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CAIE A Level Chemistry 9701 · Chemical energetics

Chemical energetics: explain question

Explain4 marksStandard

Hydrogen burns in oxygen to form water as shown.

2 HX2(g)+OX2(g)→2 HX2O(l)\ce{2H2(g) + O2(g) -> 2H2O(l)}

The bond energies are:

  • H–H: 436 kJ mol−1^{-1}
  • O=O: 498 kJ mol−1^{-1}
  • O–H: 463 kJ mol−1^{-1}

Explain why this reaction is exothermic. In your answer, refer to bond breaking and bond making, and describe how the activation energy and enthalpy change appear on a reaction pathway diagram.

0 words · 4 marks

No account needed for your first marks. Your answer is used only to mark it.

How this question is marked

4 marks, so the mark scheme is looking for about 4 separate creditworthy points. Making the same point twice in different words earns it once.

Explain means: Give the reasons or the mechanism: why or how something happens. Link each point to the next with because, so or which means. A list of facts with no links usually loses the reasoning marks.

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