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The Equilibrium constant, Keq

Most chemical processes are reversible. That is, reactant react to form products, but those products can also react to form reactants. When a reaction is at equilibrium, the rate of forward reaction is equal to the rate of the reverse reaction.

The Equilibrium Expression

aA + bB <--> cC + dD

Keq = ([C]^c [D]^d)/ ([A] ^a [B]^b)

1. [A], [B], [C] and [D] are molar concentrations or paritial pressures at equilibrium

2. Products are in the numerator, and the reactatns are in the denominator

3. Coefficients in the balanced equation become exponents in the equilibrium expression

4. solids and pure liquids are not included in the equilibium expression- only acueous reactants and products are included

5. Units are not given for Keq

Note: a. Keq is the constant for molar concentration

b. Kp is the constant for partial pressure

c. Keq is the solubility product, which has no denominator because the reactants are solids

d. Ka is the acid dissociation constant for weak bases

e. Kb is the base dissociation constant for weak bases

f. Kw is the ionization of water

The reaction Quotient, Q

The reaction quotient is determined in exactly the same way as the equilibrium constant, but initial conditions are used in place of equilibrium conditions. The reaction quotient can be used to predict the direction in which a reaction will proceed from a given set of initial condition

1. if Q is less than the calculated K for the reaction, the reaction proceeds forward, generating products

2. if Q is greater than K, the reaction proceeds backward, generating reactants

3. If Q= K, the reaction is already at equlibrium.