# Explain why the standard entropy change for the formation of sodium chloride from its elements in their standard states is positive, and describe how entropy change and enthalpy change together determine whether a reaction is feasible.

> Edexcel A-Level Chemistry (9CH0) — 13.1 Born-Haber cycles and entropy · Explain · 5 marks

> Sodium chloride is formed from sodium metal and chlorine gas. The overall reaction is highly exothermic. Chemists use both enthalpy change and entropy change to predict whether a reaction will occur spontaneously.

## Mark scheme (5 marks)

1. Entropy is a measure of the disorder (or number of ways of arranging particles) in a system
2. Chlorine gas has higher entropy than the solid sodium or solid sodium chloride because gas particles are more disordered / have more possible arrangements than solids
3. The overall entropy change is positive because a mole of gas (chlorine) is consumed but the increase in disorder from other factors / the gaseous reactant disappearing is offset — accept: one mole of gas is used so entropy of the system decreases, therefore the positive entropy change must come from the surroundings
4. A reaction is feasible when the total entropy change (ΔS_total) is positive / greater than zero
5. ΔS_total = ΔS_system + ΔS_surroundings, where ΔS_surroundings = −ΔH/T, so a large negative enthalpy change (exothermic) increases ΔS_surroundings and can make the reaction feasible even if ΔS_system is negative

## Key terms

- [entropy](https://www.gradenine.co.uk/glossary/entropy)

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