1.6 Equilibria

• what are reversible reactions?

  where reactions can go forward and backward
• what is a dynamic equilibrium?

  rate of the forwards reaction = rate of backwards reaction

  only occurs in closed systems
• what is a closed system?

  where no reactants or products can escape
• what is Le Chatelier's Principle

  If a reaction at equilibrium is subjected to a change in temperature, concentration or pressure, the position of equilibrium will shift to oppose this change
• how does an increase in concentration affect the position of equilibria?

  increase in concentration of a reactant or product will shift equilibrium to reduce the concentration
• how does a decrease in concentration affect the position of equilibria?

  decrease in concentration of a reactant or product will shift equilibrium to increase the concentration
• how does an increase in pressure affect the position of equilibria?

  increase the pressure will shift equilibria to the side with the fewest number of particles to decrease the pressure
• how does a decrease in pressure affect the position of equilibria?

  decrease the pressure will shift equilibria to the side with the most number of particles to increase the pressure
• how does an increase in temperature affect the position of equilibria?

  increase in temperature will shift equilibria in the endothermic direction (+ve) to reduce the temperature
• how does a decrease in temperature affect the position of equilibria?

  decrease the temperature will shift equilibria in the exothermic direction (-ve) to increase the temperature
• how does a catalyst affect the position of equilibria?

  catalysts have no effect on the position of equilibrium. a catalyst will speed up the rate of the forward and backward reaction equally
• how do you work out the equilibrium constant (Kc)?

  using the molar concentrations in a reaction
• how do you work out the concentration of a species in equilibrium?

  using an ICE table

  Initial moles

  Change in moles

  Equilibrium moles
• how does a change in temperature affect the value of Kc?

  - Kc is only valid for one temperature therefore changing the temperature will change equilibrium concentrations, hence Kc will change too.

  - if temperature change causes equilibrium to shift to the right then Kc will increase

  - if temperature causes equilibrium to shift left Kc will decrease
• how does a change in concentration affect the value of Kc?

  the value of Kc is unaffected by any changes in concentration
• how does a catalyst affect the value of Kc?

  - adding a catalyst has no effect on the value of Kc

  - a catalyst speeds up the forward and reverse reaction

  - a catalyst speeds up the rate at which equilibrium is established