Unit Overview
Further chemical energetics
CIE A-Level Chemistry· 5 min read 📊 n/a
1. 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
Learn the definition of lattice enthalpy, how to construct Born-Haber cycles, and how to use values to compare ionic bond strength.
★★★⏱ 12 min
Entropy
Understand entropy, calculate standard entropy changes, and predict how entropy changes for different physical and chemical processes.
★★⏱ 8 min
Gibbs free energy
Master the relationship between Gibbs free energy, enthalpy, and entropy, and use ΔG to predict reaction spontaneity.
★★★⏱ 10 min
2. Common Pitfalls
Wrong move:
Confusing the sign of lattice enthalpy: assuming lattice enthalpy of formation is positive/endothermic.
Why:
Many students mix up lattice formation and lattice dissociation; breaking is endothermic, but formation of the lattice from ions is exothermic.
Correct move:
Remember: lattice enthalpy of formation is always negative (exothermic), lattice dissociation is positive.
Wrong move:
Assuming only exothermic reactions can be spontaneous.
Why:
Enthalpy change alone does not determine spontaneity; entropy and temperature also affect whether a reaction will occur.
Correct move:
Always use the full Gibbs free energy equation ΔG = ΔH - TΔS to test for spontaneity.
3. Quick Reference 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 | (T in Kelvin) |
Spontaneity rule | Reaction is spontaneous when |
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.