Question

A flask is filled with PCl5 to a pressure of 2.00 atm at 300°C and allowed to come to equilibrium according to the reaction: PCl5(g) ⇄ PCl3(g) + Cl2(g) Analysis shows the total pressure in the flask at equilibrium is 3.96 atm. Calculate the equilibrium constant Kp for the reaction.

A)49.0

B)3.96

C)96.0

D)0.886

E)7.84

Answer 1

First, let us define the equilibrium constant for any species:

The equilibrium constant will relate product and reactants distribution. It is similar to a ratio

The equilibrium is given by

rReactants -> pProducts

Keq = [products]^p / [reactants]^r

For a specific case:

aA + bB = cC + dD

Keq = [C]^c * [D]^d / ([A]^a * [B]^b)

Same is true for pressure

Kp = P-product s/ P-reactant

get Kp

Kp = P-PCl3 * P-Cl2 / P-PCl5

initially

P-PCl3 = 0

P-Cl2= 0

P-PCl5 = 2

in equilbirum

P-PCl3 = 0 +x

P-Cl2= 0 +x

P-PCl5 = 2 -x

we know

Ptotal = P(PCl3 +Cl2+PCl5) = 3.96

that is

Ptotal = x+ x+ +2-x = 2+x

get x

2+x = 3.96

x = 3.96-2 = 1.96

substitue

P-PCl3 = 0 +1.96

P-Cl2= 0 +1.96

P-PCl5 = 2 -1.96= 0.04

substitute in Kp

Kp = P-PCl3 * P-Cl2 / P-PCl5

Kp = 1.96*1.96/0.04

Kp = 96

choose C

from the