What is a reversible reaction, and how does changing conditions shift a dynamic equilibrium?
Describe reversible reactions and dynamic equilibrium, and predict the effect of changing temperature, concentration and pressure using Le Chatelier's principle.
A focused answer to WJEC GCSE Chemistry topic 2.6, covering reversible reactions and the symbol used, dynamic equilibrium in a closed system, and how changing temperature, concentration and pressure shifts the position of equilibrium using Le Chatelier's principle.
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What this topic is asking
WJEC topic 2.6 wants you to describe reversible reactions and the symbol, explain dynamic equilibrium in a closed system, and use Le Chatelier's principle to predict how changing temperature, concentration and pressure shifts the position of equilibrium.
Reversible reactions
For example, heating hydrated copper sulfate drives off water to give white anhydrous copper sulfate, and adding water reverses it:
Dynamic equilibrium
If a reversible reaction takes place in a closed system (nothing can enter or leave), it reaches dynamic equilibrium.
The concentrations of reactants and products are constant but are usually not equal; the equilibrium can lie more towards the products or more towards the reactants.
Le Chatelier's principle
This lets you predict which way the equilibrium moves when you change a condition.
Changing temperature
Raising the temperature shifts the equilibrium in the endothermic direction (to absorb the added heat); lowering it shifts towards the exothermic direction. So if the forward reaction is exothermic, raising the temperature shifts the equilibrium backwards, reducing the amount of product.
Changing concentration
Increasing the concentration of a reactant shifts the equilibrium forwards (towards the products) to use it up; increasing a product shifts it backwards. The equilibrium always shifts away from the substance you added.
Changing pressure
For reactions involving gases, increasing the pressure shifts the equilibrium towards the side with fewer gas molecules (to reduce the pressure); decreasing the pressure shifts it towards the side with more gas molecules.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC sample3 marksExplain what is meant by a reversible reaction reaching dynamic equilibrium in a closed system.Show worked answer β
A Unit 2.6 explanation question. Reward: in a reversible reaction the products can react to re-form the reactants. Dynamic equilibrium is reached when the rate of the forward reaction equals the rate of the reverse reaction, so the concentrations of reactants and products stay constant. It is "dynamic" because both reactions are still happening, just at equal rates; it only happens in a closed system where nothing can enter or leave. Markers credit equal forward and reverse rates, constant concentrations, and the closed system. A common slip is to say the reactions have stopped.
WJEC sample3 marksFor the exothermic reaction N2 + 3H2 in equilibrium with 2NH3, predict and explain the effect of increasing the temperature on the amount of ammonia at equilibrium.Show worked answer β
A Unit 2.6 Le Chatelier question. Reward: increasing the temperature shifts the equilibrium in the endothermic direction (to take in the added heat). The forward reaction is exothermic, so the reverse reaction is endothermic; the equilibrium shifts backwards (left), so the amount of ammonia decreases. Markers credit shifting in the endothermic (reverse) direction and the conclusion that less ammonia forms. A common error is to predict more ammonia by treating it as a rate question rather than an equilibrium position question.
Related dot points
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Sources & how we know this
- WJEC GCSE Chemistry specification (from 2016) β WJEC (2016)