chemical equilibrium & factors

• a chemical reaction is in dynamic equilibrium when the rate of forward and reverse reactions are equal
• at dynamic equilibrium, the concentrations of products and reactants are constant
• the factors that effect equilibrium are:

  • temperature
  • pressure
  • concentration
• for a reaction to reach equilibrium it must take part in a closed system
• a closed system is where the chemicals are trapped, and nothing is added or removed, but heat is allowed to be transferred to or from the surroundings
• if a system is at equilibrium and a change is made in any of the conditions, then the system responds to counteract the change as much as possible
• the same equilibrium mixture is obtained whether you start with the reactants or products
• increasing concentration of reactant or removing product

  equilibrium moves right
• increasing concentration of product or removing reactant

  equilibrium moves left
• increasing temperature

  favours endothermic reaction
• decreasing the temperature

  favours exothermic reaction
• catalyst

  no effect on equilibrium position
• the equilibrium constant is given as the symbol of K
• the equilibrium constant can also be written as Kc when describing concentration
• the general expression of K is

  K= [products]/[reactants]
• square brackets must be used as they indicate the concentration of a species
• the equilibrium expression represents a ratio
• K has no units
• the greater the value of Kc, the greater the concentration of products compared to reactants, which means the reaction has gone further towards completion
• a low value of K means the equilibrium lies further to the left, and favours the reactants
• K = <1

  • equilibrium lies more to the left (reactants)
  • equilibrium concentration of reactants is large
• K = 1

  • equilibrium favours neither products nor reactants
• K = >1

  • equilibrium lies further towards the right (products)
  • equilibrium concentration of products is large
• equilibrium constant gives no indication of the rate at which equilibrium is achieved, it only indicates the ratio of products to reactants once this state is reached
• in a homogeneous equilibrium all the species are in the same gaseous phase
• in a heterogeneous equilibrium the species are in more than one phase
• the value of the equilibrium constant, K, is dependant on temperature
• increasing temperature favours the endothermic (+) reaction
• decreasing the temperature favours the exothermic (-) reaction
• at higher temperatures, the products are favoured so K increases
• at lower temperatures, reactants are favoured so K will decrease
• catalysts speed up the rate of forward and reverse reactions by providing alternative pathways with a lower activation energy
• catalysts have no effect on the position of equilibrium, therefore no change on the value of K