# Using the concept of an enthalpy cycle, explain why the standard enthalpy of formation of sodium chloride (NaCl) cannot be determined directly from its elements and how Hess's law allows this value to be obtained indirectly.

> IB DP Chemistry Standard Level (2023 syllabus) — R1.2 Energy cycles in reactions · Explain · 4 marks

> The standard enthalpy of formation of an ionic compound such as sodium chloride cannot be measured directly in a laboratory because the reaction of sodium metal with chlorine gas does not proceed in a single, controllable step.

## Mark scheme (4 marks)

1. The direct combination of sodium and chlorine does not occur in a single measurable step / the reaction cannot be carried out under standard conditions in a controlled way, so a direct calorimetric measurement is not possible.
2. Hess's law states that the total enthalpy change for a reaction is independent of the route taken, provided the initial and final states are the same.
3. An enthalpy cycle (Born–Haber cycle) is constructed linking the elements to NaCl via a series of measurable steps such as atomisation, ionisation, electron affinity, and lattice enthalpy.
4. The standard enthalpy of formation is then calculated by summing the enthalpy changes of the alternative route / applying Hess's law algebraically so that ΔHf = sum of the individual step enthalpies around the cycle.

## Key terms

- [enthalpy cycle](https://www.gradenine.co.uk/glossary/enthalpy-cycle)
- [Hess's law](https://www.gradenine.co.uk/glossary/hess-s-law)
- [standard enthalpy of formation](https://www.gradenine.co.uk/glossary/standard-enthalpy-of-formation)

## Related

- [Revision notes for IB DP Chemistry Standard Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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