CAIE A Level Chemistry 9701 · Chemical energetics
The standard enthalpy change of formation of liquid butan-1-ol, , is not measured directly by reacting its elements together. A student plans to determine it from the standard enthalpy changes of combustion of carbon (graphite), hydrogen and butan-1-ol, all measured at 298 K. Explain how the student could use Hess's law to do this, and explain why the method is valid.
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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