Equilibria

• Reversible reaction


  Reactants react to form products, which can then react to reform the original reactants
• Endothermic reaction


  Reaction that absorbs heat

  
• Exothermic reaction


  Reaction that releases heat
• Dynamic equilibrium


  When the forward and backward rate of reaction is the same and remains at a constant rate
• Dynamic equilibrium can only occur in closed systems where no substances can get in or out in order to influence the reactions
• Le Chatelier's Principle


  When a system is subject to change, the system will alter to lessen the effect of that change
• Increasing temperature


  Favours the endothermic reaction (+ve ∆H) as the excess heat needs to be removed from the system

  
• Decreasing temperature


  Favours the exothermic reaction (-ve ∆H) as heat needs to be gained

  
• Increasing pressure


  Favours the side of the reaction with fewer moles as this will help to release the build up in pressure
• Decreasing pressure


  Favours the side of the reaction with more moles as pressure has been lost
• Increasing concentration of reactants


  Favours the reaction that produces the products as more molecules are available to react
• Decreasing concentration of products

  Favours the reaction that reforms the reactants
• Catalysts appear not to affect the equilibrium position as they affect the forward and backward reaction equally, but they do allow equilibrium to be reached faster
• Equilibrium constant (Kc)


  Indicates the position of equilibrium for a reaction at a certain temperature
• Kc for a reaction always has the same value unless the reaction conditions and therefore the position of equilibrium is changed
• Finding the value of Kc

  1. Determine the concentrations of the substances in the reaction at the equilibrium position

  2. Kc = concentration of products / concentration of reactants

  3. Variations in the number of moles raises that substance to a power with the same value as the number of moles
• The equilibrium constant has varying units depending on the chemical reaction
• The value of Kc is not affected by concentration change or use of a catalyst, but it is affected by changing the reaction temperature as the equilibrium position change results in different concentrations of reactants and products