Equilibrium

chemical equilibrium

state when concentration of reactants and products remain constant with time

• dynamic state 

• rate fwd = rate rvs

characteristics of chemical equilibria systems

1. mixture of reactants and products is present

2. composition of reaction mixture no longer changes = constant

3. chemical equilibrium is dynamic (still continue on a molecular level)

4. dynamic equilibrium can be controlled; Le Chatelier’s

equilibrium constants

only temperature can change

heterogeneous equilibria

more than 1 phase (different phases)

pure solids or liquids no concentrations or pressures

magnitude of equilibrium constant

indicates the extent to which the forward and reverse reactions take place

• large K: >=10³

• intermediate K: ~10^-3 to 10²

• small K: <=10^-4

large K

• at equilibrium, large [product] small [reactant]

• reaction proceeds essentially to completion

intermediate K

• at equilibrium, significant amounts of reactants and product

small K

• at equilibrium, small [product] large [reactant]

• reaction doesnt proceed much

reaction quotient

same form as K

ratio of concentrations/pressures at a given time in reaction

Q<K

reaction proceeds forwards towards products

• products > reactants

• Q gets LARGER

Q>K

reaction proceeds reverse

• products < reactants

• Q gets SMALLER

le chatelier’s principle

change in

• concentration, pressure, volume, catalyst, temperature

imposed on a system at equilibrium will shift in a direction that tends to reduce change

• system moves out of equilibrium state (Q=K) temporarily

• shifts til equilibrium re-established

how to reach equilibrium

1. addition of substance (concentration increases) equilibrium shifts to consume it

2. removal of substance (concentration decreases) equilibrium shifts to produce it

3. addition of solids/liquids → no change; dont appear in Q or K expressions

effect of changes in presssure

• change pressure by adding/removing reactants/products

• adding inert gas

  • as long as volume stays constant, it wont make an effect

  • pressure total increases, partial pressures dont change

• change pressure by changing volume

  • reaction at equilibrium: add/remove product/reactions cause shifts

effect of changes in volume

concentrations or pressures of reactants and products change

• increase volume = shift to increase number of gas molecules

• decrease volume = shift to decrease number of gas molecules

effect of changes in temperature

equilibrium constant K changes

exothermic/endothermic reactions (released/absorbed heat)

• increase temperature = shift to consume heat

• decrease temperature = shift to produce heat