What is the difference between the effect of a catalyst and the effect of increasing temperature on the position of equilibrium?

A catalyst has no effect on the position of equilibrium because it speeds up the forward and reverse reactions equally. Increasing temperature shifts the equilibrium position toward the endothermic direction to absorb excess heat.

How does increasing the volume of a reaction vessel differ from increasing the pressure?

Increasing volume is equivalent to decreasing pressure. This causes the equilibrium to shift toward the side with more moles of gas to compensate for the lower concentration of particles.

When comparing concentration changes to temperature changes, which one affects the equilibrium constant ($K_c$)?

Only temperature changes affect the value of $K_c$. Concentration changes shift the equilibrium position to maintain the same ratio defined by the original $K_c$.

What is a common error when predicting the effect of pressure on a reaction involving solids and gases?

A common mistake is counting the moles of solids or liquids. Pressure only affects the concentration of gases, so only the coefficients of gaseous species should be compared.

Why is it incorrect to say that a catalyst increases the yield of a product in an equilibrium system?

A catalyst only increases the rate at which equilibrium is reached. It does not change the relative amounts of reactants and products present once the system is at equilibrium.

What mistake do students often make when a reaction has an equal number of gas moles on both sides?

Students often predict a shift when pressure changes, but if the number of gas moles is equal (e.g., $H_2 + I_2 \rightleftharpoons 2HI$), pressure has no effect on the equilibrium position.

Define Le Chatelier's Principle.

It is the principle stating that if a system at equilibrium is disturbed by a change in temperature, pressure, or concentration, the system will shift its position to counteract the disturbance.

What does it mean for a reaction to be 'exothermic' in the context of equilibrium?

An exothermic reaction releases heat to the surroundings ($\Delta H$ is negative). In equilibrium terms, heat acts like a product of the reaction.

What is the 'position of equilibrium'?

It refers to the relative concentrations of reactants and products in a system at equilibrium. A shift 'to the right' means more products are formed.

Why does increasing pressure favor the side with fewer moles of gas?

Fewer moles of gas exert less pressure in a fixed volume. By shifting to that side, the system reduces the total number of particles, thereby opposing the increase in external pressure.

The Effect of Changing Conditions on Equilibrium | AQA GCSE Science

Combined Science Trilogy / Chemistry

Chemical Change: Rate & Extent

The Effect of Changing Conditions on Equilibrium

Summary

Le Chatelier's Principle provides a qualitative framework for predicting how a chemical system at equilibrium responds to external stresses. By shifting the position of equilibrium to oppose changes in concentration, pressure, or temperature, the system minimizes the impact of the disturbance and establishes a new state of balance.

1. Definition & Core Concepts

Le Chatelier's Principle states that if a system at equilibrium is subjected to a change in conditions, the system will shift its equilibrium position in a direction that tends to counteract or oppose that change.
The position of equilibrium refers to the relative amounts of reactants and products present; a shift to the right favors product formation, while a shift to the left favors reactant formation.
A system is only affected by these changes if it is in a closed environment where no matter can enter or leave, allowing the forward and reverse reactions to reach a steady state.

Concentration-time graph showing a sudden spike in reactant concentration followed by a shift to a new equilibrium where product concentration increases.

2. Concentration Changes

Increasing the concentration of a reactant causes the system to shift to the right to consume the excess, thereby increasing the yield of products.
Decreasing the concentration of a product (e.g., by continuous removal) forces the system to shift to the right to replace what was lost, which is a common industrial technique to maximize output.
Conversely, adding more product or removing reactant will shift the equilibrium to the left, favoring the formation of the original starting materials.

3. Pressure and Volume in Gaseous Systems

Changes in pressure significantly affect equilibrium only when at least one reactant or product is in the gaseous state and there is a change in the total number of gas molecules.
Increasing pressure (or decreasing volume) shifts the equilibrium toward the side with the fewer number of gas molecules to reduce the total pressure of the system.
Decreasing pressure (or increasing volume) shifts the equilibrium toward the side with the greater number of gas molecules to counteract the pressure drop.
If the number of moles of gas is identical on both sides of the chemical equation, changing the pressure has no effect on the position of equilibrium.

4. Temperature Effects

5. Key Distinctions: Catalysts vs. Conditions

6. Exam Strategy & Tips