Practice Problem 6

Calculate the value of Kp for the following reaction at 500oC if the value of Kc for the reaction at this temperature is 0.040.

N2(g) + 3 H2(g) 2 NH3(g)


The value of Kp for a reaction can be calculated from Kc with the following equation.

Kp = Kc x (RT)n

In order to use this equation, we need to know the value of n for the reaction. The balanced equation for this reaction generates two moles of products for every four moles of reactants consumed. The value of n for this reaction is therefore 2 - 4, or -2.

Kp = Kc x (RT)-2

We now use the known value of the ideal gas constant and the temperature in Kelvin to calculate the value of Kp for the reaction at this temperature.

Kp = 0.040 x [(0.08206 L-atm/mol-K)(773 K)]-2 = 1.0 x 10-5