How to Calculate Kc

Kc is the equilibrium constant of a chemical reaction. The letter c implies that reagent amounts are expressed as molar concentration. For the reaction A+B=AB, the equilibrium constant Kc is defined as [AB]/[A][B]. Brackets denote reagent concentrations that must be given in order to compute Kc. As an example, we will calculate Kc for two reactions. The first is the oxidation reaction between carbon monoxide (CO) and nitrogen (II) oxide (NO), and the second is the thermal decomposition of the baking soda.

    Write down the first chemical reaction equation. It leads to carbon dioxide (CO2) and nitrogen (I) oxide (N2O) and can be written as CO+2NO=CO2+N2O. Note this is a homogeneous equilibrium, i.e. all components are gases. Suppose that concentrations of those compounds are given as 2, 0.5, 1.2 and 3 mole/L for CO, 2NO, CO2 and N2O, respectively.

    Multiply the concentrations of the initial reagents (CO and NO). Note the rule is if there is a coefficient before the compound in the chemical reaction equation than its concentration has to be raised in the power of this coefficient. There is a coefficient 2 before NO in the chemical equation, hence [CO] x [NO]^2=2 mole/L x (0.5 mole/L)^2=1 mole^3/L^3.

    Multiply the concentrations of the final reagents (CO2 and N2O). [CO2] x [N2O]=1.2 mole/L x 3 mole/L=3.6 mole^2/L^2.

    Divide the number obtained in Step 3 by the number from Step 2 to calculate Kc. Kc=([CO2] x [N2O])/ ([CO] x [NO]^2)=(3.6 mole^2/L^2)/(1 mole^3/L^3)=3.6 mole^-1/L-1.

    Write down the second chemical equation for the baking soda (NaHCO3) decomposition that occurs at 200 to 300 degrees Celsius. 2NaHCO3 =Na2CO3 + CO2 +H2O. Note this is a heterogeneous equilibrium. Carbon dioxide and water steam are gases while the other two compounds are solid. Suppose that concentrations of these two gases are 1.8 and 1.5 mole/L.

    Multiply concentrations of CO2 and H2O to get Kc. An important rule is that all components which are in the solid state are not included in the equilibrium constant equation. Thus, in this case, Kc=[CO2] x [H2O]=1.8 mole/L x 1.5 mole/L=2.7 mole^2/L^2.