A-Level · Chemistry · AQA · Mark scheme decoded
AQA A-Level Chemistry: Equilibrium Constant K p and Its Applications — mark scheme explained
The short answer
In physical chemistry, the study of chemical equilibria is crucial for understanding how reactions proceed and reach a state where the concentrations of reactants and products remain constant over time.
The question
A reaction vessel contains 2.0 mol of N 2 , 3.0 mol of H 2 , and 1.0 mol of NH 3 . The total pressure in the vessel is 5 atm. Calculate the partial pressures of each gas.
[Paraphrased for study — not reproduced from any exam paper.]
Mark scheme, decoded
How the examiner actually awards the marks on this topic.
Gradora's own decode of the marking approach — not the exam board's published mark scheme.
How marks are awarded
For calculation questions, marks are typically awarded for correct substitution of values into formulas, accurate arithmetic, and providing the final answer with appropriate units. For conceptual questions, marks are given for clear and concise explanations that demonstrate a deep understanding of the topic.
What the command words demand
- Calculate
- Perform a numerical calculation using given data and appropriate formulas.
- Construct
- Write the expression for K p based on the reaction equation.
- Predict
- Determine how changes in temperature or pressure will affect the position of equilibrium and the value of K p .
- Explain
- Provide a detailed description of the concept, including relevant principles and relationships.
Model answer
A full-mark response to the question above, worked through step by step.
Timing: Allocate approximately 5-7 minutes per question to ensure you have enough time to carefully read the problem, perform calculations, and check your work.
- Identify the number of moles of each gas.0 marksn N2 = 2.0 moln H2 = 3.0 moln NH3 = 1.0 mol
- Calculate the total number of moles.1 markTotal moles = n N2 + n H2 + n NH3 = 2.0 + 3.0 + 1.0 = 6.0 mol
- Calculate the mole fraction of each gas.1 markX N2 = n N2 / Total moles = 2.0 / 6.0 = 0.333X H2 = n H2 / Total moles = 3.0 / 6.0 = 0.500X NH3 = n NH3 / Total moles = 1.0 / 6.0 = 0.167
- Calculate the partial pressure of each gas.2 marksP N2 = X N2 × P total = 0.333 × 5 atm = 1.665 atmP H2 = X H2 × P total = 0.500 × 5 atm = 2.500 atmP NH3 = X NH3 × P total = 0.167 × 5 atm = 0.835 atm
Final answer: P N2 = 1.665 atm, P H2 = 2.500 atm, P NH3 = 0.835 atm
Work through every step correctly and you earn all 4 marks.
Another worked example
For the reaction N 2 (g) + 3H 2 (g) ⇌ 2NH 3 (g), the partial pressures at equilibrium are P N2 = 0.5 atm, P H2 = 1.5 atm, and P NH3 = 2.0 atm. Calculate K p .
- Write the expression for K p .1 markK p = (P NH3 ) 2 / ((P N2 ) × (P H2 ) 3 )
- Substitute the partial pressures into the expression.1 markK p = (2.0) 2 / ((0.5) × (1.5) 3 )
- Perform the calculation.1 markK p = 4.0 / (0.5 × 3.375) = 4.0 / 1.6875 ≈ 2.37
Final answer: K p ≈ 2.37
Work through every step correctly and you earn all 3 marks.
Common mistakes
Using the wrong units for partial pressures.
Why it happens: Students often forget to use atmospheres (atm) or pascals (Pa) consistently in their calculations.
Fix: Always ensure that all partial pressures are in the same unit, typically atmospheres (atm), when substituting into the K p expression.
Forgetting to use the correct stoichiometric coefficients in the K p expression.
Why it happens: Students may overlook the exponents in the K p expression, leading to incorrect calculations.
Fix: Always double-check that the partial pressures are raised to the power of their respective stoichiometric coefficients in the reaction equation.
Misinterpreting the effect of temperature on K p for exothermic and endothermic reactions.
Why it happens: Students sometimes confuse the direction of equilibrium shift with changes in temperature, leading to incorrect predictions.
Fix: Remember that for exothermic reactions (ΔH < 0), increasing temperature decreases K p . For endothermic reactions (ΔH > 0), increasing temperature increases K p .
Incorrectly calculating mole fractions and partial pressures.
Why it happens: Students may make arithmetic errors or use incorrect values when calculating mole fractions and partial pressures.
Fix: Practice calculating mole fractions and partial pressures step-by-step. Ensure that the total number of moles is correctly calculated before finding the mole fraction.
Failing to predict the effect of pressure changes on equilibrium positions accurately.
Why it happens: Students may not fully understand how changes in pressure affect the position of equilibrium, leading to incorrect predictions.
Fix: Practice predicting the direction of equilibrium shift based on the number of moles of gas on each side of the reaction equation. Use Le Chatelier's principle to guide your reasoning.
Confusing the role of catalysts in reaching equilibrium with changes in K p .
Why it happens: Students may think that catalysts change the value of K p , leading to incorrect conclusions.
Fix: Remember that catalysts only affect the rate at which equilibrium is reached but do not change the value of K p . The position of equilibrium remains unchanged by a catalyst.
Where the marks go
The question types you’ll meet on this topic and the marks each one carries — so you know what to expect and where to focus.
| Question type | What you’re asked to do | Marks |
|---|---|---|
| Calculate Partial Pressures | Use mole fractions and total pressure to find each gas's partial pressure. | 4 |
| Calculate Kp | Write the Kp expression, substitute equilibrium partial pressures, and calculate the value with units. | 3 |
| Calculate Equilibrium Pressure | Rearrange the Kp expression to find an unknown partial pressure from given equilibrium values. | 3 |
| Kp Equilibrium Calculation | Use the Kp expression and given partial pressures to find equilibrium partial pressures of reactants. | 5 |
| Total across these question types | 15 | |
Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.