Council for the Indian School Certificate Examinations (CISCE / ICSE / ISC) Class 11 Chemistry: Factors Affecting Equilibria (Le Chatelier's Principle)

Instructions. Answer all questions. Fill exactly one bubble per question on the OMR sheet.

1. Factors Affecting Equilibria (Le Chatelier's Principle)

   1.

   Consider the following reversible reaction: $N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)$ (exothermic). Which of the following conditions would shift the equilibrium to favor the formation of ammonia ($NH_3$)?

   1. (A)

      Increasing the temperature and decreasing the pressure.

   2. (B)

      Increasing the temperature and increasing the pressure.

   3. (C)

      Decreasing the temperature and decreasing the pressure.

   4. (D)

      Decreasing the temperature and increasing the pressure.

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   Answer: (D)

   Decreasing the temperature and increasing the pressure.

   According to Le Chatelier's Principle, for an exothermic reaction, decreasing the temperature favors product formation. For a reaction with fewer moles of gas on the product side (2 moles $NH_3$ vs 4 moles $N_2 + 3H_2$), increasing the pressure favors product formation.

   Factors Affecting Equilibria (Le Chatelier's Principle) · Medium

2. 2.

   For the reaction $PCl_5(g) \rightleftharpoons PCl_3(g) + Cl_2(g)$, what would be the effect of adding an inert gas like Helium to the reaction mixture at constant volume?

   1. (A)

      The equilibrium constant will increase.

   2. (B)

      The equilibrium position will remain unchanged.

   3. (C)

      The equilibrium will shift to the right, favoring product formation.

   4. (D)

      The equilibrium will shift to the left, favoring reactant formation.

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   Answer: (B)

   The equilibrium position will remain unchanged.

   Adding an inert gas at constant volume increases the total pressure but does not change the partial pressures or molar concentrations of the reactants and products. Therefore, the equilibrium position remains unchanged.

   Factors Affecting Equilibria (Le Chatelier's Principle) · Medium

3. 3.

   Assertion (A): Increasing the concentration of a reactant in a reversible reaction at equilibrium will shift the equilibrium towards the product side.Reason (R): According to Le Chatelier's Principle, the system will try to counteract the change by consuming the added reactant.

   1. (A)

      A is true but R is false.

   2. (B)

      Both A and R are true but R is not the correct explanation of A.

   3. (C)

      Both A and R are true and R is the correct explanation of A.

   4. (D)

      A is false but R is true.

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   Answer: (C)

   Both A and R are true and R is the correct explanation of A.

   Both the assertion and the reason are true, and the reason correctly explains why increasing reactant concentration shifts the equilibrium towards products, as the system tries to relieve the stress by consuming the added reactant.

   Factors Affecting Equilibria (Le Chatelier's Principle) · Easy

4. 4.

   Consider the equilibrium $2SO_2(g) + O_2(g) \rightleftharpoons 2SO_3(g)$ (exothermic). If a catalyst is added to this system at equilibrium, what effect will it have?

   1. (A)

      It will shift the equilibrium to the right, increasing the yield of $SO_3$.

   2. (B)

      It will increase the rate of both forward and reverse reactions equally, reaching equilibrium faster.

   3. (C)

      It will shift the equilibrium to the left, decreasing the yield of $SO_3$.

   4. (D)

      It will increase the equilibrium constant $K_c$.

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   Answer: (B)

   It will increase the rate of both forward and reverse reactions equally, reaching equilibrium faster.

   A catalyst increases the rate of both the forward and reverse reactions equally, allowing the system to reach equilibrium faster, but it does not change the position of the equilibrium or the value of the equilibrium constant.

   Factors Affecting Equilibria (Le Chatelier's Principle) · Medium

5. 5.

   The dissolution of ammonium chloride in water is an endothermic process: $NH_4Cl(s) + H_2O(l) \rightleftharpoons NH_4^+(aq) + Cl^-(aq)$. If you want to increase the solubility of ammonium chloride, what condition should you apply?

   1. (A)

      Add more $NH_4Cl(s)$.

   2. (B)

      Increase the pressure.

   3. (C)

      Increase the temperature.

   4. (D)

      Decrease the temperature.

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   Answer: (C)

   Increase the temperature.

   Since the dissolution of ammonium chloride is an endothermic process (absorbs heat), according to Le Chatelier's Principle, increasing the temperature will shift the equilibrium to the right, favoring dissolution and thus increasing solubility.

   Factors Affecting Equilibria (Le Chatelier's Principle) · Hard

— End of the paper · updated 8 October 2026 —