Equilibrium

Reversible Reactions Products can reform to make reactants Completion reaction almost all of the products make reactants Theoretically all reactions are reversible under the right conditions The products can re-make the reactants

Chemical Equilibrium The rate of the forward reaction and the rate of the reverse reaction are equal Concentrations of products and reactants stay the same Both reactions continue but no net change in the system H2 + I2 ↔ 2HI H2 + I2 → 2HI and 2HI → H2 + I2 Both reactions occur at the same rate

Many chemical reactions are reversible under ordinary conditions of temperature and concentration. Reach a state of equilibrium unless at least one of the substances escapes or is removed from the system.

products of the forward reaction favored, lies to the right Higher concentration of products products of the reverse reaction favored, lies to the left Higher concentration of reactants Neither reaction is favored Considerable concentrations of both reactants and products exist

LE CHÂTELIER’S PRINCIPLE If a system at equilibrium is subjected to a stress, the equilibrium is shifted in the direction that tends to relieve the stress. So the equilibrium position shifts in the opposite direction of the stress Changes in pressure, concentration, or temperature can change the equilibrium position change the relative amounts of reactants and products

Increase reactants equilibrium shifts right to favor products Increase products, equilibrium shifts left to favor reactants. If exothermic reaction and add energy, shifts left to favor reactants If endothermic and add energy, shifts right to favor products

Changes in pressure only apply to reactions with gases If pressure increases, equilibrium shifts to side with the least gas molecules If pressure decreases, equilibrium shifts to side with the most gas molecules If the number of molecules is equal on both sides, changing the pressure has no effect.