What is a reversible reaction, and what does dynamic equilibrium mean?
Reversible reactions and equilibria: reversible reactions and the use of the reversible arrow, the energy change in each direction, dynamic equilibrium in a closed system, and the idea that the conditions affect the position of equilibrium.
A focused answer to Edexcel GCSE Chemistry topic 4, covering reversible reactions and the reversible arrow, why the forward and backward reactions have opposite energy changes, what dynamic equilibrium means in a closed system, and the qualitative effect of changing conditions on the position of equilibrium.
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What this dot point is asking
Edexcel wants you to recognise reversible reactions and the reversible arrow, explain that the forward and backward reactions have equal and opposite energy changes, describe dynamic equilibrium in a closed system, and state qualitatively how changing the conditions shifts the position of equilibrium. This is the foundation for the Haber process and industrial equilibria in separate chemistry.
Reversible reactions
A familiar example is the dehydration of hydrated copper sulfate: heating drives off the water (blue to white), and adding water reverses it (white to blue):
Another is the thermal decomposition of ammonium chloride, which re-forms on cooling.
Energy changes are equal and opposite
If the forward reaction is exothermic (releases energy), the backward reaction is endothermic (takes in energy) by exactly the same amount, and vice versa. So in the copper sulfate example, the forward dehydration is endothermic, which means adding water back is exothermic, releasing the same amount of energy.
Dynamic equilibrium
When a reversible reaction takes place in a closed system (nothing can enter or leave), it reaches dynamic equilibrium.
It is called dynamic because the reactions never stop, even though the overall amounts no longer change. If the system were open, products could escape and equilibrium could not be reached.
Changing the conditions
The position of equilibrium tells you the relative amounts of reactants and products. Changing the conditions shifts this position:
- Changing temperature, pressure (for gases) or concentration moves the position of equilibrium one way or the other.
- Moving the equilibrium towards the products increases the yield of products; moving it towards the reactants decreases it.
At GCSE you state the direction of the shift qualitatively; the detailed rules and the link to industrial processes such as the Haber process are developed in separate chemistry.
Try this
Q1. What does the reversible arrow () show? [1 mark]
- Cue. The reaction can go in both directions; the products can re-form the reactants.
Q2. The forward reaction is endothermic. State the energy change of the backward reaction. [1 mark]
- Cue. Exothermic (equal and opposite).
Q3. Explain why the amounts of reactants and products stay constant at dynamic equilibrium. [2 marks]
- Cue. The forward and backward reactions occur at the same rate, so products form as fast as they are used up.
Exam-style practice questions
Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Edexcel 20193 marksHydrated copper sulfate can be dehydrated: . The forward reaction is endothermic. State the colour change in the forward direction, state the energy change of the reverse reaction, and explain what the reversible arrow means.Show worked answer →
A 3-mark reversible-reaction question.
In the forward (dehydration) direction the colour changes from blue (hydrated) to white (anhydrous) (1 mark). If the forward reaction is endothermic, the reverse reaction (adding water back) is exothermic, because the energy change is equal and opposite (1 mark). The reversible arrow means the reaction can go in both directions: the products can react to re-form the reactants under suitable conditions (1 mark).
Markers reward the blue-to-white change, the equal-and-opposite energy change, and the meaning of the reversible arrow.
Edexcel 20224 marksA reversible reaction reaches dynamic equilibrium in a closed system. Explain what is meant by dynamic equilibrium, including what is happening to the forward and backward reactions and to the amounts of reactants and products.Show worked answer →
A 4-mark dynamic-equilibrium explanation.
At dynamic equilibrium the forward and backward reactions are still occurring (1 mark), but they occur at the same rate (1 mark). Because the rates are equal, the amounts (concentrations) of reactants and products stay constant (1 mark). Equilibrium can only be reached in a closed system, where no reactants or products can escape (1 mark).
Markers reward "both reactions continue", "at equal rates" and "amounts constant", plus the closed-system condition.
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Sources & how we know this
- Pearson Edexcel GCSE (9-1) Chemistry (1CH0) specification — Pearson Edexcel (2016)