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).