Chemistry A2 · Chemical energetics. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
Entropy, S, is the number of possible arrangements of the particles and their energy in a system. The more ways the particles and the energy can be spread out, the higher the entropy.
Gases have much higher entropy than liquids, and liquids have higher entropy than solids. So entropy increases on melting, boiling and dissolving a solid, and when a substance is heated. It decreases on condensing and freezing.
In a reaction, look at the gases first. If the number of moles of gas increases, ΔS is positive. If it decreases, ΔS is negative. The exact value comes from standard entropies.
Students leave out the entropy of an element because they think it is zero, or forget to multiply by the balancing numbers. Every substance in the equation has an S⦵ value and must be counted.
Answer: ΔS⦵ = −198.8 J K−1 mol−1
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