Entropy and ΔS (HL)

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A measure of how **dispersed** (spread out) the matter and energy of a system are — more ways to arrange the particles and energy means higher entropy.

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When matter/energy become more dispersed: solid → liquid → gas, dissolving, and especially when the **moles of gas increase**.

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When the system becomes more ordered: gas → liquid → solid, or when the **moles of gas decrease**.

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The change in the **number of moles of gas** — gases have far higher entropy than liquids or solids.

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The entropy of **one mole** of a substance under standard conditions; units **J K⁻¹ mol⁻¹**.

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**J K⁻¹ mol⁻¹** — note joules, not kilojoules (unlike ΔH).

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**gas > liquid > solid** — a gas has the most dispersed matter and energy.

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**S = 0** — a single perfectly ordered arrangement with no thermal motion (the only zero-entropy state).

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$\Delta S^{\circ} = \sum S^{\circ}(\text{products}) - \sum S^{\circ}(\text{reactants})$, each S° × its coefficient.

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Ignoring the **stoichiometric coefficients**, and mixing **J** (entropy) with **kJ** (enthalpy).

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**Always positive** for any substance above 0 K (entropy is a measure of dispersal, never negative).

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Gas particles can occupy far more positions and share energy over more ways of moving, so there are many more arrangements → higher entropy.