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CAIE A-Level Chemistry 5.2.1 Hess’s Law Energy Cycles

Practise drawing a common-route energy cycle and using arrow directions to derive the required enthalpy change.

Syllabus
2028–2030
Course
Chemistry 9701
Level
AS

Exam points

  • place reactants, products and a common set of substances at the correct cycle vertices
  • label every arrow with direction, enthalpy type and stoichiometric multiplier
  • follow one complete route from reactants to products and reverse the sign of any arrow travelled backwards

5.2.1—Hess’s law to construct simple energy cycles question 1

[Maximum number: 3]

For many compounds the enthalpy change of formation cannot be calculated directly. An indirect method based on enthalpy changes of combustion can be used.

The enthalpy change of combustion can be found by a calorimetry experiment in which the heat energy given off during combustion is used to heat a known mass of water and the temperature change recorded.

The table gives some enthalpy change of combustion values.

Table for Question 5.2.1—Hess’s law to construct simple energy cycles question 1 — CAIE A-Level Chemistry AS

Construct a labelled energy cycle to show how these values could be used to calculate the enthalpy change of formation of C3H7OH(I),ΔHf\mathrm{C}_{3} \mathrm{H}_{7} \mathrm{OH}(\mathrm{I}), \Delta H_{\mathrm{f}}.

3C( s)+4H2( g)+12O2( g)→ΔHfC3H7OH(l)3 \mathrm{C}(\mathrm{~s})+4 \mathrm{H}_{2}(\mathrm{~g})+\frac{1}{2} \mathrm{O}_{2}(\mathrm{~g}) \xrightarrow{\Delta H_{\mathrm{f}}} \mathrm{C}_{3} \mathrm{H}_{7} \mathrm{OH}(\mathrm{l})
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