Write an equilibrium expression for each of the following reactions.

1) 3O2 ⇌ 2O3

$\mathrm{Keq}=\frac{\mathrm{\left[O3\right]2}}{\mathrm{\left[O2\right]3}}$

2) N2 + 3H2 ⇌ 2NH3

$\mathrm{Keq}=\frac{\mathrm{\left[NH3\right]2}}{\mathrm{\left[N2\right]}\mathrm{\left[H2\right]3}}$

3) H2 + I2 ⇌ 2HI

$\mathrm{Keq}=\frac{\mathrm{\left[HI\right]2}}{\mathrm{\left[H2\right]}\mathrm{\left[I2\right]}}$

4) PCl5 ⇌ PCl3 + Cl2

$\mathrm{Keq}=\frac{\mathrm{\left[PCl3\right]}\mathrm{\left[Cl2\right]}}{\mathrm{\left[PCl5\right]}}$

5) SO2 + (1/2)O2 ⇌ SO3

$\mathrm{Keq}=\frac{\mathrm{\left[SO3\right]}}{\mathrm{\left[SO2\right]}\mathrm{\left[O2\right]1/2}}$

Notice that this last problem (#5) is the second example equation in the tutorial, with the coefficients divided through by 2. The result is the square root of each concentration value and so the value to the Keq in #5 is the square root of the value in the second example problem.

What's the point? Well, the point is that, when discussing an equilibrium constant, the relevant chemical equation must also be included in the discussion.