Equilibrium calculations: Kc and ΔG (HL)

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Products over reactants, each concentration raised to the power of its **stoichiometric coefficient**: $K_{c}=\dfrac{[\text{C}]^{c}[\text{D}]^{d}}{[\text{A}]^{a}[\text{B}]^{b}}$.

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**Pure solids (s) and pure liquids (l)** — including the solvent in dilute solution — because their concentration is effectively constant.

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K_{c} ≫ 1 → equilibrium lies to the **right**: mostly **products** at equilibrium.

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K_{c} ≪ 1 → equilibrium lies to the **left**: mostly **reactants** at equilibrium.

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**Temperature** only. Changing concentration, pressure or adding a catalyst does **not** change K_{c}.

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**I**nitial, **C**hange and **E**quilibrium amounts (or concentrations) for each species.

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They are in the **ratio of the stoichiometric coefficients** — reactants decrease (−), products increase (+).

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The **same expression as K_{c}**, but evaluated with the concentrations at **any moment**, not only at equilibrium.

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**Forward** (→), making more product, until Q rises to K_{c}.

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**Backward** (←), making more reactant, until Q falls to K_{c}.

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$\Delta G^{\circ}=-RT\ln K$ (given in the data booklet); R = 8.31 J K⁻¹ mol⁻¹, T in kelvin.

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ln K > 0, so ΔG° = −RT ln K is **negative** — the reaction is **spontaneous** and favours products.