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Equilibrium - Equilibrium in Chemical Processes (Dynamic Equilibrium)

Grade 11CBSEChemistry

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Chemical equilibrium is a state in a reversible reaction where the rate of the forward reaction equals the rate of the backward reaction, denoted as rf=rbr_f = r_b.

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Equilibrium is dynamic in nature, meaning the forward and reverse reactions continue to occur at the molecular level, but the macroscopic properties (concentration, pressure, color) remain constant over time.

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For a general reaction aA+bB⇌cC+dDaA + bB \rightleftharpoons cC + dD, the Law of Mass Action states that the equilibrium constant KcK_c is the ratio of the product of molar concentrations of products to that of reactants, each raised to the power of their stoichiometric coefficients.

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In homogeneous equilibrium, all reactants and products are in the same phase (e.g., all gases or all in aqueous solution).

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In heterogeneous equilibrium, substances are in different phases. The concentrations of pure solids and pure liquids are taken as unity (11) and are omitted from the KcK_c and KpK_p expressions.

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The equilibrium constant KK is temperature-dependent. It does not change with changes in concentration, pressure, or the addition of a catalyst.

📐Formulae

Kc=[C]c[D]d[A]a[B]bK_c = \frac{[C]^c [D]^d}{[A]^a [B]^b}

Kp=(PC)c(PD)d(PA)a(PB)bK_p = \frac{(P_C)^c (P_D)^d}{(P_A)^a (P_B)^b}

Kp=Kc(RT)ΔngK_p = K_c(RT)^{\Delta n_g}

Δng=∑nproducts(g)−∑nreactants(g)\Delta n_g = \sum n_{products(g)} - \sum n_{reactants(g)}

Qc=[C]tc[D]td[A]ta[B]tb (Reaction Quotient at any time t)Q_c = \frac{[C]_t^c [D]_t^d}{[A]_t^a [B]_t^b} \text{ (Reaction Quotient at any time } t)

💡Examples

Problem 1:

For the Haber process reaction: N2(g)+3H2(g)⇌2NH3(g)N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g), calculate Δng\Delta n_g and state the relationship between KpK_p and KcK_c.

Solution:

  1. Identify the number of gaseous moles of products: np=2n_p = 2 (for NH3NH_3).
  2. Identify the number of gaseous moles of reactants: nr=1+3=4n_r = 1 + 3 = 4 (for N2N_2 and H2H_2).
  3. Calculate Δng=np−nr=2−4=−2\Delta n_g = n_p - n_r = 2 - 4 = -2.
  4. Relationship: Kp=Kc(RT)−2K_p = K_c(RT)^{-2} or Kp=Kc(RT)2K_p = \frac{K_c}{(RT)^2}.

Explanation:

The value of Δng\Delta n_g is derived only from gaseous components. Since Δng\Delta n_g is negative, Kc>KpK_c > K_p for this specific reaction.

Problem 2:

Calculate KcK_c for the reaction H2(g)+I2(g)⇌2HI(g)H_2(g) + I_2(g) \rightleftharpoons 2HI(g) if at equilibrium [H2]=0.5 mol L−1[H_2] = 0.5 \text{ mol L}^{-1}, [I2]=0.5 mol L−1[I_2] = 0.5 \text{ mol L}^{-1}, and [HI]=1.0 mol L−1[HI] = 1.0 \text{ mol L}^{-1}.

Solution:

Using the formula for KcK_c: Kc=[HI]2[H2][I2]K_c = \frac{[HI]^2}{[H_2][I_2]} Kc=(1.0)2(0.5)(0.5)K_c = \frac{(1.0)^2}{(0.5)(0.5)} Kc=1.00.25=4K_c = \frac{1.0}{0.25} = 4

Explanation:

The equilibrium constant is calculated by substituting the equilibrium molar concentrations into the expression derived from the stoichiometric equation.