Question

Phosphorus pentachloride decomposes according to the chemical equation PCl5(g) - PCl3(g) + Cl2(g) A 0.213 mol sample of PCl5(g) is injected into an empty 3.40 L reaction vessel held at 250 degrees C. Calculate the concentrations of PCl5(g) and PCl3(g) at equilibrium.

Answer 1

PCl5(g)   <-----------------> PCl3(g) + Cl2(g)

ΔG^o_rxn = ΔG^o_products - ΔG^o_reactants

               = (- 267.8 + 0) - (-305.0)

           = 37.2 kJ/mol

ΔG^o = - R T ln K

37.2 = - 8.314x 10^-3 x 523 x ln K

K = 1.926 x 10^-4

concentration of PCl5 = 0.213 / 3.40 = 0.0626

PCl5(g) -----------------> PCl3(g) + Cl2(g)

0.0626                                0            0

0.0626 - x                            x             x

K = x^2 / 0.0626 - x

1.926 x 10^-4 = x^2 / 0.0626 - x

x = 0.00337

equilibrium concentrations :

[PCl5] = 0.0626 - x = 0.0592 M

[PCl3] = x = 0.00337 M