Question

For the exothermic reaction
PCl3(g)+Cl2(g)⇌PCl5(g)

Kp = 0.160 at a certain temperature.
A flask is charged with 0.500 atm PCl3 , 0.500 atm Cl2, and 0.300 atm PCl5 at this temperature.

What are the equilibrium partial pressures of PCl3, Cl2, and PCl5?

How will the following changes affect the mole fraction of chlorine gas, X Cl2, in the equilibrium mixture.

X Cl2 increases when __________
X Cl2 decreases when _________
X Cl2 stays the same __________

Fill in the blanks above using:
"increase the temperature"
"decrease the temperature"
"decrease the volume of the container"
"increase the volume of the container"

Answer 1

Given that  

               PCl3(g)     +       Cl2(g) ⇌ PCl5(g)         Kp = 0.16

Initial                  0.5 atm              0.5 atm           0.3 atm

At equilibrium       0.5-x                   0.5-x              0.3+x

                   Kp = (0.3+x)/ [ (0.5-x) (0.5-x)]

               0.16 = (0.3+x)/ (0.5-x)²

            On solving,

              x = -0.217 atm

Therefore,

Equilibrium partial pressures are

PCl3 = 0.5-x = 0.5 - (-0.217) = 0.717 atm

Cl2 = 0.5-x = 0.5 - (-0.217) = 0.717 atm

PCl5 = 0.3+x = 0.3-0.217 = 0.083 atm

--------------------------------------------------------------

Given reaction reaction exothermic.

All exothermic reactions are favoured by decrease in temperature i.e. forward reaction reaction is favourable ( Equilibrium shifts to right)

Hence,

X Cl2 increases when     increase the temperature

X Cl2 decreases when    decrease the temperature

X Cl2 stays the same   increase the volume of the container