Equilibria

• Reversible reactions


  Reactions where the forward and backward reactions occur simultaneously
• Equilibrium


  1. Reactants used up quickly initially

  2. Reactant concentration decreases

  3. Product concentration increases

  4. Rates of forward and backward reactions become equal
• Dynamic equilibrium


  Equilibrium where the concentrations of reactants and products remain constant
• Equilibrium only occurs in closed systems
• Le Chatelier's Principle


  If a system at equilibrium is subjected to a change in concentration, temperature or pressure, the system will shift to counteract the change and establish a new equilibrium
• Effect of increasing concentration

  1. Equilibrium shifts to the right to reduce concentration

  2. More products are formed
• Effect of decreasing concentration


  1. Equilibrium shifts to the left to increase concentration

  2. More reactants are formed
• Effect of increasing pressure


  1. Equilibrium shifts to the side with fewer gas particles

  2. More products are formed
• Effect of decreasing pressure


  1. Equilibrium shifts to the side with more gas particles

  2. More reactants are formed
• Effect of increasing temperature

  1. Equilibrium shifts in the endothermic direction

  2. More reactants are formed
• Effect of decreasing temperature


  1. Equilibrium shifts in the exothermic direction

  2. More products are formed
• Catalysts have no effect on the position of equilibrium
• Equilibrium constant (Kc)


  Ratio of product concentrations to reactant concentrations, raised to the power of their stoichiometric coefficients
• Calculating Kc


  • Given concentrations of SO2, O2, and SO3, calculate the Kc for the reaction: 2SO2 + O2 ⇌ 2SO3
• Molar numbers


  Numbers in front of reactants that become powers in the KC expression
• Square brackets


  Indicate concentration, which must be used in the KC expression
• Writing the KC expression

  1. Write the products and reactants

  2. Put the concentrations in square brackets

  3. Raise the molar numbers to powers
• Calculating KC


  1. Write the KC expression

  2. Substitute the concentrations

  3. Calculate the value of KC
• To find the equilibrium concentrations before calculating KC, an ICE table can be used
• Using an ICE table

  1. Write the reactants and products

  2. Fill in the initial, change, and equilibrium values

  3. Use the change values to calculate the equilibrium concentrations
• Calculating KC from equilibrium concentrations

  1. Use the mass and volume to calculate the initial moles

  2. Use the change in O2 moles to determine the changes in other species

  3. Calculate the equilibrium concentrations

  4. Substitute into the KC expression
• Temperature affects the value of KC - an exothermic forward reaction will have KC increase with temperature, while an endothermic forward reaction will have KC decrease with temperature
• Concentration changes do not affect the value of KC
• Catalysts do not affect the value of KC, they only speed up the forward and reverse reactions
• Calculating equilibrium concentrations using KC


  1. Write the KC expression

  2. Substitute the known values

  3. Rearrange to solve for the unknown concentrations