Question detail
Which of the following is a correct expression for the equilibrium constant (Kc) for the reaction: aA + bB ⇌ cC + dD?
Try the question, check the answer, then read the explanation to understand the curriculum point.
At a glance
MCQ
Type
practice
Style
Topic
A-level practical endorsement and required practical activities
Question
- A. Kc = [C]^c [D]^d / [A]^a [B]^b
- B. Kc = [A]^a [B]^b / [C]^c [D]^d
- C. Kc = [C]^c + [D]^d / [A]^a + [B]^b
- D. Kc = [C]^c - [D]^d / [A]^a - [B]^b
Answer
Kc = [C]^c [D]^d / [A]^a [B]^b
Explanation
The correct option is Kc = [C]^c [D]^d / [A]^a [B]^b. Kc = [C]^c [D]^d / [A]^a [B]^b is the best answer because it directly supports the AQA A-Level Chemistry objective to required practical 8: measure an equilibrium constant. This reasoning is anchored to A-level required practical activities in A-level practical endorsement and required practical activities, and it separates required practical from similar A-Level ideas rather than relying on a vague recall statement. Other options are weaker if they use the wrong evidence, calculation, mechanism, observation, unit, or conclusion for this subtopic.
Common mistake
Misunderstanding Equilibrium Constants
Students often confuse the equilibrium constant expression with the reaction quotient, leading to incorrect calculations.
To fix this, remember that the equilibrium constant (Kc) is calculated using the concentrations of products raised to their coefficients divided by the concentrations of reactants raised to their coefficients at equilibrium. For example, for the reaction aA + bB ⇌ cC + dD, the formula is Kc = [C]^c[D]^d / [A]^a[B]^b. Substitute the equilibrium concentrations into the formula, perform the calculations, and ensure to include units of mol/dm³ for concentrations. Keep the correction anchored to A-level required practical activities and the objective: Required practical 8: measure an equilibrium constant.
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