HOW TO FIND EQUILIBRIUM KC AND KP

Question

2.     Consider the following heterogeneous equilibrium.

       CaCO₃ (s)                  CaO (s) + CO₂ (g)

         

         At 800  the pressure of CO₂ in 0.23 atm, calculate (a) K_p and (b) K_c for the reaction at this temperature.

                                     K_p     = PCO₂

                                            = 0.236

1.  Consider the following equilibrium at 295K

NH₄HS (s)                   NH₃ (g) + H₂S (g)

         The partial pressure of each gas is 0.265 atm. Calculate K_p and K_c for         the reaction.

2.  2A(g) +  B(g)                C (g)

When 1.0 mol A and 0.75 mol B were allowed to achieve equilibrium at 65 C. In a container of 1 dm³ capacity, the equilibrium mixture was found to contain 0.7mol A and 0.6 mol B. Calculate K_c at 65 C.

3.  Write the expression for K_p for the following reaction.

N₂(g)   +   3H₂           2NH₃ (g)       H = -184 kJmol^-1

State and explain the effect (if any) of the following changes on the K_p and the amount of ammonia in the equilibrium mixture.

(a) Temperature is decreased

(b) Increasing pressure

(c) Addition of iron catalyst

Answer 1

2) Use this equation:

K_p=K_c(RT)^Δn

R=0.0821

T=temp in kelvin

Δn=moles gaseous product - moles gaseous reactant

1) NH₄HS (s) ↔NH₃ (g) + H₂S (g)

K=(NH₃)(H₂S)

For K_P, substitute atm:

K_P=(0.265)(0.265)=0.0702

Now, convert Kp to Kc:

K_p=K_c(RT)^Δn

2)

2A(g) +  B(g) ↔C (g)

1............0.75.....0

-2x.........-x......+x

0.7........0.6........0.15

notice that x has to be 0.15 so that 1-2x=0.7 and 0.75-x=0.6.

Kc=[C]÷( [A]²[B] )

Kc=[0.15]÷(0.7²×0.6)

Kc=0.510

3) delta H is negative so it's exothermic and heat is a product

N₂(g)+3H₂(g)↔2NH₃(g)+heat

(a) shifts right to increase temp, [Products] increases, KP increases and amount of ammonia (NH3) increases

(b) shifts right to reduce pressure towards ammonia, Kp unchanged because K only changes with temperature, amount of NH3 increases

(c) No effect on K, no effect on NH3. Catalysts do not cause a shift or affect the equilibrium.