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
What each mark is really for — in plain English — and the wording trap that loses it.
- S1
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)
- S2
Use the formula for enthalpy change: ΔH reaction = Σ ΔH f ° (products) - Σ ΔH f ° (reactants)
- S3
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)]
- S4
Calculate the enthalpy change: ΔH reaction = [-1130.9 - 285.8 - 393.5] - [-1901.6] = -1810.2 + 1901.6 = 91.4 kJ/mol
Model answer
Worked through, with each step tagged to the mark it earns.
- S1
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)
- S2
Use the formula for enthalpy change: ΔH reaction = Σ ΔH f ° (products) - Σ ΔH f ° (reactants)
- S3
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)]
- S4
Calculate the enthalpy change: ΔH reaction = [-1130.9 - 285.8 - 393.5] - [-1901.6] = -1810.2 + 1901.6 = 91.4 kJ/mol
Final answer: The enthalpy change for the thermal decomposition of NaHCO 3 is 91.4 kJ/mol.
Common mistakes
- Using the wrong units for enthalpies of formation or combustion. — 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. — 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. — Always double-check that the chemical equation is balanced before substituting values into the formula.
- Using the wrong formula for calculating enthalpy change. — Always use the correct formula: ΔH reaction = Σ ΔH f ° (products) - Σ ΔH f ° (reactants).
- Failing to account for stoichiometric coefficients in the calculation. — 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. — 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
- Full worked solution (all marking points)5 marks