Class 9 Chemistry Notes – Chapter 6

Subject: Chemistry

Class: 9th

Board: Punjab Board

Chapter 6: Equilibria

Exercise: Multiple Choice Questions (MCQs)

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

Answer: (a) The equilibrium point will reach very soon.

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

Answer: (d) N₂ and O₂

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

Answer: (a) Both containers will have the same composition of mixtures.

(iv) CaO or lime is used extensively in steel, glass, and paper industries. It is produced in an endothermic reversible reaction by the decomposition of lime (CaCO₃). Choose the conditions to produce the maximum amount of lime.

Answer: (b) Heating at high temperature in an open vessel.

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

Answer: (c) The concentrations of all the reactants and products should become constant.

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

Answer: (c) Because the gas is dissolved under pressure; hence, it comes out when pressure is decreased.

(vii) The following reaction is performed in a closed vessel:

CaCO₃(s) ⇌ CaO(s) + CO₂(g)

How will the equilibrium be affected if you increase the pressure?

Answer: (b) The backward reaction will be favoured.

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

Answer: (a) If the activation energy of the forward reaction is comparable to that of the backward reaction.

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

Answer: (c) They are useful only when equilibrium lies far to the right side.

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

Answer: (a) They will remain constant.

Exercise: Questions for Short Answers

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

Answer: Dynamic equilibrium occurs in reversible chemical reactions where both forward and reverse processes continue at equal rates, keeping reactant and product concentrations constant. Static equilibrium occurs when all physical movement or chemical reactions have completely ceased, such as a building standing at rest.

(ii) How will the following reversible reaction be affected if its temperature is increased?

2H₂O(l) ⇌ 2H₂(g) + O₂(g)

Answer: Since the decomposition of water into hydrogen and oxygen is an endothermic process, increasing the temperature will shift the equilibrium position in the forward direction, producing more H₂ and O₂ gases.

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

Answer: Maximum yield can be obtained by continuously removing products from the reaction mixture, increasing reactant concentrations, and optimizing temperature and pressure conditions according to Le Chatelier’s principle.

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

Answer: The time taken to reach equilibrium can be decreased by using a suitable catalyst, increasing the temperature, or increasing the concentrations or surface area of the reactants to speed up both forward and reverse reaction rates equally.

(v) What is the effect of increasing pressure on the following reaction at equilibrium?

N₂(g) + O₂(g) ⇌ 2NO(g)

Answer: Increasing pressure will have no effect on the position of equilibrium because the total number of moles of gaseous reactants (2 moles) is equal to the total number of moles of gaseous products (2 moles).

Exercise: Constructed Response Questions

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

Answer: Irreversible reactions proceed to completion in one direction because the products are highly stable or escape from the reaction mixture, requiring enormous activation energy for the reverse reaction. Reversible reactions have comparable activation energies for both forward and backward pathways, allowing products to recombine into reactants under identical conditions.

(ii) Why are combustion reactions generally irreversible?

Answer: Combustion reactions are strongly exothermic and release vast amounts of energy while producing gaseous products like CO₂ and H₂O that escape into the atmosphere, making recombination impossible under ambient conditions.

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

Answer: Yes, an irreversible reaction involving a gas can often be made reversible by carrying it out in a closed container to prevent gas escape. Conversely, a reversible reaction can be rendered irreversible by allowing one of the products to continuously escape or precipitate out of the system.

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

Answer: A reaction is reversible if it does not go to completion, shows double arrows (⇌) in its chemical equation, maintains constant concentrations of reactants and products in a closed system, and yields products that can react back to form the original reactants. An irreversible reaction proceeds in a single direction (→) to complete consumption of the limiting reactants.

(v) Are the phase changes in water (solid to liquid, liquid to vapour) reversible or irreversible?

Answer: Physical phase changes in water (ice ⇌ liquid water ⇌ water vapour) are fully reversible processes because they only involve alterations in physical state and weak intermolecular forces, allowing complete interconversion by changing temperature or pressure.

Exercise: Descriptive Questions

(i) How can you drive the reversible reaction at equilibrium (a) in the forward direction, (b) in the backward direction?

Answer:

(a) Forward direction: Add more reactants, continuously remove products, or adjust temperature and pressure according to Le Chatelier’s principle to favour the forward process.

(b) Backward direction: Add more products, remove reactants, or alter temperature and pressure in the direction that favours the reverse path.

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

Answer: Initially, the forward reaction rate is at its maximum while the backward reaction rate is zero. As reactants are consumed, the forward rate gradually decreases, and as products accumulate, the backward rate steadily increases. Eventually, both rates become equal, establishing a state of dynamic equilibrium.

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

Answer: A catalyst lowers the activation energy equally for both forward and reverse reactions. Therefore, it speeds up both pathways equally, allowing the system to reach equilibrium much faster without altering the final yield or equilibrium composition.

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

Answer: A reversible reaction can be driven to near completion by continuously removing one or more products from the reaction vessel, such as allowing a gaseous product to escape in an open container or forming an insoluble precipitate.

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

Answer: According to Le Chatelier’s principle, an increase in temperature shifts an equilibrium in the endothermic direction to absorb excess thermal energy, while a decrease in temperature shifts the equilibrium in the exothermic direction.

Exercise: Investigative Questions

(i) Study the effect of heat on hydrated copper sulphate. Why does this salt look coloured, and why does it lose colour upon heating?

Answer: Hydrated copper sulphate (CuSO₄·5H₂O) is blue because of water molecules coordinated to Cu²⁺ ions that cause d–d electron transitions. Upon heating, water of crystallization is driven off to yield anhydrous copper sulphate (CuSO₄), which appears white due to the loss of coordinated water ligands.

(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:

For the reaction:

N₂(g) + 3H₂(g) ⇌ 2NH₃(g) (Exothermic)

• High Pressure: Applied (around 200 atm) because the product side has fewer gas moles (2 moles compared with 4 moles).

• Moderate Temperature: Applied (around 400–450°C) as an optimum balance between high yield (favoured by lower temperature) and an acceptable reaction rate.

• Catalyst: An iron catalyst with a promoter is used to attain equilibrium quickly.

• Product Removal: Ammonia is liquefied and continuously removed from the system.

Notes by Sir Dilawar

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