Chapter 9: Chemical Equilibrium

1. Encircle the correct answer.

(i) Which is true about the equilibrium state?

(a) The forward reaction stops.

(b) The reverse reaction stops.

(c) Both forward and reverse reactions stop.

(d) Both forward and reverse reactions continue at the same rate.

Answer: (d) Both forward and reverse reactions continue at the same rate.

Explanation: Chemical equilibrium is a dynamic state where the rate of the forward reaction equals the rate of the reverse reaction. Both reactions continue to occur, but there is no net change in the concentrations of reactants and products.

(ii) When a mixture of I₂ and H₂ is sealed in a flask and temperature is kept at 25°C, following equilibrium is established. H₂(g) + I₂(g) [source symbol unavailable] 2HI(g) Which substance or substances will be present in the equilibrium mixture?

(a) H₂ and I₂

(b) HI only

(c) H₂ only

(d) H₂, I₂ and HI

Answer: (d) H₂, I₂ and HI

Explanation: At equilibrium, both reactants and products are present in the mixture. Since the reaction is reversible, H₂ and I₂ will not be completely converted to HI. Therefore, the equilibrium mixture contains H₂, I₂, and HI.

(iii) Concentration of reactants and products at equilibrium remains unchanged if

(a) concentration of any reactant or product is not changed.

(b) temperature of the reaction is not changed.

(c) pressure or volume of the system is not changed.

(d) all of the above are observed

Answer: (d) all of the above are observed

Explanation: The equilibrium position can be disturbed by changes in concentration, temperature, pressure, or volume. If none of these factors change, the concentrations of reactants and products remain constant at equilibrium.

(iv) Which of the following does not happen, when a system is at equilibrium state?

(a) forward and reverse reactions stop.

(b) forward and reverse rates become equal.

(c) concentration of reactants and products stop changing.

(d) reaction continues to occur in both the directions.

Answer: (a) forward and reverse reactions stop.

Explanation: At equilibrium, the reactions do not stop; they continue at equal rates. This is why it is called a dynamic equilibrium.

(v) In an irreversible reaction equilibrium is

(a) established quickly

(b) established slowly

(c) never established

(d) established when reaction stops.

Answer: (c) never established

Explanation: Irreversible reactions go to completion. The reactants are completely converted into products, and the reverse reaction does not occur. Therefore, equilibrium is never established.

2. Give short answer.

(i) Differentiate between forward and reverse reactions.

Answer:

(ii) What is chemical equilibrium?

Answer: Chemical equilibrium is a dynamic state in a reversible chemical reaction where the rate of the forward reaction equals the rate of the reverse reaction. At this point, the concentrations of reactants and products remain constant over time, although both reactions continue to occur. The system appears static macroscopically but is highly dynamic at the molecular level. It can be represented as: Reactants [source symbol unavailable] Products.

(iii) Write two chemical equations of reversible reactions.

Answer:

1. Haber Process: N₂(g) + 3H₂(g) [source symbol unavailable] 2NH₃(g)

2. Formation of Hydrogen Iodide: H₂(g) + I₂(g) [source symbol unavailable] 2HI(g)

(iv) Write down the conditions for equilibrium.

Answer: The conditions for equilibrium are:

1. Reversible Reaction: The reaction must be reversible ([source symbol unavailable]).

2. Closed System: The reaction must occur in a closed system where no reactants or products can escape.

3. Constant Temperature: The temperature must remain constant.

4. Equal Rates: The rate of the forward reaction must equal the rate of the reverse reaction.

5. Constant Concentrations: The concentrations of reactants and products must remain constant.

3. Coal reacts with hot steam to form CO and H₂. These substances react further in the presence of a catalyst to give methane and water vapour. CO(g) + 3H₂(g) [source symbol unavailable] CH₄(g) + H₂O(g) Write forward and reverse reactions for it.

Answer:

Forward Reaction: CO(g) + 3H₂(g) [source symbol unavailable] CH₄(g) + H₂O(g) Reverse Reaction: CH₄(g) + H₂O(g) [source symbol unavailable] CO(g) + 3H₂(g)

4. How does temperature affect cobalt chloride equilibrium?

Answer: The equilibrium for cobalt chloride in water is: [Co(H₂O)₆]²⁺(aq) + 4Cl⁻(aq) [source symbol unavailable] [CoCl₄]²⁻(aq) + 6H₂O(l) (Pink) (Blue)

Effect of Increasing Temperature: The forward reaction is endothermic (absorbs heat). According to Le Chatelier's principle, increasing temperature shifts the equilibrium to the right (towards the blue [CoCl₄]²⁻ complex). The solution turns blue. Effect of Decreasing Temperature: Decreasing temperature shifts the equilibrium to the left (towards the pink [Co(H₂O)₆]²⁺ complex). The solution turns pink.

1. Bromine chloride (BrCl) decomposes to form chlorine and bromine. Write reversible chemical reaction for this reaction.

Answer: The reversible chemical reaction for the decomposition of bromine chloride is: 2BrCl(g) [source symbol unavailable] Br₂(g) + Cl₂(g)

(Note: No specific Concept Assessment Exercises were provided in the images for this unit. If they were provided, they would be added here.)