Final answer:
The equilibrium constant expressions for the given reactions are K = [H2][Cl2] / [HCl]^2, K = [NO2]^4 [O2] / [N2O5]^2, and K = [NH3] [HCl] / [NH4Cl].
Step-by-step explanation:
The equilibrium constant expression, also known as K, is a way to express the ratio of products to reactants at equilibrium for a given chemical reaction. It is written using the concentrations (or pressures) of the reactants and products, raised to the power of their stoichiometric coefficients.
For the reaction 2 HCl (g) ⇌ H2 (g) + Cl2 (g), the equilibrium constant expression is K = [H2][Cl2] / [HCl]^2.
Similarly, for the reaction 2 N2O5 (g) ⇌ 4 NO2 (g) + O2 (g), the equilibrium constant expression is K = [NO2]^4 [O2] / [N2O5]^2.
Finally, for the reaction NH4Cl (s) ⇌ NH3 (g) + HCl (g), the equilibrium constant expression is K = [NH3] [HCl] / [NH4Cl].