Question detail
For the equilibrium reaction: aA + bB ⇌ cC + dD, how is the Kc expression formulated?
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 determine units for Kc. This reasoning is anchored to Equilibrium constant Kc in Chemical equilibria, Le Chatelier's principle and Kc, and it separates units 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
Common Mistake in Kc Units
Students often confuse the units of Kc by not considering the stoichiometry of the balanced equation.
To determine the units for Kc, use the formula: Kc = [products]^coefficients / [reactants]^coefficients. Substitute the concentrations in mol/dm³ for each species in the equilibrium expression. For example, if the balanced equation is aA + bB ⇌ cC + dD, then Kc = ([C]^c * [D]^d) / ([A]^a * [B]^b). The units will be (mol/dm³)^(c+d) / (mol/dm³)^(a+b), leading to the final units of Kc being mol^(c+d-a-b) dm^(-3(c+d-a-b)). This shows how the stoichiometry affects the units.
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