C6 equilibria

• a reversible reaction carried out in a closed system will produce an equilibrium mixture, which has all reactants and products in constant proportions
• dynamic equilibrium is when the forward and reverse reactions are happening at the same rate, so the proportion of reactants and products stays constant
• a closed system means the reactants and products cannot escape
• there are 4 conditions that apply to all equilibria:

  • can only be reached in a closed system
  • can be approached from either direction and the equilibrium position would be the same
  • the rates of the forward reaction and reverse reaction are the same
  • visible properties do not change
• the position of equilibrium can be changed in order to produce a different proportion of reactants and products
• if proportion of products increases, the equilibrium has shifted to the right
• if proportion of products decreases, the equilibrium has shifted to the left
• le chatelier’s principle states that if a system at equilibrium is disturbed, the equilibrium moves in the direction that tends to reduce the disturbance
• if surrounding temperature is increased, the equilibrium shifts to favour the endothermic reaction

  if surrounding temperature is decreased, the equilibrium shifts to favour the exothermic reaction
• if pressure is increased, the equilibrium shifts to favour the reaction that produces fewer moles of gas

  if pressure is decreased, the equilibrium shifts to favour the reaction that produces more moles of gas
• if concentration of reactants is increased, the equilibrium shifts to the right to favour the forwards reaction

  if concentration of products is decreased, the equilibrium shifts to the left to favour the reverse reaction
• the presence of a catalyst does not affect the position of equilibrium as it increases the rate of both reactions at equal proportions by reducing the activation energy
• many industrial processes involve reversible reactions, but to find the best conditions for the reaction to take place, compromises must be made
• in industry, the more product you make, the more money you will get from selling it, but a high yield may require a high temperature or pressure which would cost a lot of money to maintain, or a high yield may require a low temperature or pressure which slows the reaction down, reducing profits
• the equilibrium constant for reversible reactions shows the position of equilibrium for a reaction at a specific temperature, it is represented by $K_c$
• Kc is equal to the product of the concentrations of the products, divided by the product of the concentrations of the reactants

  if there are multiple moles of a reactant or product, raise it to the power of the number of moles
• Kc has different units depending on the reaction, to find them use the expression but replace the concentrations with units and cancel anything that can be cancelled
• Kc is only affected by changes in temperature
• if temperature is increased, the equilibrium will shift to favour the endothermic reaction, if this is forwards then Kc increases, if it is reverse then Kc decrease

  if temperature is decreased, the equilibrium will shift to favour the exothermic reaction, if this is forwards then Kc increases, if it is reverse then Kc decreases
• the expression for Kc always has products on the top, so if more products are produced, it will increase, and if more reactants are produced, it will decrease
• changes in concentration do not affect Kc
• changes to the presence of a catalyst do not affect Kc