Quick questions on The equilibrium constant Kc: TCE Chemistry (Tasmania)

Most $K_c$ calculations follow the same routine: build an Initial, Change, Equilibrium table in moles, use the stoichiometry to fill the change row, convert to concentrations by dividing by the volume, then substitute. Always check the volume because the powers in the expression may not cancel.

The units of $K_c$ depend on the equation, because the powers may not cancel. Substitute $\text{mol L}^{-1}$ for each concentration and simplify. For $\text{N}_2 + 3\text{H}_2 \rightleftharpoons 2\text{NH}_3$ the units are $\dfrac{(\text{mol L}^{-1})^2}{(\text{mol L}^{-1})(\text{mol L}^{-1})^3} = (\text{mol L}^{-1})^{-2}$.

The form of $K_c$ depends on how the equation is written, so altering the equation alters the constant in predictable ways. If you reverse an equation, the new constant is the reciprocal, $K' = 1/K_c$. If you multiply every coefficient by a factor $n$, the new constant is $K_c$ raised to the power $n$, $K' = (K_c)^n$. If you add two equilibria, the overall constant is the product of the individual constants.