AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
The effect of changing conditions on equilibrium (HT only): The relative amounts of all the reactants and products at equilibrium depend on the conditions of the reaction.
This Higher Tier only topic explores how the relative amounts of reactants and products at dynamic equilibrium depend on the conditions imposed.
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
This Higher Tier only topic explores how the relative amounts of reactants and products at dynamic equilibrium depend on the conditions imposed. At equilibrium in a closed system, forward and reverse reactions continue at equal rates, so concentrations stay constant. According to Le Chatelier's principle, changing temperature, pressure, or concentration shifts the position of equilibrium. For example, if the forward direction is exothermic, an increase in temperature shifts the equilibrium to favour the endothermic reverse reaction, decreasing the relative amount of products. For gases, increasing pressure favours the side with fewer molecules. A catalyst speeds both directions equally and does not change relative amounts.
What earns the marks
- States that at equilibrium the rates of the forward and reverse reactions are equal and concentrations remain constant in a closed system.
- Explains that the relative amounts of reactants and products are determined by the conditions, not by a fixed ratio.
- Identifies temperature as a condition that changes the position of equilibrium, favouring the endothermic or exothermic direction.
- Identifies pressure as a condition affecting gaseous equilibria by favouring the side with fewer or more gas molecules.
- Explains that changing the concentration of a reactant or product shifts the equilibrium to counteract the change.
- States that a catalyst does not change the position of equilibrium or the relative amounts, only the rate at which equilibrium is reached.
Common mistakes and how to put them right
Thinking equilibrium means equal amounts of reactants and products.
Put it right: Equilibrium means equal rates, and amounts depend on the conditions.
Believing a catalyst increases the yield of product.
Put it right: A catalyst speeds both forward and reverse reactions equally and does not change relative amounts.
Assuming the equilibrium position is fixed for a reaction.
Put it right: Changing temperature, pressure or concentration changes the relative amounts present.
Exam tips
- Use the phrase 'position of equilibrium' when describing how relative amounts change with conditions.
- For gaseous equilibria, count gas molecules on each side of the balanced equation to justify the effect of pressure.
Try the lesson
The start of the The effect of changing conditions on equilibrium (HT only): The relative amounts of all the reactants and products at equilibrium depend on the conditions of the reaction. lesson, made from checked teaching notes for this specification point.
Have a go first
A reversible reaction reaches equilibrium in a closed system. Which statement correctly describes how the relative amounts of reactants and products at equilibrium are determined?
Listen
This Higher Tier only topic explores how the relative amounts of reactants and products at dynamic equilibrium depend on the conditions imposed. At equilibrium in a closed system, forward and reverse reactions continue at equal rates, so concentrations stay constant. According to Le Chatelier's principle, changing temperature, pressure, or concentration shifts the position of equilibrium. For example, if the forward direction is exothermic, an increase in temperature shifts the equilibrium to favour the endothermic reverse reaction, decreasing the relative amount of products. For gases, increasing pressure favours the side with fewer molecules. A catalyst speeds both directions equally and does not change relative amounts. A strong answer earns marks for these points. States that at equilibrium the rates of the forward and reverse reactions are equal and concentrations remain constant in a closed system. Explains that the relative amounts of reactants and products are determined by the conditions, not by a fixed ratio. Identifies temperature as a condition that changes the position of equilibrium, favouring the endothermic or exothermic direction. Identifies pressure as a condition affecting gaseous equilibria by favouring the side with fewer or more gas molecules. A common slip: Thinking equilibrium means equal amounts of reactants and products. Instead, equilibrium means equal rates, and amounts depend on the conditions. Exam tip: Use the phrase 'position of equilibrium' when describing how relative amounts change with conditions.
Key words
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Dynamic equilibrium
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Le Chatelier's principle
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Effect of temperature on equilibrium
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Effect of pressure and concentration on equilibrium
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See every definition
- Dynamic equilibrium
- In a closed system the forward and reverse reactions continue at equal rates, so concentrations stay constant.
- Le Chatelier's principle
- Changing temperature, pressure or concentration shifts the position of equilibrium to counteract the change.
- Effect of temperature on equilibrium
- If the forward direction is exothermic, an increase in temperature shifts the equilibrium to favour the endothermic reverse reaction, decreasing the relative amount of products.
- Effect of pressure and concentration on equilibrium
- For gases, increasing pressure favours the side with fewer molecules, and changing the concentration of a reactant or product shifts the equilibrium to counteract the change.
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