What is a reversible reaction, and what does equilibrium mean?
Reversible reactions, the symbol used, energy changes in reversible reactions, and the idea of a dynamic equilibrium in a closed system.
A focused answer to the WJEC GCSE Science Double Award Unit 5 topic on reversible reactions, covering what a reversible reaction is and its symbol, the energy changes in each direction, and the idea of a dynamic equilibrium in a closed system.
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What this dot point is asking
WJEC Double Award Unit 5 wants you to describe reversible reactions and their symbol, the energy changes in each direction, and the idea of a dynamic equilibrium in a closed system.
Reversible reactions
An everyday example is heating hydrated copper sulfate (blue) to drive off water, leaving anhydrous copper sulfate (white); adding water turns it blue again. Many industrial reactions are reversible.
Energy changes in reversible reactions
This is a direct result of energy being conserved: re-forming the reactants needs back exactly the energy that was released.
Dynamic equilibrium
At equilibrium the concentrations do not change, but it does not mean the reaction has stopped, or that there are equal amounts of reactants and products - just that the two rates are balanced.
Why a closed system is needed
A closed system is needed for equilibrium because if products could escape (an open system), the backward reaction could not happen and the reaction would just go forwards. Keeping everything contained lets the products build up enough to react back, so the forward and backward rates can become equal. This is why equilibrium is studied in sealed containers, and it matters for industrial processes where conditions are controlled to get the most product.
Changing the conditions
The position of an equilibrium can be shifted by changing the conditions, which industry uses to get more product. In general, changing the temperature, pressure (for gases) or concentration can move the balance towards the reactants or the products. For example, for the exothermic Haber reaction, a lower temperature shifts the equilibrium towards more ammonia, and a higher pressure also gives more ammonia. This is why the conditions in industrial processes are chosen carefully, balancing the better yield against the cost and rate, which links equilibrium to the Haber process.
A familiar reversible example
A clear example of a reversible reaction is hydrated and anhydrous copper sulfate. Blue hydrated copper sulfate can be heated to drive off the water, leaving white anhydrous copper sulfate (an endothermic change); adding water to the white solid turns it blue again and releases heat (exothermic). This colour change is also used as a test for water. It shows neatly that a reversible reaction goes both ways and that the energy change is reversed in each direction, making it a popular exam example.
Try this
Q1. What symbol shows a reversible reaction? [1 mark]
- Cue. A double arrow.
Q2. At equilibrium, how do the forward and backward rates compare? [1 mark]
- Cue. They are equal.
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 style3 marksWhat is a reversible reaction, and what symbol is used to show one?Show worked answer →
A Unit 5 recall question. A reversible reaction is one in which the products can react to re-form the reactants, so it can go in both directions (1); it is shown by a special double arrow (the reversible-reaction symbol) instead of a single arrow (1); for example, the reaction can go forwards and backwards depending on the conditions (1). Markers credit the idea of going both ways and the double-arrow symbol. A common error is to say a reversible reaction can never reach a steady state.
WJEC style4 marksExplain what is meant by a dynamic equilibrium in a closed system.Show worked answer →
A Unit 5 explain question worth 4 marks. Reward: in a closed system (nothing enters or leaves), a reversible reaction reaches equilibrium when the forward and backward reactions happen at the same rate (2); the amounts (concentrations) of reactants and products stay constant (1); but the reactions are still happening (it is dynamic, not stopped) (1). Markers credit equal forward and backward rates, constant amounts, and that the reactions continue. A common error is to say the reaction has stopped at equilibrium.
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