them right

• Thinking that all the added reactant is converted into product.

  Put it right: Only some is converted; a new equilibrium is reached with some added reactant remaining.

• Believing that the reverse reaction stops.

  Put it right: The reverse reaction continues but is initially slower than the forward reaction.

• Confusing rate with yield.

  Put it right: Increasing reactant concentration increases both the rate at which equilibrium is approached and the equilibrium yield of products, but these are distinct ideas.

Exam tips

• Link your answer to the reaction equation: name the reactant whose concentration increases and the product whose amount increases.
• Use the phrase 'the equilibrium shifts to the right' or 'in the forward direction' and then explain why in terms of rates.
• As this is a Higher Tier only topic, be prepared to explain the shift in terms of unequal forward and reverse reaction rates.