Reversible reactions; dynamic equilibrium in a closed system; the energy changes of reversible reactions; and Le Chatelier's principle for changes in concentration, temperature and pressure.

What this dot point is asking

AQA wants you to describe reversible reactions, explain dynamic equilibrium in a closed system, state that the forward and backward reactions have opposite energy changes, and use Le Chatelier's principle to predict the effect of changing concentration, temperature and pressure. The pressure and detailed Le Chatelier work is Higher tier and is closely tied to the industrial production of ammonia.

Reversible reactions

A classic example is the thermal decomposition of ammonium chloride, or the hydration of anhydrous copper sulfate (white) to the hydrated form (blue), which can be reversed by heating. The energy changes are equal and opposite: if the forward reaction is exothermic, the backward reaction is endothermic by the same amount, so heating drives the reaction in the endothermic direction.

Dynamic equilibrium

"Dynamic" stresses that the reactions never stop; they simply balance. The concentrations of reactants and products are constant but not necessarily equal: the equilibrium can lie towards the products or towards the reactants depending on the conditions.

Le Chatelier's principle (Higher)

• Concentration: increasing a reactant's concentration shifts the equilibrium towards the products (to use up the added reactant); increasing a product's concentration shifts it towards the reactants.
• Temperature: increasing the temperature shifts the equilibrium in the endothermic direction (to absorb the added energy); decreasing it favours the exothermic direction.
• Pressure (gases): increasing the pressure shifts the equilibrium towards the side with fewer molecules of gas (to reduce the pressure).

Using equilibrium in industry

Industry adjusts conditions to maximise the yield of a product, balancing yield against the rate and the cost of the conditions. In the Haber process ($N_2 + 3H_2 \rightleftharpoons 2NH_3$), a high pressure increases yield (fewer gas molecules on the product side) and a low temperature would increase yield (the forward reaction is exothermic), but a low temperature makes the reaction too slow, so a compromise temperature of about 450 degrees Celsius is used with an iron catalyst.

Try this

Q1. Describe what is meant by dynamic equilibrium. [2 marks]

• Cue. The forward and backward reactions occur at the same rate, so concentrations stay constant.

Q2. State the effect of increasing temperature on the position of equilibrium. [1 mark]

• Cue. It shifts in the endothermic direction.

Q3. For $N_2 + 3H_2 \rightleftharpoons 2NH_3$, state and explain the effect of increasing pressure on the yield of ammonia. [2 marks]

• Cue. The yield increases, because the equilibrium shifts to the side with fewer gas molecules (2 molecules on the right versus 4 on the left).