Reversible reactions — AQA GCSE Combined Science
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Reversible reactions explained
Most reactions you meet are shown as one-way: reactants change into products and stop.
Read the full explanation
In a reversible reaction, the products can react together to reform the original reactants, so both changes happen in the same mixture. We show this with the special symbol ⇌ instead of a single arrow. For example, heating hydrated copper(II) sulfate drives off water to give white anhydrous copper(II) sulfate; adding water to the white solid regenerates the blue hydrated salt. The forward change and the reverse change are separate reactions, and the symbol ⇌ reminds you that both are possible. Reversibility does not mean the reaction never finishes; it means the system can move in either direction depending on conditions such as temperature, pressure or concentration.
A + B C + D
This equation is a general model of a reversible reaction. A and B are the reactants on the left; C and D are the products on the right. The symbol ⇌ between them shows that the forward reaction (A + B → C + D) and the reverse reaction (C + D → A + B) can both happen. The letters are placeholders, so you can replace them with real substances. For example, in the Haber process, nitrogen + hydrogen ⇌ ammonia, and in the hydration of copper(II) sulfate, anhydrous copper(II) sulfate + water ⇌ hydrated copper(II) sulfate. The equation does not tell you how fast either direction goes or how far the reaction shifts; those depend on temperature, pressure and concentration.
The direction of reversible reactions can be changed by changing the conditions.
A reversible reaction can proceed in both the forward and reverse directions, shown by the ⇌ symbol. The direction in which the reaction appears to go is not fixed: it depends on the conditions. If you change a condition such as temperature, pressure or concentration, you change the balance between the two directions, so the reaction may shift towards making more products or more reactants. For example, heating hydrated copper(II) sulfate drives off water to form white anhydrous copper(II) sulfate and water vapour; adding water to the white solid reverses this, regenerating the blue hydrated salt and releasing heat. In a closed system the reaction eventually reaches equilibrium, where forward and reverse rates are equal, but the position of that equilibrium still moves when conditions change.
For example:
This statement introduces a worked example of a reversible reaction whose direction changes with conditions. A suitable example is the thermal decomposition of ammonium chloride. When ammonium chloride is heated, it decomposes into ammonia gas and hydrogen chloride gas: NH₄Cl(s) ⇌ NH₃(g) + HCl(g). On cooling, the gases recombine to form solid ammonium chloride again, often seen as a white deposit. The forward reaction is endothermic, so heating favours decomposition; the reverse reaction is exothermic, so cooling favours recombination. This shows that changing the temperature changes the direction in which the reaction proceeds. The example also shows that the reaction is reversible and that the direction can be reversed by changing conditions.
Your focus
- Define a reversible reaction as one where products can reform the original reactants.
- Use the symbol ⇌ correctly in a word or symbol equation.
- Describe a named example of a reversible reaction and the conditions that favour each direction.
Show all 12 objectives
- Interpret the general equation A + B ⇌ C + D in terms of reactants and products.
- Explain that the ⇌ symbol means both the forward and reverse reactions can occur.
- Apply the general pattern to a named reversible reaction and write a balanced equation for it.
- State that reversible reactions can go in both directions and are represented by the ⇌ symbol.
- Explain how changing temperature, pressure or concentration can change the direction of a reversible reaction.
- Describe a named example, such as the hydration and dehydration of copper(II) sulfate, to show a change in direction.
- Write a balanced equation with the ⇌ symbol for a named reversible reaction.
- Explain how heating and cooling change the direction of the ammonium chloride reaction.
- Describe the observable evidence that shows the reverse reaction has taken place.
Reversible reactions exam tips
Marking Points
- A reversible reaction is one in which the products can react to form the original reactants.
- The reaction is represented using the symbol ⇌ between reactants and products, not a single arrow.
- The forward reaction converts reactants into products; the reverse reaction converts products back into reactants.
- Both directions can occur in the same closed system, and the direction favoured depends on the conditions.
- An example is the hydration and dehydration of copper(II) sulfate: blue hydrated ⇌ white anhydrous + water.
- Recognising ⇌ in an equation shows the reaction is reversible rather than complete in one direction.
- A and B represent the reactants, and C and D represent the products.
- The ⇌ symbol shows the reaction is reversible, so products can react to reform reactants.
- The forward reaction is A + B → C + D; the reverse reaction is C + D → A + B.
- The equation is a general template that can be replaced by real formulae or names.
- The position of ⇌ does not by itself show which direction is favoured under given conditions.
- A balanced symbol equation for a reversible reaction must have the same number of each type of atom on both sides.
- A reversible reaction is represented by the ⇌ symbol, meaning the forward and reverse reactions occur simultaneously rather than the reaction simply stopping.
- Changing a condition such as temperature, pressure or concentration alters the relative rates of the forward and reverse reactions, so the direction of the overall change can be reversed.
- In a closed system, reversible reactions can reach dynamic equilibrium, where the rates of the forward and reverse reactions are equal and the concentrations of reactants and products remain constant.
- Changing conditions shifts the position of equilibrium, so the yield of product or reactant changes even though the reaction remains reversible.
- A concrete example is the hydration and dehydration of copper(II) sulfate: heating blue hydrated copper(II) sulfate produces white anhydrous copper(II) sulfate plus water vapour, and adding water reverses the change.
- The direction observed depends on which set of conditions is applied, so a statement about direction must be linked to the specific conditions used.
- The example must be a genuine reversible reaction, written with the ⇌ symbol, showing both forward and reverse changes.
- Heating ammonium chloride causes it to decompose into ammonia and hydrogen chloride gases, which is the forward, endothermic direction.
- Cooling the gaseous products causes ammonia and hydrogen chloride to recombine into solid ammonium chloride, which is the reverse, exothermic direction.
- The observed white deposit on cooling is evidence that the reverse reaction has occurred and that the direction has changed.
- The example demonstrates that changing the temperature changes which direction is favoured, because heating favours the endothermic direction and cooling favours the exothermic direction.
- A full answer links the symbol, the named substances and the condition change to the direction of reaction.
Examiner Tips
- 💡When you see ⇌ in an equation, state clearly that the reaction is reversible and name the forward and reverse changes.
- 💡Use a named example such as hydrated copper(II) sulfate ⇌ anhydrous copper(II) sulfate + water to explain the idea.
- 💡If asked to explain, link the direction of the reaction to a change in conditions such as heating or adding water.
- 💡When a question gives a general equation, translate it into a named example to show you understand the pattern.
- 💡Label the forward and reverse reactions clearly if asked to explain the equation.
- 💡Check that any real equation you write from the template is balanced and uses ⇌, not →.
- 💡Always name the specific condition you are changing, such as temperature or pressure, and state how the direction of the reaction changes as a result.
- 💡Use the ⇌ symbol correctly in equations and explain that it shows reversibility, not that the reaction has stopped.
- 💡When describing equilibrium, include the idea that forward and reverse rates are equal and that concentrations stay constant in a closed system.
- 💡Link any example to observable evidence, such as a colour change, so your explanation shows how the direction change can be detected.
- 💡Choose a named example you can write accurately, including the ⇌ symbol and correct state symbols.
- 💡State the condition change, such as heating or cooling, and then state which direction is favoured and why.
- 💡Describe the observable change, such as a white solid forming on cooling, to support your explanation.
- 💡Keep the explanation focused on direction and conditions rather than giving unnecessary detail about laboratory apparatus.
Common Mistakes
- Writing a single arrow → for a reversible reaction; correction: use ⇌ to show both directions are possible.
- Thinking reversible means the reaction never reaches an end point; correction: reversible reactions can still settle at a balance point where forward and reverse rates are equal.
- Assuming the reverse reaction always happens at the same rate as the forward reaction; correction: the rates depend on the conditions, so one direction may be favoured.
- Treating A, B, C and D as fixed chemicals rather than placeholders; correction: they stand for any reactants and products in a reversible change.
- Reading the equation as only going left to right; correction: the ⇌ shows both directions are possible.
- Forgetting to balance a real equation written from this template; correction: check that atoms are conserved on both sides.
- Thinking a reversible reaction stops when it reaches equilibrium; the correction is that both forward and reverse reactions continue at equal rates, so it is dynamic.
- Assuming the ⇌ symbol means the reaction can only go one way at a time; the correction is that both directions occur together, and conditions decide which direction dominates.
- Writing that heating always pushes a reversible reaction forwards; the correction is that heating favours the endothermic direction, which may be the reverse reaction depending on the reaction.
- Treating an open system as if it reaches the same equilibrium as a closed system; the correction is that escaping gases can prevent true equilibrium being reached.
- Writing the equation without the ⇌ symbol; the correction is to use ⇌ to show that the reaction is reversible.
- Claiming that ammonium chloride melts when heated; the correction is that it decomposes into ammonia and hydrogen chloride gases, which recombine on cooling.
- Saying that cooling drives the forward reaction; the correction is that cooling favours the reverse, exothermic reaction, forming solid ammonium chloride again.
- Omitting state symbols; the correction is to include (s) and (g) so the change of state and the recombination are clear.