Chemical Equlibrium

1. Gases enters in the mass action expression as partial pressures; usually in atm. Use P for pressure.
2. Dissolved species enters as concentration (mols / L). Use brakets for concentration.
3. Pure solids and liquids (e.g. water) do not enter.
4. Solvents are omitted from the mass action expression.
5. At equilibrium the mass action quotient is equal to K, the equilibrium constant.
6.
a) The reverse of a reaction gives the reciprocal of the K.
b) The sum of reactions results in products of K.
c) The differece of reactions results in quotients of K.

aA + bB <==> cC + dD

Given the general reaction, the mass action quotient can be written,

The concentration and pressure of a gas is related by the ideal gas law,


where [A] is the concentration of A (mols/L) and n the moles of A. The equilibrium expression in terms of concentration of gasses is,

LeChatelier's principle states that if a stress is applied to a system at equilibrium, then the position of the equilibrium will shift in the direction that counteracts the stress.

The kinds of stresses that occur:

1. Product removed. Shifts to the right to produce more product.
2. Reactant removed. Shifts to the left to compensate the loss.
3. Product added. Shifts to the left to partially consume the added product.
4. Reactant added. Shifts to the right to partially consume the added reactant.
5. Temperature raised/lowered. Depends on reaction.

endothermic: aA + bB + heat <==> cC + dD

exothermic: aA + bB <==> cC + dD + heat

6. Volume decrease. Volume dcrease causes a pressure increase. Shifts to the side that would relieve the pressure (less moles of gas).
7. Volume increase causes a pressure decrease. Shifts to the side to increase the pressure (more moles of gas).

In general if Q < K, the reaction procedes to the right. If Q > K, the recations procedes to the left.