Question

a. Calculate Kp for the decomposition of ammonia at 25c and 1.000 bar: 2NH3 = N2 + 3H2

b. What is Kp at 525c?

c. Calculate the degree of dissociation (alpha) of NH3 at 25c (alpha is defined as a fraction of NH3 molecules that have dissociated).

Answer 1

2 NH3 ---------> N2 + 3H2

a)

delta Gorxn = delta Gof products - delta Go reactants

                    = 0 + 0 - ( 2 x - 16.6)

                    = 33.2 kJ/mol

delta Go = - RT ln Kp

33.2 = - 8.312 x 10^-3 x 298 x ln Kp

Kp = 1.52 x 10^-6

b)

temperature = 525 oC = 798.15 K

delta Go = - RT ln Kp

33.2 = - 8.312 x 10^-3 x 798 x ln Kp

Kp = 6.71 x 10^-3

c)

2 NH3 ---------> N2   + 3 H2

1.00                     0         0

1 - 2x                    x           3x

Kp = x (3x)^3 / (1-2x)^2

1.52 x 10^-6 = 27x^4 / (1-2x)^2

x = 0.0154

degree of dissociation = 0.0154 / 1 x 100

                                   = 1.54 %