Equilibria Chapter

Equilibria Class 9th Chemistry – Chapter 6 Solved Exercises (2025 Punjab Boards)

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his web page provides detailed solutions and explanations for the Equilibria chapter (Chapter 6) of Class 9th Chemistry, designed according to the 2025 Punjab Board syllabus. The content includes solved exercises, short and long answer questions, and additional tips to help students prepare for exams. Specifically crafted for Lahore Board and other Punjab Boards, it ensures clarity and understanding with simplified explanations.

Solutions to the given MCQs:

(i) What will happen if the rates of forward and reverse reactions are very high?

  • Options:
    1. The equilibrium point will reach very soon.
    2. The equilibrium point will reach very late.
    3. The reaction will not attain the state of dynamic equilibrium.
    4. The reaction will be practically irreversible.
  • Correct Answer: (a) The equilibrium point will reach very soon.
  • Explanation:
    When the rates of the forward and reverse reactions are very high, equilibrium is established faster as the reaction quickly balances the forward and reverse processes.

(ii) Predict which components of the atmosphere react in the presence of lightning.

  • Options:
    1. N2 and H2O
    2. O2 and H2O
    3. CO2 and O2
    4. N2 and O2
  • Correct Answer: (d) N2 and O2
  • Explanation:
    Lightning provides energy for the nitrogen and oxygen in the atmosphere to react, forming nitrogen oxides (NO and NO2).

(iii) An inorganic chemist places one mole of PCl5 in container A and one mole each of Cl2 and PCl3 in container B. Both containers are sealed and heated to the same temperature to reach equilibrium. Guess the composition of mixtures in both containers.

  • Options:
    1. Both containers will have the same composition of mixtures.
    2. Container A will have more concentration of PCl5 than B.
    3. Container A will have less concentration of PCl5 than B.
    4. Both containers will have zero concentration of its reactants.
  • Correct Answer: (a) Both containers will have the same composition of mixtures.
  • Explanation:
    Regardless of the initial concentrations, equilibrium is defined by the same equilibrium constant at a given temperature. Therefore, both containers will reach the same composition of mixtures.

(iv) Choose the conditions to produce the maximum amount of lime (CaO) in the decomposition of lime (CaCO3).

  • Options:
    1. Heating at high temperature in a closed vessel.
    2. Heating at high temperature in an open vessel.
    3. Cooling in a closed vessel.
    4. Cooling in an open vessel.
  • Correct Answer: (b) Heating at high temperature in an open vessel.
  • Explanation:
    Decomposition of CaCO3 into CaO and CO2 is favored by removing CO2 gas (a product) continuously. This is best achieved in an open system.

(v) What condition should be met for the reversible reaction to achieve the state of equilibrium?

  • Options:
    1. All the reactants should be converted into products.
    2. 50% of the reactants should be converted into products.
    3. The concentration of all the reactants and products should become equal.
    4. The rate of the forward and reverse reactions should become equal.
  • Correct Answer: (d) The rate of the forward and reverse reactions should become equal.
  • Explanation:
    At equilibrium, the forward and reverse reaction rates are equal, leading to constant concentrations of reactants and products (though not necessarily equal in value).

(vi) Why does gas start coming out when you open a can of fizzy drink?

  • Options:
    1. Because the solubility of the gas increases.
    2. Because the gas is insoluble in water.
    3. Because the gas is dissolved under pressure and comes out when the pressure is decreased.
    4. Because the solubility of the gas decreases at high pressure.
  • Correct Answer: (c) Because the gas is dissolved under pressure and comes out when the pressure is decreased.
  • Explanation:
    Carbon dioxide is dissolved in the drink under high pressure. When the can is opened, the pressure decreases, causing the gas to come out of the solution.

(vii) The following reaction is performed in an open vessel: CaCO3(s)↔CaO(s)+CO2(g). How will the equilibrium be affected if you increase the pressure?

  • Options:
    1. The forward reaction will be favored.
    2. The backward reaction will be favored.
    3. No effect on the backward reaction.
    4. No effect on forward or backward reaction.
  • Correct Answer: (b) The backward reaction will be favored.
  • Explanation:
    Increasing pressure favors the reaction that decreases the number of gaseous molecules. In this case, the backward reaction reduces CO2, which is a gas.

(viii) When will a reaction become a reversible one?

  • Options:
    1. If the activation energy of the forward reaction is comparable to that of the backward reaction.
    2. If the activation energy of the forward reaction is higher than that of the backward reaction.
    3. If the activation energy of the forward reaction is lower than that of the backward reaction.
    4. If the enthalpy change of both the reactions is zero.
  • Correct Answer: (a) If the activation energy of the forward reaction is comparable to that of the backward reaction.
  • Explanation:
    For a reaction to be reversible, both the forward and backward reactions must occur at comparable rates, which requires similar activation energies.

(ix) Is a reversible reaction useful for preparing compounds on a large scale?

  • Options:
    1. No.
    2. Yes.
    3. They are useful only when equilibrium lies far to the right side.
    4. They are useful only when equilibrium lies far to the left side.
  • Correct Answer: (c) They are useful only when equilibrium lies far to the right side.
  • Explanation:
    To maximize product yield in a reversible reaction, the equilibrium must favor product formation, meaning it must lie far to the right.

(x) What will happen to the concentrations of the products if a reversible reaction at equilibrium is not disturbed?

  • Options:
    1. They will remain constant.
    2. They will keep on increasing.
    3. They will keep on decreasing.
    4. They will first increase and then decrease.
  • Correct Answer: (a) They will remain constant.
  • Explanation:
    At equilibrium, the rates of the forward and reverse reactions are equal, so the concentrations of products and reactants remain unchanged.

Answers to the Questions:

2. Questions for Short Answers

(i) How is dynamic equilibrium different from static equilibrium?

  • Answer:
    In dynamic equilibrium, the forward and backward reactions occur at the same rate, maintaining constant concentrations of reactants and products. In static equilibrium, there is no movement or reaction, and the system is at rest.

(ii) How will the following reversible reaction be affected if its temperature is increased?
Reaction: 2H2O(l)→Electricity 2H2(g)+O2(g)

  • Answer:
    Increasing temperature will favor the forward reaction because the decomposition of water into hydrogen and oxygen is an endothermic process, which absorbs heat.

(iii) How can you get the maximum yield in a reversible reaction?

  • Answer:
    Maximum yield can be obtained by:
    • Adjusting temperature and pressure to favor the desired reaction.
    • Removing the product as it forms to shift equilibrium toward the product side.

(iv) How can you decrease the time to attain the position of equilibrium in a reversible reaction?

  • Answer:
    • Increase the concentration of reactants.
    • Use a catalyst to speed up the forward and reverse reactions.
    • Raise the temperature to increase reaction rates (if temperature is favorable).

(v) What is the effect of increasing pressure on the following reaction?
Reaction: N2+O2↔2NO

  • Answer:
    There is no change because the number of moles of gas on both sides of the reaction is the same. Pressure changes do not affect equilibrium in such cases.

3. Constructed Response Questions

(i) Why are some reactions irreversible while others are reversible?

  • Answer:
    Irreversible reactions go to completion because their products are stable and cannot revert to reactants (e.g., combustion). Reversible reactions occur when products can react back to form the reactants, maintaining equilibrium.

(ii) Why are combustion reactions generally irreversible?

  • Answer:
    Combustion reactions are highly exothermic, releasing large amounts of energy. The products (e.g., CO2 and H2O) are stable and cannot revert to the reactants under normal conditions.

(iii) Can you make an irreversible reaction reversible and vice versa?

  • Answer:
    Some irreversible reactions can become reversible under specific conditions, such as high pressure or low temperature. However, most irreversible reactions (e.g., combustion) cannot be reversed due to product stability.

(iv) How do you know if a reaction is reversible or irreversible?

  • Answer:
    A reaction is reversible if:
    • The products can convert back into reactants.
    • It reaches equilibrium.
      A reaction is irreversible if it goes to completion without equilibrium.

(v) Do the phase changes in water (solid to liquid, liquid to vapor) reverse or irreversible?

  • Answer:
    Phase changes in water are reversible because the process can go back and forth under appropriate temperature and pressure conditions.

4. Descriptive Questions

(i) How can you drive the reversible reaction at equilibrium?

  • Answer:
    • In the forward direction: Increase the concentration of reactants, remove products, or adjust temperature/pressure to favor the forward reaction.
    • In the backward direction: Increase the concentration of products or decrease reactants.

(ii) Explain how the forward and backward reactions change when the system approaches equilibrium.

  • Answer:
    Initially, the forward reaction rate is high, and the backward rate is low. As products form, the backward reaction rate increases. At equilibrium, both reaction rates become equal.

(iii) Describe the effect of a catalyst on the reversible reaction.

  • Answer:
    A catalyst speeds up both the forward and backward reactions equally, helping the system reach equilibrium faster without changing the equilibrium position.

(iv) How can a reversible reaction be forced to go to completion?

  • Answer:
    • Continuously remove the product.
    • Adjust temperature and pressure to favor the desired reaction completely.
    • Use excess reactants.

(v) How does a change in temperature affect the reaction at equilibrium?

  • Answer:
    • For an exothermic reaction: Increasing temperature shifts equilibrium to the reactants.
    • For an endothermic reaction: Increasing temperature shifts equilibrium to the products.

5. Investigate

(i) Study the effect of heat on hydrated CuSO4. Why does this salt look colored, and why does it lose color upon heating?

  • Answer:
    Hydrated CuSO4CuSO_4 (blue in color) loses its water molecules on heating, forming anhydrous CuSO4 (white). The color is due to the presence of water of crystallization, which is lost upon heating.

(ii) Synthesis of ammonia gas is very important industrially because it is used in the preparation of urea fertilizer. Explain the conditions you will use to get the maximum yield of ammonia.

  • Answer:
    • Use high pressure to favor the forward reaction (fewer moles of gas).
    • Use moderate temperature (450°C) to balance rate and yield (since the reaction is exothermic).
    • Use an iron catalyst to speed up the reaction.

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