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Le Chatelier's Principles on Equilibrium MCQ - Practice Questions with Answers

Edited By admin | Updated on Sep 25, 2023 25:23 PM | #NEET

Quick Facts

  • Le Chatelier’s principle(P, C), Le Chatelier’s principle(T, Innert Gas) is considered one of the most asked concept.

  • 31 Questions around this concept.

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For the reversible reaction :

N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g) + heat

The equilibrium shifts in forward direction :

If the value of an equilibrium constant for a particular reaction is 1.6 \times 10^{12}, then at equilibrium the system will contain:

Concepts Covered - 3

Reaction coefficient/quotient

It is defined as the ratio of concentration of products to the concentration of the reacting species raised to their stoichiometric coefficient at any point of time other than the equilibrium stage. It has the exact same expression as that of the Equilibrium constant except that the concentration values are at any instant. Mathematically, it can be determined as follows:


If we consider a reaction
\mathrm{m A +n B \rightleftharpoons pC +q D}

\mathrm{Q\: =\: \frac{[C]^{p}[D]^{q}}{[A]^{m}[B]^{n}}}
Q can be denoted as Qc or Qp if we use concentration in terms of mole per litre or partial pressure respectively.

The value of Q is useful to determine the direction in which the equlibrium will shift at any instant for a particular set of activity of the species involved.

  • When Q = K, the reaction is at equilibrium and rate of forward and backward reactions are equal.
  • When Q > K, the reaction will proceed or favour backward direction. This means products convert into reactant to attain equilibrium.
  • When Q < K, the reaction will proceed or favour forward direction. This means that the reactants convert into product to attain equilibrium.

Relation between K, q and \mathrm{\Delta G}

\mathrm{\Delta G = \Delta G^0 + RTlnQ}\ \ \ \ \rightarrow (1)

where 

\mathrm{\Delta G} = \text{Change in Gibbs Free energy}

\mathrm{\Delta G^0} = \text{Change in Gibbs Free energy under Standard Conditions}

\text{Q = Reaction quotient}

Now,  we know that 

\mathrm{\Delta G^0= -RTlnK_{eq}}

Putting this value in equation (1)

\mathrm{\Delta G = -RT lnK_{eq} + RTlnQ}

which can be simplified to 

\mathrm{\Delta G = RT ln\left ( \frac{Q}{K_{eq}}\right )}\ \ \ \ \rightarrow (2)

From Equation (2) it is clear that 

  • \mathrm{When\ Q = K_{eq},\ \Delta G = 0} \text{ and the reaction is at equilibrium}
  • \mathrm{When\ Q < K_{eq},\ \Delta G < 0} \text{ the reaction will move in the forward direction}
  • \mathrm{When\ Q > K_{eq},\ \Delta G > 0} \text{ the reaction will move in the backward direction}
Le Chatelier’s principle(P, C)

It describes the effect of change in concentration, pressure and temperature on the reversible system.
According to it, "If the system at equilibrium is subjected to a change of concentration or temperature or pressure, the system adjusts itself in such a way as to nul the effect of that change i.e., the effect of these changes can be neglected or minimized."

Effect of Concentration

  • An increase in the concentration of any substance favours the reaction in which it is used up i.e, in the opposite direction.

  • An increase of concentration of reactant favours the formation of more product i.e., forward reaction. Increase in concentration of product favours.

  • Increase in concentration of product favours backward reaction. I.e., its continuous removal is essential for more formation of it.

Effect of Pressure

  • High pressure is favourable for the reaction in which there is a decrease in volume or nr > np.

  • Low pressure is favourable for the reaction in which there is an increase in volume or nr < np.

  • Pressure is kept constant when the volume is constant or nr = np.

Here nr = moles of gaseous reactant

np = moles of gaseous product

Le Chatelier’s principle(T, Innert Gas)

Effect of change in temperature

On increasing the temperature, equilibrium shifts to that direction which proceeds with the absorption of heat.

  • \mathrm{A\: +\: B\:+\: \Delta \: \rightleftharpoons \: C\: +\: D}
    This is an endothermic reaction. Thus, on increasing the temperature, equilibrium shifts in the forward direction.
     
  • \mathrm{A\: +\: B\:: \rightleftharpoons \: C\: +\: D\: +\: \Delta }
    This is an exothermic reaction. Thus on increasing the temperature, equilibrium shifts in backward direction.

For example:

\mathrm{H_{2}\: +\: I_{2}\: \rightleftharpoons \: 2HI+\: 3000\, calories }
In this reaction, the product formed is HI and the release of 3000 calories of energy. Thus if temperature is increased then equilibrium will shift backward and form the reactants. 

Effect of Adding Inert Gas on Equilibrium

  • When np is equal to nr there is no effect of adding an inert gas either at constant volume or pressure.
  • When np nr there is no effect of adding an inert gas at constant volume.
  • When np nr at constant pressure on adding inert gas equilibrium will shift towards more volume side. e.g., dissociation of ammonia will be more at constant pressure by adding inert gas like argon (Ar).

Study it with Videos

Reaction coefficient/quotient
Le Chatelier’s principle(P, C)
Le Chatelier’s principle(T, Innert Gas)

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Books

Reference Books

Reaction coefficient/quotient

Physical Chemistry

Page No. : 714

Line : 10

Le Chatelier’s principle(P, C)

Chemistry Part I Textbook for Class XI

Page No. : 209

Line : 25

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