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Chemistry · Glossary

What is Reaction entropy?

Also known as: standard reaction entropy

Definition 2.5 University Chemistry — Year 2 · Chapter 2 — Entropy and Free Energy of Reaction

The reaction entropy is ΔrS=(∂S/∂ξ)T,p\Delta_r S = (\partial S/\partial\xi)_{T,p}, and the standard reaction entropy is ΔrS∘(T)=∑iνiSm,i∘(T)\Delta_r S^\circ(T) = \sum_i\nu_iS^\circ_{m,i}(T), in J/(K mol)\mathrm{J}/(\mathrm{K}\,\mathrm{mol}).

Examples

Example 2.6 (Three reaction entropies)

At 298.15 K298.15\,\mathrm{K}:

  • CaCOX3(s)→CaO(s)+COX2(g)\ce{CaCO3(s) -> CaO(s) + CO2(g)}: ΔrS∘=39.75+213.79−92.9=+160.6 J/(K mol)\Delta_r S^\circ = 39.75 + 213.79 - 92.9 = +160.6\,\mathrm{J}/(\mathrm{K}\,\mathrm{mol}), a gas made from a solid;
  • NX2(g)+3 HX2(g)→2 NHX3(g)\ce{N2(g) + 3H2(g) -> 2NH3(g)}: ΔrS∘=2(192.77)−191.61−3(130.68)=−198.1 J/(K mol)\Delta_r S^\circ = 2(192.77) - 191.61 - 3(130.68) = -198.1\,\mathrm{J}/(\mathrm{K}\,\mathrm{mol}), four molecules of gas becoming two;
  • NX2OX4(g)→2 NOX2(g)\ce{N2O4(g) -> 2NO2(g)}: ΔrS∘=2(240.03)−304.38=+175.7 J/(K mol)\Delta_r S^\circ = 2(240.03) - 304.38 = +175.7\,\mathrm{J}/(\mathrm{K}\,\mathrm{mol}).
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