Equilibrium Constant, Kc

Equilibrium expression is written using information of “equal rate of forward and reverse reactions”.

For the reaction:            aA + bB ⇔ cC + dD

Rate expression for forward reaction is        R_f = k_f.[A]^a[B]^b

Rate expression for reverse reaction is         R_r = k_r.[C]^c[D]^d

At equilibrium                   R_f = R_r

k_f.[A]^a[B]^b = k_r.[C]^c[D]^d

(EQUILIBRIUM EXPRESSION)

K_c: the equilibrium constant in terms of concentrations.

THE REACTION QUOTIENT, Q

The reaction quotient, Q, is the ratio of the concentrations of the reactants and products as in the equilibrium expression at any point in time.

For example;        The reaction quotient for the reaction

H_2(g) + I_2(g) ⇔ 2HI_(g)

• Q = Kc ⇒ the reaction is at equilibrium; no net reaction occurs.
• Q < Kc ⇒ reaction proceeds to the right in favor of products.
• Q > Kc ⇒ reaction proceeds to the left in favor of reactants.

 

Relationships between Kc for different equations of a reaction

aA + bB ⇔ cC + dD        Kc

• Inverse of this reaction:

cC + dD ⇔ aA + bB        Kc’

has an equilibrium constant as Kc’ = 1/Kc = Kc^-1

• Double of this reaction:

2aA + 2bB ⇔ 2cC + 2dD          Kc’’

has an equilibrium constant as Kc’’ = (Kc)²

• If the reactions are added to each other,

aA + bB ⇔ cC + dD        Kc₁

cC + dD ⇔ eE + fF Kc₂

the net reaction

aA + bB ⇔ eE + fF          Kc’’’

has an equilibrium constant as Kc’’’ = (Kc₁) * (Kc₂).