A-Level · Chemistry · AQA · Mark scheme decoded
AQA A-Level Chemistry: Equilibrium Constant K in Homogeneous Systems — mark scheme explained
The short answer
In physical chemistry, understanding equilibrium is crucial for predicting the behavior of chemical systems. The equilibrium constant, denoted as K , is a fundamental concept that helps us quantify the extent to which a reversible reaction proceeds.
The question
For the reaction N 2 (g) + 3H 2 (g) ⇌ 2NH 3 (g), write the equilibrium constant expression and calculate K c if [N 2 ] = 0.5 mol dm -3 , [H 2 ] = 1.5 mol dm -3 , and [NH 3 ] = 0.8 mol dm -3 . [Paraphrased for study — not reproduced from any exam paper.]
Mark scheme, decoded
What each mark is really for — in plain English — and the wording trap that loses it.
- S1
Step 1: Write the equilibrium constant expression for the reaction.
- S2
K c = [NH 3 ] 2 / [N 2 ][H 2 ] 3
- S3
Step 2: Substitute the given equilibrium concentrations into the expression.
- S4
K c = (0.8) 2 / (0.5)(1.5) 3
- S5
Step 3: Calculate the value of K c .
- S6
(0.8) 2 = 0.64
- S7
(1.5) 3 = 3.375
- S8
K c = 0.64 / (0.5 × 3.375) = 0.64 / 1.6875 ≈ 0.38
Model answer
Worked through, with each step tagged to the mark it earns.
- S1
Step 1: Write the equilibrium constant expression for the reaction.
- S2
K c = [NH 3 ] 2 / [N 2 ][H 2 ] 3
- S3
Step 2: Substitute the given equilibrium concentrations into the expression.
- S4
K c = (0.8) 2 / (0.5)(1.5) 3
- S5
Step 3: Calculate the value of K c .
- S6
(0.8) 2 = 0.64
- S7
(1.5) 3 = 3.375
- S8
K c = 0.64 / (0.5 × 3.375) = 0.64 / 1.6875 ≈ 0.38
Final answer: 0.38
Common mistakes
- Using the wrong units for concentrations. — Always ensure that concentrations are in mol dm -3 when substituting into the equilibrium constant expression.
- Forgetting to use stoichiometric coefficients as exponents in the equilibrium constant expression. — Always include the stoichiometric coefficients as exponents in the equilibrium constant expression.
- Misinterpreting the effect of temperature on K for endothermic and exothermic reactions. — Remember that increasing temperature increases K for endothermic reactions and decreases K for exothermic reactions.
- Incorrectly applying the Van't Hoff equation. — Double-check your calculations and ensure you are using the correct values for ΔH°, R, T 1 , and T 2 .
- Believing that changes in concentration or catalysts affect K. — Understand that changes in concentration only shift the position of equilibrium, and catalysts do not affect K; they speed up both forward and reverse reactions equally.
- Failing to explain why changes in concentration or catalysts do not affect K clearly. — Practice explaining that changes in concentration shift the position of equilibrium but do not change K, and catalysts speed up both forward and reverse reactions equally without affecting K.
Where the marks go
- Full worked solution (all marking points)4 marks