Phosphorus pentachloride, PCl₅, is used to convert alcohols (such as CH₃CH₂OH) to alkyl chlorides (such as CH₃CH₂Cl). If you were an industrial chemist, you might be asked to prepare some PCl₅ by the reaction of PCl₃ and Cl₂. At elevated temperatures, however, PCl₅ decomposes to these starting materials, so it will be important to estimate the equilibrium composition of the reaction mixture. Suppose that you place 3.12 g of PCl₅ in a reaction vessel of volume 500. mL and allow the sample to reach equilibrium with its decomposition products PCl₃ and Cl₂ at 250°C, when K = 78.3 for the reaction PCl₅ (g) ⇌ PCl₃ (g) + Cl₂(g). Find the composition of the equilibrium mixture.

ANTICIPATE Because the equilibrium constant is neither very small nor very large, you should expect that the partial pressures of reactants and products at equilibrium will be similar to each other.

PLAN Use the general procedure set out in Toolbox 5I.1. Because data are in liters and pressures are expected in bar, use the value of R that matches these units (namely, L · bar · K^–1 · mol^–1).

Transcribed Image Text:

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