# Further chemical energetics

> CIE A-Level Chemistry · 9701
> Source: https://www.owlsprep.com/study/cie-9701-u17-overview/
> Weight: n/a

This unit extends basic energetics concepts to advanced thermodynamics, covering ionic lattice stability, disorder in chemical systems, and predicting reaction spontaneity — core content for CIE 9701 exams.

**Prerequisites:** [Basic chemical energetics (enthalpy changes, Hess' law)](https://www.owlsprep.com/study/cie-9701-u05-basic-energetics/)

## Learning objectives

- Relate lattice enthalpy to ionic bond strength and calculate it using Born-Haber cycles
- Explain the concept of entropy and calculate standard entropy changes for chemical and physical processes
- Calculate Gibbs free energy change and use it to predict reaction spontaneity under different conditions

## Unit at a glance

This unit builds on your foundational knowledge of energetics to answer two key questions: why are some ionic compounds more stable than others, and when will a reaction happen spontaneously, regardless of enthalpy change? The learning arc progresses step-by-step: we start with lattice enthalpy for ionic solids, move to entropy (the measure of disorder in a system), and end with Gibbs free energy, the core function that lets you predict reaction spontaneity.

This unit contains three core sub-topics:
- [Lattice enthalpy](https://www.owlsprep.com/study/cie-9701-u17-lattice-enthalpy/) — Learn the definition of lattice enthalpy, how to construct Born-Haber cycles, and how to use values to compare ionic bond strength.
- [Entropy](https://www.owlsprep.com/study/cie-9701-u17-entropy/) — Understand entropy, calculate standard entropy changes, and predict how entropy changes for different physical and chemical processes.
- [Gibbs free energy](https://www.owlsprep.com/study/cie-9701-u17-gibbs-free-energy/) — Master the relationship between Gibbs free energy, enthalpy, and entropy, and use ΔG to predict reaction spontaneity.

## Common pitfalls

- **Wrong:** Confusing the sign of lattice enthalpy: assuming lattice enthalpy of formation is positive/endothermic.
  - Why it fails: Many students mix up lattice formation and lattice dissociation; breaking is endothermic, but formation of the lattice from ions is exothermic.
  - Correct: Remember: lattice enthalpy of formation is always negative (exothermic), lattice dissociation is positive.
- **Wrong:** Assuming only exothermic reactions can be spontaneous.
  - Why it fails: Enthalpy change alone does not determine spontaneity; entropy and temperature also affect whether a reaction will occur.
  - Correct: Always use the full Gibbs free energy equation ΔG = ΔH - TΔS to test for spontaneity.

## Cheatsheet

| Concept | Key Rule/Formula |
| --- | --- |
| Lattice enthalpy (Born-Haber) | ΔH_lattice = sum of all other enthalpy changes (from Hess' law) |
| Standard entropy change of reaction | ΔS° = ΣS°(products) - ΣS°(reactants) |
| Gibbs free energy | $\Delta G = \Delta H - T\Delta S$ (T in Kelvin) |
| Spontaneity rule | Reaction is spontaneous when $\Delta G < 0$ |
| Entropy trend | Entropy increases: solid → liquid → gas, with increasing temperature, and with increasing number of moles of gas |

## What's next

Start your study of this unit with the first sub-topic on lattice enthalpy, where you will learn Born-Haber cycle calculations. Once you complete all three sub-topics in this unit, you can proceed to the next unit on reaction kinetics to continue building your physical chemistry knowledge.

- [Lattice enthalpy](https://www.owlsprep.com/study/cie-9701-u17-lattice-enthalpy/)
- [Entropy](https://www.owlsprep.com/study/cie-9701-u17-entropy/)
- [Gibbs free energy](https://www.owlsprep.com/study/cie-9701-u17-gibbs-free-energy/)

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