# 8.4 Types of Equilibrium Based on the physical states of the reactants and products involved in a reversible reaction, chemical equilibria can be classified into two main types. ## i. Homogeneous Equilibria A **homogeneous equilibrium** is an equilibrium system in which all reactants and products are in the *same phase* (e.g., all gaseous or all aqueous). **Example Reactions (Gaseous Phase):** - Haber process for ammonia synthesis: $$ \mathrm{N_{2(g)} + 3H_{2(g)} \rightleftharpoons 2NH_{3(g)}} $$ - Contact process for sulfuric acid production: $$ \mathrm{2SO_{2(g)} + O_{2(g)} \rightleftharpoons 2SO_{3(g)}} $$ - Formation of nitrosyl chloride: $$ \mathrm{2NO_{(g)} + Cl_{2(g)} \rightleftharpoons 2NOCl_{(g)}} $$ **Example Reaction (Liquid Phase):** - Esterification of acetic acid with ethanol: $$ \mathrm{CH_3COOH_{(l)} + C_2H_5OH_{(l)} \rightleftharpoons CH_3COOC_2H_{5(l)} + H_2O_{(l)}} $$ ## ii. Heterogeneous Equilibria A **heterogeneous equilibrium** is an equilibrium system in which the reactants and products are present in *more than one phase* (e.g., a mix of solids, liquids, and gases). **Example Reactions:** - Thermal decomposition of calcium carbonate: $$ \mathrm{CaCO_{3(s)} \rightleftharpoons CaO_{(s)} + CO_{2(g)}} $$ - Reaction of carbon with steam (water gas production): $$ \mathrm{C_{(s)} + H_2O_{(g)} \rightleftharpoons CO_{(g)} + H_{2(g)}} $$ - Reaction of iron with steam: $$ \mathrm{3Fe_{(s)} + 4H_2O_{(g)} \rightleftharpoons Fe_3O_{4(s)} + 4H_{2(g)}} $$ ### Equilibrium Constants for Heterogeneous Equilibria A key rule for writing equilibrium constant expressions ($K_c$ or $K_p$) is that the concentrations (or partial pressures) of **pure solids** and **pure liquids** are considered constant and are *not included* in the expression. This is because their concentration (density) does not change significantly during the reaction. **Example 1: Decomposition of Water** - **Reaction:** $$ \mathrm{2H_2O_{(l)} \rightleftharpoons 2H_{2(g)} + O_{2(g)}} $$ - Since $\mathrm{H_2O}$ is a pure liquid, it is omitted from the expression. - **Equilibrium Constant ($K_c$):** $$ K_c = [\mathrm{H_2}]^2 [\mathrm{O_2}] $$ - **Equilibrium Constant ($K_p$):** $$ K_p = (P_{\mathrm{H_2}})^2 (P_{\mathrm{O_2}}) $$ **Example 2: Reaction of Iron with Steam** - **Reaction:** $$ \mathrm{3Fe_{(s)} + 4H_2O_{(g)} \rightleftharpoons Fe_3O_{4(s)} + 4H_{2(g)}} $$ - The pure solids, $\mathrm{Fe_{(s)}}$ and $\mathrm{Fe_3O_{4(s)}}$, are omitted. - **Equilibrium Constant ($K_c$):** $$ K_c = \frac{[\mathrm{H_2}]^4}{[\mathrm{H_2O}]^4} $$ - **Equilibrium Constant ($K_p$):** $$ K_p = \frac{(P_{\mathrm{H_2}})^4}{(P_{\mathrm{H_2O}})^4} $$ ## Concept Assessment Exercise 8.3 Write the $K_c$ and $K_p$ expressions for each of the following reactions. **i. $\mathrm{FeO_{(s)} + CO_{(g)} \rightleftharpoons Fe_{(s)} + CO_{2(g)}}$** 1. Identify phases: $\mathrm{FeO}$ and $\mathrm{Fe}$ are solids. $\mathrm{CO}$ and $\mathrm{CO_2}$ are gases. 2. Write expression: Exclude the solids from the equilibrium constant expressions. 3. Answer: - $K_c = \frac{[\mathrm{CO_2}]}{[\mathrm{CO}]}$ - $K_p = \frac{P_{\mathrm{CO_2}}}{P_{\mathrm{CO}}}$ **ii. $\mathrm{P_{4(s)} + 5O_{2(g)} \rightleftharpoons P_4O_{10(s)}}$** 1. Identify phases: $\mathrm{P_4}$ and $\mathrm{P_4O_{10}}$ are solids. $\mathrm{O_2}$ is a gas. 2. Write expression: Exclude the solids. The products side only contains a solid, so it is represented by 1 in the numerator. 3. Answer: - $K_c = \frac{1}{[\mathrm{O_2}]^5}$ - $K_p = \frac{1}{(P_{\mathrm{O_2}})^5}$ **iii. $\mathrm{CH_{4(g)} + 4Cl_{2(g)} \rightleftharpoons CCl_{4(l)} + 4HCl_{(g)}}$** 1. Identify phases: $\mathrm{CH_4}$, $\mathrm{Cl_2}$, and $\mathrm{HCl}$ are gases. $\mathrm{CCl_4}$ is a pure liquid. 2. Write expression: Exclude the pure liquid ($\mathrm{CCl_4}$). 3. Answer: - $K_c = \frac{[\mathrm{HCl}]^4}{[\mathrm{CH_4}][\mathrm{Cl_2}]^4}$ - $K_p = \frac{(P_{\mathrm{HCl}})^4}{(P_{\mathrm{CH_4}})(P_{\mathrm{Cl_2}})^4}$ | Type | Definition | Example | $K_c$ Expression | | :--- | :--- | :--- | :--- | | **Homogeneous** | All species in the same phase. | $\mathrm{N_{2(g)} + 3H_{2(g)} \rightleftharpoons 2NH_{3(g)}}$ | $K_c = \frac{[\mathrm{NH_3}]^2}{[\mathrm{N_2}][\mathrm{H_2}]^3}$ | | **Heterogeneous** | Species in multiple phases. | $\mathrm{CaCO_{3(s)} \rightleftharpoons CaO_{(s)} + CO_{2(g)}}$ | $K_c = [\mathrm{CO_2}]$ |