For the reaction 2 + 2, the units of Kc are — Chemical Equilibrium Chemistry Question

💡 Solution & Explanation

# Units of Kc for 2$SO_2$ + $O_2$ ⇌ 2$SO_3$ **Step 1: Write the equilibrium expression** $$K_c = \frac{[SO_3]^2}{[SO_2]^2[O_2]}$$ **Step 2: Identify concentration units** All concentrations are expressed in mol/L (or M), so: $$K_c = \frac{(\text{mol/L})^2}{(\text{mol/L})^2 \times (\text{mol/L})} = \frac{(\text{mol/L})^2}{(\text{mol/L})^3}$$ **Step 3: Simplify the units** $$K_c = (\text{mol/L})^{2-3} = (\text{mol/L})^{-1} = \text{L/mol}$$ Or equivalently: $K_c$ has units of $\mathbf{L \cdot mol^{-1}}$ or $\mathbf{M^{-1}}$ **Key concept:** For any reaction, $K_c$ units depend on the change in number of moles of gases: $(\text{mol/L})^{\Delta n}$, where $\Delta n = n_{\text{products}} - n_{\text{reactants}} = 2 - (2+1) = -1$