Suppose you are designing a chemical plant that is providing phosphorus compounds to other industries and you need to explore the equilibrium properties for the reaction of PCl₅(g) ⇌ PCl₃(g) + Cl₂(g). The reaction has reached equilibrium at 250 °C (the equilibrium partial pressures of the components are PPCl₅ = 0.02 bar, PPCl₃ = 1.28 bar, PCl₂ = 1.28 bar, and K = 78.3). You add 0.0100 mol Cl₂(g) to the equilibrium mixture in the container (of volume 500. mL); then the system is once again allowed to reach equilibrium. Use this information to calculate the new composition of the equilibrium mixture.

ANTICIPATE Because a product has been added, you should expect the reaction to respond by forming reactants at the expense of products.

PLAN The general procedure is like that set out in Toolbox 5I.1, except that the direction of reaction might not be toward products. Write the expression for the equilibrium constant, and then set up an equilibrium table. In this case, use as the initial partial pressures those found immediately after addition of the reagent but before the reaction has responded.

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Toolbox 51.1 HOW TO SET UP AND USE AN EQUILIBRIUM TABLE CONCEPTUAL BASIS The composition of a reaction mixture tends to adjust until the molar concentrations of solutes or partial pressures of gases ensure that Q = K. A change in the abundance of any one component is linked to changes in the others by the reaction stoichiometry. PROCEDURE Step 1 Write the equilibrium expression for the balanced chemical equation. Then set up an equilibrium table as shown here, with columns labeled by the species taking part in the reaction. If K, is given for a reaction involving gases (Topic 5H), then proceed in the same way, but with molar concentrations for all the species. The first line in the table shows how the reaction system was pre- pared before reaction began. As always, use molar concentrations (in moles per liter) and partial pressures (in bar), but strike out the units. Omit pure solids and liquids, as they do not appear in the equilibrium expression. initial composition change in composition equilibrium composition Species 1 Species 2 Species 3