The reversible reaction PCl₅(g) ⇌ PCl₃(g) + Cl₂(g) (at ) is carried out in a rigid…

CHEMISTRY · Multi-part · A Level · Chemistry · Chemical equilibria (Le Chatelier’s principle and Kc) · Chemical equilibria (Le Chatelier’s principle and Kc)

Problem

The reversible reaction PCl₅(g) ⇌ PCl₃(g) + Cl₂(g)

K_c = 1.80

(at

523\,\text{K}

) is carried out in a rigid reaction vessel at

523\,\text{K}

. The mixture is allowed to reach equilibrium, after which a portion of Cl₂ is removed rapidly at

t = 1\,\text{min}

. A record of the concentrations following the disturbance is given below. Concentration Data (all units =

\text{mol L}^{-1}

)

Time (min)	[PCl₅]	[PCl₃]	[Cl₂]
0 (just before Cl₂ removal)	0.200	0.300	1.20
1 (immediately after Cl₂ removal)	0.200	0.300	0.60
5	0.180	0.320	0.68
10	0.160	0.340	0.76
15	0.150	0.350	0.80

(1) Part 1: (a) Calculate the value of the reaction quotient

Q_c

t = 5\,\text{min}

. Indicate whether the net reaction is proceeding in the forward or reverse direction during the interval just after

t = 5\,\text{min}

. (2) Part 2: (b) Repeat the

Q_c

calculation for

t = 10\,\text{min}

. Compare the value of

Q_c

10\,\text{min}

with that at

5\,\text{min}

and with

K_c

, and state whether the system is closer to or farther from equilibrium at

10\,\text{min}

. (3) Part 3: (c) Using the concentration data provided, determine the earliest time after the disturbance at which the system can be considered to have re-established equilibrium. Justify your answer quantitatively.

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