Chemical Equilibria

• Dynamic equilibrium

  The rate of the forward reaction is equal to the rate of the reverse reaction. (Hence, the concentrations of reactant and product do not change)
• Conditions for equilibrium mixture

  • Equilibrium occurs in a closed system (where reactants and products cannot escape)
  • Macroscopic properties do not change with time
• Le Chatelier's principle

  If a system at equilibrium is disturbed, the equilibrium moves in the direction that tends to reduce the disturbance
• Increasing temperature in endothermic reaction

  Equilibrium position shifts to the right
• Increasing pressure in reaction with more moles of gas in products

  Equilibrium position shifts to the left
• Reasons for using high pressure in industrial production of hydrogen

  • High pressure increases the collision frequency, increasing the rate of reaction
  • This is a compromise pressure between an economically viable rate of reaction and a slightly lower yield of hydrogen
• Catalyst has no effect on the position of equilibrium
• Conditions that affect the value of Kc

  • Concentration
  • Catalyst
  • Pressure
  • Temperature
• Expression for Kc

  Kc = [D][E]^4 / [A]^2[B]^3[C]
• Units for Kc

  mol^-1 dm^3
• Kc is relevant for

  Homogeneous systems in equilibrium
• Kc greater/less than 1

  • Greater than 1 = equilibrium position shifted to the right
  • Less than 1 = equilibrium position shifted to the left
• Decreasing temperature in endothermic reaction

  Kc decreases
• Increasing temperature in endothermic reaction

  Kc increases
• Decreasing temperature in exothermic reaction

  Kc increases
• Increasing temperature in exothermic reaction

  Kc decreases