Chemical Equilibrium

Overview of Chemical Equilibrium

  • Chemical Equilibrium: A state in which the concentrations of reactants and products no longer change; the rates of the forward and reverse reactions are equal.

    • Dynamic Equilibrium: Reactants and products are continually formed and consumed.

Key Characteristics of Chemical Equilibrium

  • Equilibrium Condition:

    • Concentrations of reactants and products are constant.

    • Rate of formation of products equals the rate of breakdown of products back to reactants.

  • Equilibrium Constants:

    • Kc: Equilibrium constant for concentrations, defined for a reaction of the form

    aA+bB⇌cC+dDaA + bB \rightleftharpoons cC + dD

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

Factors Affecting Chemical Equilibrium

  • Reactant/Product Preference:

    • Product favored: More products than reactants at equilibrium.

    • Reactant favored: More reactants than products at equilibrium.

  • Effect of Catalysts:

    • Catalysts speed up both forward and reverse reactions without changing equilibrium concentrations or the value of Kc.

Determining the Equilibrium Constant (Kc)

  • Example: For the reaction:

    N<em>2(g)+3H</em>2(g)⇌2NH3(g)N<em>2(g) + 3H</em>2(g) \rightleftharpoons 2NH_3(g)

    K<em>c=[NH</em>3]2[N<em>2][H</em>2]3K<em>c = \frac{[NH</em>3]^2}{[N<em>2][H</em>2]^3}

  • Exclusion of Solids and Pure Liquids:

    • Pure solids and liquids do not appear in the expression for Kc because their concentrations remain constant.

Calculating Equilibrium Concentrations

  • Problem Approach:

    • Use initial concentrations and change (x) at equilibrium to express concentrations, then solve for x using Kc to find [Reactants] and [Products].

Le Chatelier’s Principle

  • Principle:

    • A system at equilibrium will adjust concentrations of reactants and products to counteract any changes made to the system (concentration, temperature, or pressure).

  • Changing Concentration:

    • Adding a reactant shifts equilibrium toward products; removing a reactant shifts it toward reactants.

Effects of Changes in Pressure and Volume

  • Changing pressure or volume affects gaseous equilibria, with the reaction shifting to minimize changes. If the number of gas particles changes, the equilibrium will shift toward the side with fewer moles of gas when pressure increases.

Temperature Effects on Equilibrium Constants

  • Temperature Change:

    • Changing temperature changes Kc; for endothermic reactions, Kc increases with temperature, while for exothermic reactions, Kc decreases with temperature.

Practical Applications: The Haber-Bosch Process

  • Haber-Bosch Process:

    • Industrial synthesis of ammonia from atmospheric nitrogen and hydrogen.

    • Optimized conditions:

    • High Pressure (around 200 atm) and moderate temperatures (450°C) to favor product formation.

    • Catalysts are used to speed up the reactions.

    • This process is exothermic:
      N<em>2(g)+3H</em>2(g)⇌2NH3(g)ΔH°=−92.2kJN<em>2(g) + 3H</em>2(g) \rightleftharpoons 2NH_3(g) \Delta H° = -92.2 kJ