A-Level · Chemistry · AQA · Mark scheme decoded
AQA A-Level Chemistry: Hess's Law and Enthalpy Changes — mark scheme explained
The short answer
Hess’s law is a fundamental principle in physical chemistry that allows us to calculate the enthalpy change of a reaction by using known enthalpy changes of other reactions.
The question
Calculate the enthalpy change for the thermal decomposition of NaHCO 3 using the following standard enthalpies of formation: ΔH f ° (NaHCO 3 ) = -950.8 kJ/mol, ΔH f ° (Na 2 CO 3 ) = -1130.9 kJ/mol, ΔH f ° (H 2 O) = -285.8 kJ/mol, and ΔH f ° (CO 2 ) = -393.5 kJ/mol.
[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.
- Analyze
- Examine and interpret data or graphs to draw conclusions about the behavior of the system.
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.
- Write the balanced equation for the thermal decomposition of NaHCO 3 : 2 NaHCO 3(s) → Na 2 CO 3(s) + H 2 O (l) + CO 2(g)1 mark
- Use the formula for enthalpy change: ΔH reaction = Σ ΔH f ° (products) - Σ ΔH f ° (reactants)1 mark
- Substitute the values: ΔH reaction = [1 × (-1130.9 kJ/mol) + 1 × (-285.8 kJ/mol) + 1 × (-393.5 kJ/mol)] - [2 × (-950.8 kJ/mol)]2 marks
- Calculate the enthalpy change: ΔH reaction = [-1130.9 - 285.8 - 393.5] - [-1901.6] = -1810.2 + 1901.6 = 91.4 kJ/mol1 mark
Final answer: The enthalpy change for the thermal decomposition of NaHCO 3 is 91.4 kJ/mol.
Work through every step correctly and you earn all 5 marks.
Another worked example
Calculate the enthalpy change for the hydration of MgSO 4 using the following standard enthalpies of formation: ΔH f ° (MgSO 4 ) = -1278.5 kJ/mol, ΔH f ° (H 2 O) = -285.8 kJ/mol, and ΔH f ° (MgSO 4 ·7H 2 O) = -3106.1 kJ/mol.
- Write the balanced equation for the hydration of MgSO 4 : MgSO 4(s) + 7 H 2 O (l) → MgSO 4 ·7H 2 O (s)0 marks
- Use the formula for enthalpy change: ΔH reaction = Σ ΔH f ° (products) - Σ ΔH f ° (reactants)1 mark
- Substitute the values: ΔH reaction = [1 × (-3106.1 kJ/mol)] - [1 × (-1278.5 kJ/mol) + 7 × (-285.8 kJ/mol)]2 marks
- Calculate the enthalpy change: ΔH reaction = -3106.1 - [-1278.5 - 2000.6] = -3106.1 + 3279.1 = 173 kJ/mol2 marks
Final answer: The enthalpy change for the hydration of MgSO 4 is 173 kJ/mol.
Work through every step correctly and you earn all 5 marks.
Common mistakes
Using the wrong units for enthalpies of formation or combustion.
Why it happens: Students often forget to use consistent units (kJ/mol) when substituting values into calculations.
Fix: Always ensure that all enthalpy values are in kJ/mol before performing any calculations.
Forgetting to reverse the sign of ΔH f ° for reactants when using the formula.
Why it happens: Students may overlook the need to change the sign of the enthalpy of formation for reactants in the calculation.
Fix: Always use the negative sign for the enthalpy of formation of reactants when applying the formula ΔH reaction = Σ ΔH f ° (products) - Σ ΔH f ° (reactants).
Incorrectly balancing the chemical equation.
Why it happens: Students may not balance the equation correctly, leading to incorrect calculations of enthalpy changes.
Fix: Always double-check that the chemical equation is balanced before substituting values into the formula.
Using the wrong formula for calculating enthalpy change.
Why it happens: Students may confuse the formula for enthalpy change with other thermodynamic equations, leading to incorrect answers.
Fix: Always use the correct formula: ΔH reaction = Σ ΔH f ° (products) - Σ ΔH f ° (reactants).
Failing to account for stoichiometric coefficients in the calculation.
Why it happens: Students may overlook the need to multiply enthalpy values by the appropriate stoichiometric coefficients.
Fix: Always include the stoichiometric coefficients when substituting values into the formula. For example, if 2 moles of a substance are involved, use 2 × ΔH f ° .
Misinterpreting the sign of the enthalpy change.
Why it happens: Students may not understand the significance of a positive or negative enthalpy change, leading to incorrect conclusions about whether a reaction is exothermic or endothermic.
Fix: Remember that a positive ΔH reaction indicates an endothermic reaction (heat absorbed), while a negative ΔH reaction indicates an exothermic reaction (heat released).
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 Enthalpy Change | Use standard enthalpies of formation to find the reaction enthalpy via Hess's law. | 5 |
| Calculate Enthalpy Change | Use Hess's law and formation enthalpies to find the enthalpy of hydration. | 5 |
| Hess's Law Cycle | Use enthalpies of formation to calculate the enthalpy change for hydrating anhydrous copper sulfate. | 5 |
| Calculate Enthalpy Change | Use standard enthalpies of formation to calculate the enthalpy change of combustion. | 5 |
| Total across these question types | 20 | |
Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.