Homogeneous and Heterogeneous Equilibria — Revision Notes

⚡ 30-Second Revision

Homogeneous Equilibrium — All species in same phase (g, l, aq).
Heterogeneous Equilibrium — Species in $\ge 2$ phases (s, l, g, aq).
$K_c$ Expression — Products in numerator, reactants in denominator, coefficients as exponents.
$K_p$ Expression — Same as $K_c$, but with partial pressures for gases.
Key Rule for Heterogeneous — Pure solids (s) and pure liquids (l) are EXCLUDED from $K_c$ and $K_p$ expressions (activity = 1).
$K_p = K_c(RT)^{\Delta n_g}$ — $\Delta n_g = \sum n_{products(g)} - \sum n_{reactants(g)}$. Only gaseous species count for $\Delta n_g$.
Effect of adding pure solid/liquid — No effect on equilibrium position (Le Chatelier's principle).

2-Minute Revision

Chemical equilibrium is categorized into homogeneous and heterogeneous types based on the physical states of reactants and products. In homogeneous equilibrium, all species are in the same phase, such as all gases (e.

g., $N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)$) or all in a single liquid solution. For these, all species are included in the equilibrium constant ($K_c$ or $K_p$) expression, with their stoichiometric coefficients as exponents.

For example, $K_c = [NH_3]^2 / ([N_2][H_2]^3)$.

Heterogeneous equilibrium involves species in two or more different phases, like solids and gases (e.g., $CaCO_3(s) \rightleftharpoons CaO(s) + CO_2(g)$). The critical rule here is that pure solids and pure liquids are *excluded* from the $K$ expression because their concentrations (activities) are constant.

So, for the $CaCO_3$ example, $K_c = [CO_2]$ and $K_p = P_{CO_2}$. The relationship $K_p = K_c(RT)^{Delta n_g}$ still applies, but $Delta n_g$ must only consider gaseous species. Adding more pure solid or liquid has no effect on the equilibrium position.

5-Minute Revision

Let's consolidate our understanding of homogeneous and heterogeneous equilibria, crucial for NEET. Chemical equilibrium is a dynamic state where forward and reverse reaction rates are equal, leading to constant concentrations. The classification depends on the phases of the species involved.

1. Homogeneous Equilibrium:

Definition: — All reactants and products are in the same physical phase. This can be gaseous or a single liquid solution.
Example (Gas-phase): — $2NO(g) + O_2(g) \rightleftharpoons 2NO_2(g)$

* $K_c = \frac{[NO_2]^2}{[NO]^2[O_2]}$ * $K_p = \frac{(P_{NO_2})^2}{(P_{NO})^2(P_{O_2})}$

Example (Liquid-phase): — $CH_3COOH(aq) + C_2H_5OH(aq) \rightleftharpoons CH_3COOC_2H_5(aq) + H_2O(aq)$

* $K_c = \frac{[CH_3COOC_2H_5][H_2O]}{[CH_3COOH][C_2H_5OH]}$ (Note: Water is included if its concentration changes significantly).

Key Point: — All species are included in the $K$ expression.

2. Heterogeneous Equilibrium:

Definition: — Reactants and products exist in two or more different physical phases.
Example (Solid-Gas): — $C(s) + H_2O(g) \rightleftharpoons CO(g) + H_2(g)$

* $K_c = \frac{[CO][H_2]}{[H_2O]}$ (Note: $C(s)$ is excluded) * $K_p = \frac{P_{CO}P_{H_2}}{P_{H_2O}}$ (Note: $C(s)$ is excluded)

Example (Solid-Aqueous): — $AgCl(s) \rightleftharpoons Ag^+(aq) + Cl^-(aq)$

* $K_{sp} = [Ag^+][Cl^-]$ (Note: $AgCl(s)$ is excluded)

Key Point: — Pure solids and pure liquids are *excluded* from the $K$ expression because their concentrations (activities) are constant and effectively absorbed into the $K$ value. Their presence is necessary, but their *amount* doesn't shift equilibrium.

3. Relationship between $K_c$ and $K_p$:

For gaseous equilibria (both types, if gases are involved): $K_p = K_c(RT)^{Delta n_g}$
$Delta n_g = (\text{sum of stoichiometric coefficients of gaseous products}) - (\text{sum of stoichiometric coefficients of gaseous reactants})$.
Crucial: — Only gaseous species are considered for $Delta n_g$. For $CaCO_3(s) \rightleftharpoons CaO(s) + CO_2(g)$, $Delta n_g = 1 - 0 = 1$.

4. NEET Focus: Be able to identify the type of equilibrium, write correct $K$ expressions (especially for heterogeneous systems), calculate $Delta n_g$, and understand that adding/removing pure solids/liquids does not shift equilibrium.

Prelims Revision Notes

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Chemical Equilibrium — A dynamic state where forward reaction rate ($R_f$) equals reverse reaction rate ($R_r$). Concentrations of reactants and products remain constant. $R_f = R_r$.
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Homogeneous Equilibrium — All reactants and products are in the *same* physical phase.

* Examples: All gases ($N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)$), all in aqueous solution ($CH_3COOH(aq) + C_2H_5OH(aq) \rightleftharpoons CH_3COOC_2H_5(aq) + H_2O(aq)$). * **$K_c$ and $K_p$ expressions**: All species are included. Stoichiometric coefficients become exponents. * For $aA(g) + bB(g) \rightleftharpoons cC(g) + dD(g)$: $K_c = \frac{[C]^c[D]^d}{[A]^a[B]^b}$ $K_p = \frac{(P_C)^c(P_D)^d}{(P_A)^a(P_B)^b}$

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Heterogeneous Equilibrium — Reactants and products are in *two or more different* physical phases.

* Examples: Solid-gas ($CaCO_3(s) \rightleftharpoons CaO(s) + CO_2(g)$), solid-aqueous ($AgCl(s) \rightleftharpoons Ag^+(aq) + Cl^-(aq)$), liquid-gas ($H_2O(l) \rightleftharpoons H_2O(g)$). * **$K_c$ and $K_p$ expressions: Pure solids (s) and pure liquids (l) are EXCLUDED.

** Their concentrations/activities are constant (unity) and are absorbed into the $K$ value. Only gaseous and aqueous species are included.

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Relationship between $K_c$ and $K_p$ — Applicable for equilibria involving gases.

* $K_p = K_c(RT)^{Delta n_g}$ * $R = 0.0821,L cdot atm/(mol cdot K)$ (or $8.314,J/(mol cdot K)$ if using SI units for pressure/volume). * $T$ must be in Kelvin. * $Delta n_g = (\text{sum of moles of gaseous products}) - (\text{sum of moles of gaseous reactants})$. * Crucial: Only gaseous species are counted for $Delta n_g$, even in heterogeneous equilibria. * Example: For $C(s) + H_2O(g) \rightleftharpoons CO(g) + H_2(g)$, $Delta n_g = (1+1) - 1 = 1$.

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Effect of adding/removing pure solids/liquids — According to Le Chatelier's principle, adding or removing a pure solid or pure liquid (as long as some is present) has NO EFFECT on the equilibrium position because their concentrations are constant and not part of the $K$ expression. Only changes in concentration/partial pressure of gaseous or aqueous species, temperature, or total pressure (for gaseous systems) can shift the equilibrium.

Vyyuha Quick Recall

HETERO-K: 'HETERO' means 'EXCLUDE the SOLIDS and LIQUIDS!' (from K expression, only include gases and aqueous species).