ExpertEasy1 markMultiple Choice
AQA GCSE · Question 03.3 · The rate and extent of chemical change
Water molecules break down into hydrogen ions and hydroxide ions.
The equation for the reaction is:
H₂O ⇌ H⁺ + OH⁻
Which sentence describes this reaction at equilibrium?
Water molecules break down into hydrogen ions and hydroxide ions.
The equation for the reaction is:
H₂O ⇌ H⁺ + OH⁻
Which sentence describes this reaction at equilibrium?
Answer options:
A.
Water molecules break down at a higher rate than they reform.
B.
Water molecules break down and reform at the same rate.
C.
Water molecules break down at a lower rate than they reform.
How to approach this question
The question is about the definition of dynamic equilibrium. The symbol ⇌ indicates a reversible reaction at equilibrium.
1. At equilibrium, are the reactions still happening? Yes.
2. How do the rates of the forward reaction (breaking down) and the reverse reaction (reforming) compare at equilibrium?
Full Answer
B.Water molecules break down and reform at the same rate.✓ Correct
The equation H₂O ⇌ H⁺ + OH⁻ represents the autoionization of water, a reversible reaction. The double arrow (⇌) signifies that the reaction is at dynamic equilibrium. In a dynamic equilibrium:
- Both the forward reaction (dissociation of H₂O) and the reverse reaction (formation of H₂O from H⁺ and OH⁻) are continuously occurring.
- The rate of the forward reaction is exactly equal to the rate of the reverse reaction.
- Because the rates are equal, the concentrations of the reactants (H₂O) and the products (H⁺ and OH⁻) remain constant.
Therefore, the correct description is that the breakdown and reformation of water molecules happen at the same rate.
Common mistakes
✗ Thinking that the reaction stops at equilibrium.
✗ Believing that the concentrations of reactants and products must be equal at equilibrium (it's the rates that are equal).
Practice the full AQA GCSE Chemistry Higher Tier Paper 2
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