Chemistry: Chemical Equilibria and ICE Box Method

Reversible Reactions and Dynamic Equilibrium

  • Chemical reactions can be reversible, meaning reactants can form products which can revert back to reactants.

  • At dynamic equilibrium:

    • Forward and reverse reaction rates are equal.

    • Though reactions are ongoing, there is no net change in concentrations, indicating stability.

Stoichiometry and Equilibrium Calculations

  • Traditional stoichiometry deals with unidirectional reactions, where reactants completely convert to products.

  • In equilibria, calculating concentrations is more complex due to reversibility.

ICE Box Method for Equilibrium Concentrations

  • ICE Box Explanation:

    • I: Initial concentrations.

    • C: Change in concentrations.

    • E: Equilibrium concentrations.

  • Example of using ICE Box:

    • Starting with 1 mole of PCl₅ and at equilibrium finding 0.135 moles of PCl₃:

    • Setup for ICE Box:

      • Initial: PCl₅ = 1, PCl₃ = 0

      • Change: PCl₅ = -X, PCl₃ = +X

      • At Equilibrium: PCl₅ = (1 - X), PCl₃ = X

    • Given: PCl₃ = 0.135 moles (X = 0.135)

    • Calculate PCl₅: 1 - 0.135 = 0.865 moles

Equilibrium Constant (Kc)

  • Equilibrium Expression:

    • Kc=[Products]stoichiometric coefficient[Reactants]stoichiometric coefficientK_c = \frac{[Products]^{\text{stoichiometric coefficient}}}{[Reactants]^{\text{stoichiometric coefficient}}}

  • Kc indicates:

    • If K_c >> 1 : Products favored (more products formed).

    • If K_c << 1 : Reactants favored (more reactants present).

  • Only gases and aqueous species are considered in the Kc expression; solids and pure liquids are excluded.

Reaction Quotient (Q)

  • Used to predict the direction of the reaction based on non-equilibrium concentrations.

  • Calculating Q:

    • Substitute non-equilibrium values into the Kc expression to find Q.

  • Implications:

    • If K_c > Q : Q is reactant-favored; products will form.

    • If K_c < Q : Q is product-favored; reactants will form.

    • If Kc=QK_c = Q: System is at equilibrium.

Advanced ICE Box Example

  • When calculating the equilibrium concentrations using ICE boxes:

    • Initial concentrations defined (initial value for reactant, zero for products).

    • Changes: Reflect stoichiometry (subtracting for reactants and adding for products).

    • Example:

    • 2 moles of reactant form 1 mole of each product: Use 2X and X in the changes.

    • Set up equilibrium expression and solve for X. Square roots may facilitate finding X easily; otherwise, the quadratic equation may be needed.