Write the equilibrium constant expression for each of the following reactions.
a CO(g) + H2O(g) ⇄ CO2(g) + H2(g)
b CH4(g) + Cl2(g) ⇄ CH3 Cl(g) + HCl(g)
c 2SO2(g) + O2(g) ⇄ 2SO3(g)
d FeO(s) + CO(g) ⇄ Fe(s) + CO2(g)