Chemqueries: Thermodynamics of equilibrium

Friday, 9 November 2018

Thermodynamics of equilibrium

Reaction Quotient(Q):

         At each point in a reaction ,we can formulate a ratio of concentration terms having the same form as the equilibrium constant expression. This ratio is called the reaction quotient denoted by symbol Q.
It helps in predicting the direction of a reaction.

For a general reaction:

     aA(g) + bB(g) ⇔ cC(g) + dD(g) 

      Q=[C]tc[D]td/ [A]ta[B]tb

  1. If the net reaction is at equilibrium,Q= Kc
  2. A the net reaction proceeding from right to left(the reverse direction) if  ,Q > Kc

Thermodynamics of equilibrium:

     For a general reaction, mA + nB ⇔ pC + qD 

∆G =∆G⁰ + 2.303RTlog10Q

Where,    ∆G   = Gibb's free energy change.
                ∆G⁰ = Standart Gibb's free energy change.
                Q     =  Reaction quotient.
 Since at equilibrium,Q=K
 Here K is the thermodynamic equilibrium constant replacing  Kc or  Kp 
Here aX   denotes the activity of x.
In fact,is the ratio of activity of substance at equilibrium and its activity in standard condition.That is why it is unitless and K is also unitless. 


  1. Thermodynamics equilibrium constant is unitless since activity is unitless.
  2. For pure solids and pure liquids activityis unity.
  3. For gases (ideal behavior),the activity is partial pressure(in atm)
  4. For components in solution, activity is molar concentration
  5. For endothermic reaction()value of equilibrium constant increases with the rise in temperature.
  6. For exothermic reaction, value of equilibrium constant decreases with increase in temperature.

Condition of spontaneity: 

  •    ∆G<0 for a spontaneous process or reaction.                 
             Since, ∆G = ∆H - T∆S
                   = > ∆H - T∆S < 0
                   = >  T >  ∆H/T∆S
  • ∆G > 0 for a non spontaneous process or reaction
  • ∆G = 0 for a equilibrium reaction

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