Chemqueries: Le-Chatelier's principle

Saturday, 10 November 2018

Le-Chatelier's principle

Le-Chatelier's principle:

  • This is based on the fundamental of a stable equilibrium.
  • It states,when a system at equilibrium is subjected to a change in temperature,pressure or concentration of a reacting species,the system react in a way that partially offset the change while reaching a new state of equilibrium.

Effect of concentration:

If the concentration of a reacting component is increased,reaction shifts in a direction which tends to decrease its concentration.e.g. in the following example.

N2(g) + 3H2(g)  2NH3
[reactant] ↑         foreward shift
[product] ↑          backward shift
  • If the concentration of reactant is increased at equilibrium then reaction shifts in the foreward direction.
  • If concentration of product is increased then reaction shifts in backward direction.
Note: The addition of any solid component doesnot effect the concentration. 

Effect of volume:

  • If volume is increased,pressure decreases hence reaction will shift in the direction in which pressure increases that is in the direction in which number of moles of gases increases and vice versa.
  • If volume is increased then for,
          1. ∆ng > 0 ,reaction will shift in the foreward direction 
                PCl5(g)  PCl3(g) + Cl2(g)  , ∆ng = 2-1 =1 

          2. ∆ng < 0 ,reaction will shift in the backward direction
                 N2(g) + 3H2(g) = 2NH3(g)  , ∆ng =2-4= -2 < 0

          3. ∆ng= 0 ,reaction will not shift e.g.
              H2(g) + I2(g)   2HI(g)  (no effect)

Effect of pressure:

  • On increasing pressure,equilibrium will shift in the direction in which pressure decreases,i.e. number of moles in the reaction decreases and vice versa.
  • Pressure is directly proportional to number of moles.
           1. For ∆ng= 0 ,no effect
           2. For  ∆ng > 0
               If pressure decreases equilibrium shifts to forward direction.
               If pressure increases equilibrium shifts to backward direction.
           3.For  ∆ng < 0
               If pressure  increases equilibrium shifts to forward direction.
               If pressure decreases equilibrium shifts to backward direction.

Effect of catalyst:

  • Due to catalyst ,state of equilibrium is not affected i.e.no shift will occur as catalyst lowers the activation energy of both the forward and reverse reaction by same amount.
  • Thus alternating the forward and reverse rate equally and hence, the equilibrium will be attained faster i.e.time taken to reach the equilibrium is less.

Effect of inert gas addition:

  • at constant volume: Inert gas addition has no effect at constant volume  
  • at constant pressure: If inert gas is added at constant pressure,volume is increased and hence equilibrium will shift in that direction in which larger number of gaseous moles are formed.
           1.∆ng > 0 , reaction will shift in forward direction.
           2.∆ng < 0 , reaction will shift in backward direction.
           3.∆ng= 0 ,  no effect

Effect of temoerature:

  • :Exothermic reaction:The reaction in which heat is evolved.
            1.If temperature increases equilibrium will shift in backward direction.
            2.If temperature decreases equilibrium will shift in forward direction.
  • Endothermic reaction: The reaction in which heat is consumed
           1.If  temperature increases equilibrium will shift in forward direction.
           2..If  temperature decreases equilibrium will shift in backward direction.

No comments:

Post a Comment