Reversible
Chemical
Reactions
BIOB111
Session 4

Reversible
Chemical
Reaction

  • can proceed in both directions
  • need to consider what forces the reaction to go backwards/forwards

Chemical
Equilbrium

the state in which forward and reverse reactions occur simultaneously at the same rate

concentrations of reactants and products no longer change
(amounts of products and reactants don't have to be equal)

Dynamic equilibrium
there is frantic activity, things are moving around
reactions are still proceeding

Le Châtelier's Principle

if stress is applied to a system in dynamic equilibrium the system will adjust in the direction that best reduces stress, reaching a new equilibrium

when the equilibrium disruption leads to the formation of more products than reactants - equilibrium will shift to the LEFT
e.g. add more product. Will start to use the product at a higher rate, therefore increase reverse reaction

when the equilibrium disruption causes the formation of more reactants than products - the equilibrium will shift to the RIGHT
e.g. add more reactants. Will cause rate of the forward reaction to use up extra reactants

more products = left

more reactants = right

Concentration
change

addition of any component (reactant or product) shifts the equilibrium to the opposite side

removal of any component (reactant or product) shifts the equilibrium to the same side

Temperature
change

Exothermic
reaction

Endothermic
reaction

energy is treated as one of the reactants/products

increase in temperature → reaction shifts to LEFT

  • equivalent of adding a component

decrease in temperature → reaction shifts to RIGHT

  • equivalent of removing a component

increase in temperature → reaction shifts to the RIGHT

  • equivalent of adding a component

decrease in temperature → reaction shifts to the LEFT

  • equivalent of removing a component

Catalysts

no effect on the position of the equilibrium

will help the system reach equilibrium faster

increase reaction rates in both forward and reverse reactions - lower activation energies

Pressure
changes

only affect chemical reactions in gaseous state

ONLY has an affect if the number of moles changes in the reaction

Pressure increase = volume decrease
equilibrium will shift towards side with smaller number of moles

Pressure decrease = volume increase
equilibrium will shift towards side with greater number of moles