AQA A-Level Chemistry: Enthalpy Changes and Standard Enthalpies — mark scheme explained

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

What each mark is really for — in plain English — and the wording trap that loses it.

• S1

  Step 1: Identify the given values.

• S2

  ∆H c Ɵ for CH 4(g) = -890 kJ mol -1

• S3

  Step 2: Write the balanced equation for the combustion of methane.

• S4

  CH 4(g) + 2O 2(g) → CO 2(g) + 2H 2 O (l)

• S5

  Step 3: The standard enthalpy change for the reaction is given by ∆H c Ɵ .

• S6

  ∆H = -890 kJ mol -1

Model answer

Worked through, with each step tagged to the mark it earns.

1. S1

   Step 1: Identify the given values.

2. S2

   ∆H c Ɵ for CH 4(g) = -890 kJ mol -1

3. S3

   Step 2: Write the balanced equation for the combustion of methane.

4. S4

   CH 4(g) + 2O 2(g) → CO 2(g) + 2H 2 O (l)

5. S5

   Step 3: The standard enthalpy change for the reaction is given by ∆H c Ɵ .

6. S6

   ∆H = -890 kJ mol -1

7. Final answer: -890 kJ mol -1

Common mistakes

• Using the wrong units for enthalpy. — Always ensure that enthalpy values are in kJ mol -1 when substituting into calculations.
• Forgetting to use standard conditions (298 K, 100 kPa). — 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 . — 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. — 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. — Always double-check your calculations and ensure that you are summing the enthalpy changes of intermediate steps correctly.
• Failing to balance chemical equations properly. — 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

• Full worked solution (all marking points)2 marks

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