Learning Objectives

  • Write the expression for the equilibrium constant for a given reversible reaction.
  • Understand what is meant by reaction quotient.
  • Understand the connection between the size of the equilibrium constant and the extent of reaction.
  • Understand that it is possible to write more than one equilibrium constant for a particular reaction.

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        Rf = kf.[A]a[B]b

Rate expression for reverse reaction is         Rr = kr.[C]c[D]d

At equilibrium                   Rf = Rr

kf.[A]a[B]b = kr.[C]c[D]d

(EQUILIBRIUM EXPRESSION)

Kc: the equilibrium constant in terms of concentrations.

Write the equilibrium expression for the following reactions.

  • 2H2(g) + O2(g) ⇔ 2H2O(g)
  • Cu2+(aq) + 4NH3(aq) ⇔ [Cu(NH3)4]2+(aq)

Write the equilibrium expression for the following reactions.

  • 2H2(g) + O2(g) ⇔ 2H2O(g)

  • Cu2+(aq) + 4NH3(aq) ⇔ [Cu(NH3)4]2+(aq)

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

H2(g) + I2(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).