Question detail
What is the 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
Chemical equilibria, Le Chatelier's principle and Kc
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 construct Kc expressions for homogeneous equilibria. This reasoning is anchored to Equilibrium constant Kc in Chemical equilibria, Le Chatelier's principle and Kc, and it separates Kc expression 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
Incorrect Kc Expression Construction
Students often confuse the products and reactants when constructing Kc expressions, leading to incorrect formulas.
To construct the Kc expression, use the formula Kc = [products]^[coefficients] / [reactants]^[coefficients]. For example, for the reaction aA + bB ⇌ cC + dD, the correct expression is Kc = [C]^c [D]^d / [A]^a [B]^b. Substitute the equilibrium concentrations into the formula and calculate Kc.
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