Practice Problem 6

Calculate the value of K_p for the following reaction at 500^oC if the value of K_c for the reaction at this temperature is 0.040.

N₂(g) + 3 H₂(g) 2 NH₃(g)

Solution

The value of K_p for a reaction can be calculated from K_c with the following equation.

K_p = K_c 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.

K_p = K_c x (RT)^-2

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

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