What is Reaction quotient? — Chemistry

Definition 43.5 School Chemistry — Grades 1 to 12 · Chapter 43 — Chemical Equilibrium: Quotient and Constant

For a reaction in solution $a\,\ce{A} + b\,\ce{B} \rightleftharpoons c\,\ce{C} + d\,\ce{D}$ , the reaction quotient in a given state of the system is

Q = \frac{\left([\ce{C}]/c^\circ\right)^c \left([\ce{D}]/c^\circ\right)^d} {\left([\ce{A}]/c^\circ\right)^a \left([\ce{B}]/c^\circ\right)^b} .

Solids, and the solvent (water in an aqueous solution), do not appear in $Q$ .

Examples

Example 43.6 (Writing quotients)

For $\ce{CH3COOH + H2O <=> CH3COO- + H3O+}$ in water (the ion $\ce{H3O+}$ is introduced in the next chapter), water is the solvent: $Q = \dfrac{[\ce{CH3COO-}]\,[\ce{H3O+}]}{[\ce{CH3COOH}]\,c^\circ}$ . For the dissolving of a solid, $\ce{AgCl(s) <=> Ag+ + Cl-}$ , the solid is left out: $Q = [\ce{Ag+}][\ce{Cl-}]/(c^\circ)^2$ . For the esterification, where all four species are in the same liquid mixture of volume $V$ , the volumes cancel and $Q = \dfrac{n_{\text{ester}}\, n_{\text{water}}}{n_{\text{acid}}\, n_{\text{alcohol}}}$ .

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Definition 7.13 University Chemistry — Year 1 · Chapter 7 — Describing a Chemical System: Extent, Activities, Q and K

The reaction quotient of a reaction $0 = \sum_i \nu_i\,\mathrm{A}_i$ in a given state of the system is

Q = \prod_i a_i^{\nu_i} ,

the product of the activities of the products divided by that of the reactants, each raised to the power of its stoichiometric coefficient.

Examples

Example 7.14 (Three quotients)

For $\ce{N2(g) + 3H2(g) <=> 2NH3(g)}$ : $Q = \dfrac{(p_{\ce{NH3}}/p^\circ)^2} {(p_{\ce{N2}}/p^\circ)(p_{\ce{H2}}/p^\circ)^3}$ . For $\ce{CH3COOH(aq) + H2O(l) <=> CH3COO^-(aq) + H3O+(aq)}$ : $Q = \dfrac{[\ce{CH3COO-}][\ce{H3O+}]}{[\ce{CH3COOH}]\,c^\circ}$ , the solvent having activity 1. For $\ce{CaCO3(s) <=> CaO(s) + CO2(g)}$ : $Q = p_{\ce{CO2}}/p^\circ$ , the solids having activity 1.

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Examples

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