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Question

What is Le Chatelier's principle?


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Solution

Le Chatelier's principle states that change in any of the factors that characterize the equilibrium of a system will cause the system to change in such a way as to reduce the effect of the change.

  1. Effect of concentration change
    H2(g)+I2(g)2HI(g)
    i) If the concentration of the reactant increases, equilibrium will shift towards the direction where the reactants are consumed. i.e., in the forward direction.
    ii) If the concentration of products increases, Equilibrium will shift towards the direction where the concentration of the product reduces i.e., in the reverse direction.
  2. Effect of pressure change
    i) Pressure change will not affect the equilibrium concentration of solids and liquids.
    ii) It will only affect the gaseous reactions.
    iii) For example - CO(g)+3H2(g)CH4(g)+H2O(g)4moles2moles
    iv) If the volume is halved, then the total pressure will be doubled as pressure is inversely proportional to volume.
    v) We know that pressure is directly proportional to the number of moles.
    vi) As the pressure is increased, the equilibrium will shift in a direction to neutralize the effect i.e, in the direction of a decrease in the number of moles. So, the reaction will shift in the forward direction.
    vii) If the pressure is decreased, the equilibrium will shift in a direction to neutralize the effect i.e, in the direction of an increase in the number of moles. So, the reaction will shift in the reverse direction.
  3. Effect of Temperature change
    i) In an exothermic reaction, heat is released. i.e., ∆H = negative. The equilibrium constant for an exothermic reaction decreases as the temperature increases.
    ii) In an endothermic reaction, heat is absorbed. i.e., ∆H = positive. The equilibrium is constant for an endothermic reaction increases as the temperature increases.
    For example - N2(g)+3H2(g)2NH3(g)H=-92.38kJmol-1
    It is an exothermic reaction, so low temperature facilitates Ammonia production.
  4. Effect of a catalyst
    i) Catalyst does not affect the equilibrium concentration of reactants and products.
    ii) It increases the rate of forward and reverse reactions by providing a low-energy pathway for the reaction.
    iii) It reduces the activation energy for the forward and reverse reactions.
    For example - Haber's process
    N2(g)+3H2(g)2NH3(g)
    Iron is used as a catalyst in this reaction which increases the rate of the reaction.

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