Equilibrium Review Notes

Reversible Reactions & Equilibrium

  • Reversible reactions proceed in both directions (forward and reverse).
  • Equilibrium is when the rates of forward and reverse reactions are equal.
  • Equilibrium occurs in a closed system (no exchange of reactants/products, constant temperature/pressure).

Equilibrium Constant (Keq)

  • Keq=[products][reactants]K_{eq} = \frac{[products]}{[reactants]}
  • Balancing coefficients are exponents in the Keq expression.
  • Keq=[C]c×[D]d[A]a×[B]bK_{eq} = \frac{[C]^c \times [D]^d}{[A]^a \times [B]^b}
  • Liquids and solids are excluded from the Keq expression.
  • If Keq>1K_{eq} > 1, products are favored.
  • If Keq<1K_{eq} < 1, reactants are favored.
  • If Keq≈1K_{eq} ≈ 1, neither side is strongly favored.

Collision Theory

  • Reactions require collisions with sufficient energy and correct orientation.
  • Reaction mechanisms involve multiple steps and reaction intermediates.
  • The slowest step in a reaction mechanism is the rate-determining step (RDS).

Calculating Reaction Rates

  • Related to the change in concentration of reactants or products over time: rate=ΔMΔsecrate = \frac{ΔM}{Δsec}.
  • Increased reactant concentration speeds up the reaction.

Energy Diagrams

  • Activation energy is the minimum energy for a successful collision.
  • Activated complex (transition state) is the unstable molecule at the peak of the energy diagram.
  • Exothermic reactions have a negative ΔHΔH, endothermic reactions have a positive ΔHΔH.

Factors Affecting Reaction Rates

  • Nature of reactants: Ionic bonds react faster than covalent bonds.
  • State of reactants: liquids/aqueous/gases react faster than solids.
  • Increased surface area and mechanical mixing increase reaction rate.
  • Increased temperature increases reaction rate.
  • Increased concentration and pressure increase reaction rate.
  • Catalysts lower activation energy, speeding up the reaction, but are not consumed.

ICE Box Problems

  • Use an ICE (Initial, Change, Equilibrium) table to calculate equilibrium concentrations.
  • Changes in concentration are related by stoichiometry (e.g., if [H<em>2][H<em>2] changes by "x", then [I</em>2][I</em>2] changes by "x", and [HI][HI] changes by "2x").

Le Chatelier's Principle

  • Concentration: Increasing concentration on one side shifts the equilibrium to the other side.
  • Pressure/Volume: Increasing pressure favors the side with fewer moles of gas.
  • Temperature: Only factor that changes Keq; treat heat as a reactant (endothermic) or product (exothermic) when determining shift.
  • Catalysts: Have no effect on Keq or equilibrium position.