Chemical equilibria: reversible reactions, dynamic equilibrium — common mistakes

When answering exam questions about Le Chatelier's principle, be precise. State the change, state how the system opposes the change, and state the direction of the shift in the position of equilibrium (e.g., 'to the right' or 'favouring the products'). For pressure, always count and state the number of moles of gas on each side of the equation.

Remember that industrial conditions are often a compromise. While low temperature might give the best yield for an exothermic reaction, the rate might be too slow to be economical. Examiners may ask you to explain why the actual conditions used differ from the theoretical optimum yield conditions.

A catalyst works by providing an alternative reaction pathway with a lower activation energy. Crucially, it lowers the activation energy for both the forward and the reverse reactions by the same amount. This means it speeds up both reactions equally. Since the position of equilibrium depends on the relative rates, and both are increased by the same factor, the position of equilibrium itself remains unchanged. The only difference is that the system reaches this equilibrium state much faster.

The 'rate of reaction' refers to how fast reactants are converted into products (or vice versa). The 'position of equilibrium' describes the relative amounts of reactants and products once equilibrium is reached. If the equilibrium 'lies to the right', it means there are more products than reactants at equilibrium. Le Chatelier's principle describes how the position of equilibrium shifts, not how the rate changes (except for temperature, which affects both).

For gas-phase reactions, yes, increasing the pressure increases the concentration of the gas molecules. This leads to more frequent collisions, so the rate of reaction increases for both the forward and reverse reactions. This is a separate concept from the effect of pressure on the position of equilibrium, which is determined by the relative number of moles of gas on each side of the equation.