A-Level · Chemistry · AQA · Mark scheme decoded
AQA A-Level Chemistry: Enthalpy Changes and Standard Enthalpies — mark scheme explained
The short answer
In physical chemistry, understanding enthalpy changes is crucial for analyzing the energy transformations that occur during chemical reactions. This section focuses on defining standard enthalpy of combustion (∆H c Ɵ ) and standard enthalpy of formation (∆H f Ɵ ), as well as their significance in thermodynamics.
The question
Calculate the standard enthalpy change for the combustion of methane (CH 4 ) using the given ∆H c Ɵ value: CH 4(g) + 2O 2(g) → CO 2(g) + 2H 2 O (l) , ∆H c Ɵ = -890 kJ mol -1 .
[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.
- Explain
- Provide a detailed description of the concept, including relevant principles and relationships.
- Determine
- Find or derive a specific value or quantity based on given information.
- Apply
- Use a principle or method to solve a problem or interpret data.
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 given values.0 marks∆H c Ɵ for CH 4(g) = -890 kJ mol -1
- Write the balanced equation for the combustion of methane.0 marksCH 4(g) + 2O 2(g) → CO 2(g) + 2H 2 O (l)
- The standard enthalpy change for the reaction is given by ∆H c Ɵ .2 marks∆H = -890 kJ mol -1
Final answer: -890 kJ mol -1
Work through every step correctly and you earn all 2 marks.
Another worked example
Calculate the standard enthalpy change for the formation of water (H 2 O) using the given ∆H f Ɵ value: H 2(g) + ½O 2(g) → H 2 O (l) , ∆H f Ɵ = -286 kJ mol -1 .
- Identify the given values.0 marks∆H f Ɵ for H 2 O (l) = -286 kJ mol -1
- Write the balanced equation for the formation of water.1 markH 2(g) + ½O 2(g) → H 2 O (l)
- The standard enthalpy change for the reaction is given by ∆H f Ɵ .1 mark∆H = -286 kJ mol -1
Final answer: -286 kJ mol -1
Work through every step correctly and you earn all 2 marks.
Common mistakes
Using the wrong units for enthalpy.
Why it happens: Students often forget to use kilojoules per mole (kJ mol -1 ) when expressing enthalpy changes.
Fix: Always ensure that enthalpy values are in kJ mol -1 when substituting into calculations.
Forgetting to use standard conditions (298 K, 100 kPa).
Why it happens: Students may overlook the importance of using standard conditions in their calculations.
Fix: Always specify that the enthalpy values are under standard conditions (298 K, 100 kPa) when solving problems.
Confusing ∆H c Ɵ and ∆H f Ɵ .
Why it happens: Students sometimes mix up the definitions of standard enthalpy of combustion and formation, leading to incorrect answers.
Fix: Remember that ∆H c Ɵ is always negative (exothermic) and refers to combustion, while ∆H f Ɵ can be either positive or negative and refers to formation.
Misinterpreting the sign of enthalpy changes.
Why it happens: Students may not fully understand that a negative ∆H indicates an exothermic process, while a positive ∆H indicates an endothermic process.
Fix: Practice identifying whether reactions are exothermic or endothermic based on the sign of ∆H. A negative ∆H means heat is released, and a positive ∆H means heat is absorbed.
Incorrectly applying Hess's Law.
Why it happens: Students may make arithmetic errors or misinterpret the steps when using Hess's Law to calculate enthalpy changes.
Fix: Always double-check your calculations and ensure that you are summing the enthalpy changes of intermediate steps correctly.
Failing to balance chemical equations properly.
Why it happens: Students may not balance chemical equations correctly, leading to incorrect enthalpy calculations.
Fix: Practice balancing chemical equations and ensure that the number of atoms of each element is the same on both sides of the equation before performing any calculations.
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 |
|---|---|---|
| State Enthalpy Change | Identify the standard enthalpy of combustion for methane and state it with correct units. | 2 |
| State Formation Enthalpy | Identify and state the standard enthalpy of formation for liquid water with correct units. | 2 |
| Enthalpy Of Reaction | Use Hess's law with formation enthalpies to calculate the standard reaction enthalpy. | 4 |
| Calculate Enthalpy Change | Use the given standard enthalpy of combustion to find the reaction's enthalpy change. | 2 |
| Total across these question types | 10 | |
Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.