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
What each mark is really for — in plain English — and the wording trap that loses it.
- S1
Step 1: Identify the number of moles of each gas.
- S2
n N2 = 2.0 mol
- S3
n H2 = 3.0 mol
- S4
n NH3 = 1.0 mol
- S5
Step 2: Calculate the total number of moles.
- S6
Total moles = n N2 + n H2 + n NH3 = 2.0 + 3.0 + 1.0 = 6.0 mol
- S7
Step 3: Calculate the mole fraction of each gas.
- S8
X N2 = n N2 / Total moles = 2.0 / 6.0 = 0.333
- S9
X H2 = n H2 / Total moles = 3.0 / 6.0 = 0.500
- S10
X NH3 = n NH3 / Total moles = 1.0 / 6.0 = 0.167
- S11
Step 4: Calculate the partial pressure of each gas.
- S12
P N2 = X N2 × P total = 0.333 × 5 atm = 1.665 atm
- S13
P H2 = X H2 × P total = 0.500 × 5 atm = 2.500 atm
- S14
P NH3 = X NH3 × P total = 0.167 × 5 atm = 0.835 atm
Model answer
Worked through, with each step tagged to the mark it earns.
- S1
Step 1: Identify the number of moles of each gas.
- S2
n N2 = 2.0 mol
- S3
n H2 = 3.0 mol
- S4
n NH3 = 1.0 mol
- S5
Step 2: Calculate the total number of moles.
- S6
Total moles = n N2 + n H2 + n NH3 = 2.0 + 3.0 + 1.0 = 6.0 mol
- S7
Step 3: Calculate the mole fraction of each gas.
- S8
X N2 = n N2 / Total moles = 2.0 / 6.0 = 0.333
- S9
X H2 = n H2 / Total moles = 3.0 / 6.0 = 0.500
- S10
X NH3 = n NH3 / Total moles = 1.0 / 6.0 = 0.167
- S11
Step 4: Calculate the partial pressure of each gas.
- S12
P N2 = X N2 × P total = 0.333 × 5 atm = 1.665 atm
- S13
P H2 = X H2 × P total = 0.500 × 5 atm = 2.500 atm
- S14
P 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
Common mistakes
- Using the wrong units for partial pressures. — 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. — 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. — 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. — 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. — 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 . — 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
- Full worked solution (all marking points)4 marks