Learning objective
Use Hess's law to calculate reaction enthalpy changes.
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Topic
Energetics
Subtopic
Applications of Hess's law
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Short explanation
Use Hess's law to calculate reaction enthalpy changes. sits within Applications of Hess's law for Energetics in AQA A-Level Chemistry 7405. A strong exam response should identify the relevant chemical relationship, choose the correct formula or representation where needed, substitute values or evidence carefully, and state the final conclusion with appropriate units or terminology. When revising this objective, focus on the distinction between the concept being tested and neighbouring ideas so that calculation steps, organic representations, or equilibrium reasoning do not collapse into a generic answer.
Key concepts
Why it matters
This objective helps connect Applications of Hess's law to exam-style questions, flashcards, and revision notes for Energetics.
Common mistakes
1 linked- Common Mistake in Hess's Law Calculations: To fix this, remember that exothermic reactions have negative enthalpy changes and endothermic reactions have positive enthalpy changes. Use the formula ΔH = Σ(ΔH_f products) - Σ(ΔH_f reactants) and ensure you apply the correct signs during substitution. For example, if you have ΔH_f for products as -100 kJ/mol and for reactants as -50 kJ/mol, the calculation would be: ΔH = (-100) - (-50) = -100 + 50 = -50 kJ/mol. Thus, the final answer is -50 kJ/mol, indicating an exothermic reaction.
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Open revision notesRelated learning objectives
- Define enthalpy change under constant pressure.
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- Distinguish exothermic and endothermic reactions using enthalpy change signs.
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- Interpret reaction profile diagrams.
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- Define standard enthalpy changes for formation, combustion and neutralisation where appropriate.
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- Use q = mcΔT to calculate heat energy transferred.
Calorimetry and enthalpy measurements