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Cambridge AS & A Level Chemistry

5.2Hess's law

Hess's law in Cambridge AS & A Level Chemistry: what the syllabus expects you to know, the learning objectives and how to practise.
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Learning objectives

What the syllabus expects you to be able to do.

  1. 5.2.1Apply Hess's law to construct simple energy cycles.
  2. 5.2.2Calculate enthalpy changes using cycles, including ones that cannot be measured directly and ones that use bond energies.

Lessons

  • Hess's law and bond energy calculations

    45 min

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Sample question

A sample question on this topic. Answer it, check it, then read the worked answer.

Hess's law: sample question

1 mark

Data: ΔH⦵f FeX2OX3(s)\ce{Fe2O3(s)} = −824.2 kJ mol⁻¹; ΔH⦵f AlX2OX3(s)\ce{Al2O3(s)} = −1675.7 kJ mol⁻¹. What is ΔH for FeX2OX3(s)+2 Al(s)→AlX2OX3(s)+2 Fe(s)\ce{Fe2O3(s) + 2Al(s) -> Al2O3(s) + 2Fe(s)}?

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