Question

The equilibrium constant, K , of a reaction at a particular temperature is determined by the concentrations or pressures of the reactants and products at equilibrium.

For a gaseous reaction with the general form

aA+bB⇌cC+dD

the Kc and Kp expressions are given by

Kc=[C]c[D]d[A]a[B]b

Kp=(PC)c(PD)d(PA)a(PB)b

The subscript c or p indicates whether K is expressed in terms of concentrations or pressures. Equilibrium-constant expressions do not include a term for any pure solids or liquids that may be involved in the reaction.

Part A

Phosgene (carbonyl chloride), COCl2 , is an extremely toxic gas that is used in manufacturing certain dyes and plastics. Phosgene can be produced by reacting carbon monoxide and chlorine gas at high temperatures:

CO(g)+Cl2(g)⇌COCl2(g)

Carbon monoxide and chlorine gas are allowed to react in a sealed vessel at 450 ∘C . At equilibrium, the concentrations were measured and the following results obtained:

Gas	Partial Pressure (atm )
CO	0.890
Cl2	1.18
COCl2	0.140

What is the equilibrium constant, Kp , of this reaction?

Express your answer numerically.

Hints

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Deriving concentrations from data

In Part A, you were given the equilibrium pressures, which could be plugged directly into the formula for K . In Part B however, you will be given initial concentrations and only one equilibrium concentration. You must use this data to find all three equilibrium concentrations before you can apply the formula for K .

Part B

The following reaction was performed in a sealed vessel at 724 ∘C :

H2(g)+I2(g)⇌2HI(g)

Initially, only H2 and I2 were present at concentrations of [H2]=3.05M and [I2]=2.85M . The equilibrium concentration of I2 is 0.0400 M . What is the equilibrium constant, Kc , for the reaction at this temperature?

Express your answer numerically.

Hints

Kp =

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Answer 1

Part A)

for,

CO(g) + Cl2(g) --> COCl2(g)

at equilibrium,

[CO] = 0.890 atm

[Cl2] = 1.18 atm

[COCl2] = 0.140 atm

So,

Equilibrium constant Kp = [COCl2]/[CO][Cl2]

                                       = (0.140)/(0.890 x 1.18) = 0.133

Part B)

For,

H2(g) + I2(g) --> 2HI(g)

at equilibrium [I2] = 0.04 M

change in concentration of I2 = 2.85 - 0.04 = 2.81 M

So,

equilibrium [H2] = 3.05 - 2.81 = 0.24 M

equilibrium [HI] = 2 x 2.81 = 5.62 M

Equilibrium constant Kc = [HI]^2/[H2][I2]

                                       = (5.62)^2/(0.24 x 0.04) = 3290.042