Equilibria and Reversible Reactions

Dynamic Equilibrium

• All reversible reactions reach a dynamic equilibrium.
• Characteristics:
  • Forward and backward reactions occur at equal rates.
  • Concentrations of reactants and products remain constant.
• Position of Equilibrium:
  • Describes the composition of the equilibrium mixture.
  • If it favors reactants (towards the left), the mixture contains mostly reactants.

Le Chatelier's Principle

• Used to predict the effect of changing external conditions on equilibrium.
• Principle:
  • If an external condition changes, the equilibrium shifts to oppose the change.
  • Temperature: Increase favors endothermic direction; decrease favors exothermic direction.
  • Pressure: Increase shifts to side with fewer moles of gas; decrease shifts to side with more moles.
  • Concentration: Increasing a reactant shifts equilibrium to use up the added reactant.

Effects on Equilibrium

• Temperature:
  • Increasing temperature shifts equilibrium to endothermic side.
  • Decreasing temperature shifts it to exothermic side.
  • Industrial compromise: Low temp gives good yield but slow rate - compromises are made.
• Pressure:
  • Increasing pressure favors side with fewer gas moles.
  • High pressure gives good yield and rate but is costly due to energy and equipment expense.
• Concentration:
  • Shifts to use up added reactants and stabilize equilibrium.
• Catalysts:
  • Speed up rate to reach equilibrium but do not affect the position of equilibrium.

Industrial Applications

• Haber Process:
  • Conditions: 450°C, 200-1000 atm, iron catalyst.
  • High pressure is costly; compromise temperatures used.
• Contact Process:
  • Conditions: 450°C, 1-2 atm, V2O5 catalyst.
  • Slight yield increase with higher pressure; compromise temperature used.
• Methanol Production:
  • Conditions: 400°C, 50 atm, chromium and zinc oxides.
• Ethanol Production:
  • Conditions: 300°C, 70 atm, H3PO4 catalyst.
  • High pressure can cause unwanted polymerization.

Equilibrium Constant (Kc)

• Expression:
  • Kc = [products]^coefficients / [reactants]^coefficients
• Calculating Kc: Steps to Find Equilibrium Concentrations:
  • Determine equilibrium moles from initial conditions and changes.
  • Calculate equilibrium concentrations.
  • Substitute into the Kc expression.
• Unit Calculation:
  • Kc units depend on the balanced equation.
• Effect of Changing Conditions:
  • Temperature affects both equilibrium position and Kc.
  • Pressure and catalysts do not affect Kc.

Practical Experiments

• Titration and Equilibrium Constants:
  • Use burettes to prepare equilibrium mixtures.
  • Titrate the mixture to determine equilibrium constants.
• Calculations Involving Equilibrium:
  • Work out initial moles, equilibrium amounts, and then calculate Kc.
  • Experiments confirm if equilibrium is established by consistent Kc values.

These notes provide a comprehensive guide to understanding and calculating equilibria in chemical reactions, including the effects of various factors on the position and constants of equilibrium in both theoretical and industrial contexts.