9th Class Chemistry Chapter # 6 Exercise Solutions – Punjab Board
Equilibria
Multiple Choice Questions (MCQs)
(i) What will happen if the rates of forward and reverse reactions are very high?
Options:
(a) The equilibrium point will reach very soon.
(b) The equilibrium point will reach very late.
(c) The reaction will not attain the state of dynamic equilibrium.
(d) The reaction will be practically irreversible.
✅ Correct Answer: (a) The equilibrium point will reach very soon.
📘 Explanation:
If both forward and reverse reactions are fast, equilibrium will be established quickly. The system will still reach dynamic equilibrium, just faster.
(ii) Predict which components of the atmosphere react in the presence of lightning.
Options:
(a) N₂ and H₂O
(b) O₂ and H₂O
(c) CO₂ and O₂
(d) N₂ and O₂
✅ Correct Answer: (d) N₂ and O₂
📘 Explanation:
Nitrogen and oxygen in the air react during lightning because high energy provides enough activation energy to form nitric oxide (NO):
N₂ + O₂ → 2NO
(iii) An inorganic chemist places one mole of Cl₂ and one mole of PCl₅ in container A and one mole each of Cl₂ and PCl₃ in container B… Guess the composition.
Options:
(a) Both the containers will have the same composition of mixtures.
(b) Container A will have more concentration of PCl₅ than B.
(c) Container A will have less concentration of PCl₅ than B.
(d) Both the containers will have zero concentration of its reactants.
✅ Correct Answer: (a) Both the containers will have the same composition of mixtures.
📘 Explanation:
In a reversible reaction, regardless of starting point, the system reaches the same equilibrium composition (at same temperature and pressure).
PCl₅ ⇌ PCl₃ + Cl₂ — no matter where you start, the equilibrium position is the same.
(iv) CaO is produced from CaCO₃ in an exothermic reversible reaction… Choose condition to produce max CaO.
Options:
(a) Heating at high temperature in a closed vessel
(b) Heating at high temperature in an open vessel
(c) Cooling it in a closed vessel
(d) Cooling it in an open vessel
✅ Correct Answer: (b) Heating at high temperature in an open vessel
📘 Explanation:
The reaction: CaCO₃ ⇌ CaO + CO₂ is endothermic in forward direction (although exothermic in total energy release from formation of CaO).
High temperature favors decomposition.
Open vessel allows CO₂ to escape, shifting equilibrium forward (Le Chatelier’s Principle).
→ So, more CaO is formed.
(v) What condition should be met for the reversible reaction to achieve equilibrium?
Options:
(a) All the reactants should be converted into the products.
(b) 50% of the reactants should be converted into products.
(c) The concentrations of all the reactants and the products should become constant.
(d) One of the products should be removed from the reaction mixture.
✅ Correct Answer: (c) The concentrations of all the reactants and the products should become constant.
📘 Explanation:
At dynamic equilibrium, the rate of forward reaction = rate of reverse reaction, so the concentration of reactants and products remains constant, even though reactions are still occurring.
(vi) Why does gas start coming out when you open a can of fizzy drink?
Options:
(a) Because the solubility of the gas increases
(b) Because the gas is insoluble in water
(c) Because the gas is dissolved under pressure hence it comes out when pressure is decreased
(d) Because the solubility of the gas decreases at high pressure
✅ Correct Answer: (c)
📘 Explanation:
In fizzy drinks, CO₂ is dissolved under high pressure. When the can is opened, pressure drops, so the solubility of gas decreases and CO₂ escapes as bubbles.
(vii) The following reaction is performed in a closed vessel:
CaO (solid) + CO₂ (gas) ⇌ CaCO₃ (solid)
How is equilibrium affected if pressure is increased?
Options:
(a) The forward reaction will be favoured
(b) The backward reaction will be favoured
(c) No effect on backward reaction
(d) No effect on forward or backward reaction
✅ Correct Answer: (a)
📘 Explanation:
Since CO₂ is a gas and other substances are solids, increasing pressure favors the side with fewer gas molecules, i.e., the forward reaction (formation of CaCO₃).
(viii) When will a reaction become reversible?
Options:
(a) If the activation energy of the forward reaction is comparable to that of backward reaction
(b) If the activation energy of the forward reaction is higher than that of backward reaction
(c) If the activation energy of the forward reaction is lower than that of backward reaction
(d) If the enthalpy change of both the reactions is zero
✅ Correct Answer: (a)
📘 Explanation:
A reaction is reversible if both forward and backward reactions can occur. This is possible when their activation energies are comparable, allowing both directions to proceed under the same conditions.
(ix) Is reversible reaction useful for preparing compounds on large scale?
Options:
(a) No
(b) Yes
(c) They are useful only when equilibrium lies far to the right side
(d) They are useful only when equilibrium lies far to the left side
✅ Correct Answer: (c)
📘 Explanation:
A reversible reaction is useful only if equilibrium lies far to the right, meaning products are favored. That way, maximum yield of the desired product can be obtained.
(x) What will happen to the concentrations of the products if a reversible reaction at equilibrium is not disturbed?
Options:
(a) They will remain constant
(b) They will keep on increasing
(c) They will keep on decreasing
(d) They will remain constant for some time and then start decreasing
✅ Correct Answer: (a)
📘 Explanation:
At equilibrium, the rate of forward and reverse reactions are equal, so the concentrations of products and reactants remain constant (but not necessarily equal in value).
Short Answer Questions
i. How is dynamic equilibrium different from static equilibrium?
Answer:
Dynamic equilibrium occurs in reversible reactions, where the forward and reverse reactions continue to occur, but at equal rates, so the concentrations of reactants and products remain constant.
✅ Reaction is active, but balanced.Static equilibrium occurs in systems where no changes happen over time, and there is no movement or reaction taking place at all.
❌ No chemical activity continues.
🔁 Dynamic = Moving but balanced
⛔ Static = No movement, no change
ii. How will the following reversible reaction be affected if temperature is increased?
2H2O(liquid)→Electricity2H2(gas)+O2(gas)
Answer:
This reaction is endothermic (requires energy/electricity to break water into hydrogen and oxygen).
According to Le Chatelier’s Principle, increasing temperature favors the endothermic direction (forward reaction).
✅ So, increasing temperature will increase the production of H₂ and O₂ gases.
iii. How can you get the maximum yield in a reversible reaction?
Answer:
To get maximum yield in a reversible reaction, you must apply Le Chatelier’s Principle:
Change conditions (temperature, pressure, concentration) to favor the forward reaction.
Remove products continuously to shift the equilibrium forward.
Use catalysts to reach equilibrium faster (though they don’t affect yield directly).
✅ The ideal is 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:
To reduce the time to reach equilibrium:
Add a catalyst — it lowers activation energy for both forward and reverse reactions.
Increase temperature (if applicable) — raises kinetic energy, increasing reaction speed.
Increase concentration of reactants — faster collision rate.
Increase pressure (for gases) — especially if fewer gas molecules are on one side.
✅ These changes speed up both reactions, making equilibrium reach more quickly.
v. What is the effect of increasing pressure on the following reaction at equilibrium?
N2+O2\⇌2NO
Answer:
On the left side: 1 mol N₂ + 1 mol O₂ = 2 moles of gas
On the right side: 2 mol NO = 2 moles of gas
Since the number of gas molecules is equal on both sides, increasing pressure has no effect on the position of equilibrium.
✅ Equilibrium remains unchanged.
Constructed Response Questions
i. Why are some reactions irreversible while others are reversible?
Detailed Answer:
Chemical reactions are classified as irreversible or reversible based on whether the products can change back into reactants under the same conditions.
Irreversible Reactions:
In these reactions, the products are so stable or the energy released is so high that the reverse reaction does not occur. Also, if a gas escapes or a precipitate forms, it removes the product from the reaction mixture, pushing the reaction in only one direction.
Example: Combustion of methane → CO₂ and H₂O (cannot be reversed).Reversible Reactions:
N2+3H2⇌2NH3
In these reactions, both forward and reverse reactions can take place. The system can reach a point called dynamic equilibrium, where the rate of the forward and reverse reactions become equal.
Example:Here, ammonia can decompose back into nitrogen and hydrogen under suitable conditions.
✅ Key Point:
Whether a reaction is reversible or not depends on bond energies, system conditions (like temperature and pressure), and the stability of products.
ii. Why are combustion reactions generally irreversible?
Detailed Answer:
Combustion reactions involve the burning of a substance in oxygen, producing heat, light, and stable products such as carbon dioxide and water.
These reactions are highly exothermic, meaning they release a large amount of energy.
The products formed (like CO₂ and H₂O) are very stable and do not easily react to form the original substance again.
Most combustion occurs in open systems, so gases escape, and it’s practically impossible to reverse the process.
Example:
CH4+2O2→CO2+2H2O+Heat
Once methane is burned, it cannot be recovered from carbon dioxide and water under normal conditions.
✅ Conclusion:
Due to the stability of products, gas evolution, and high energy release, combustion reactions are generally irreversible.
iii. Can you make an irreversible reaction reversible and vice versa?
Detailed Answer:
Irreversible to Reversible:
Generally, no. If a reaction is naturally irreversible (like combustion or neutralization), we cannot reverse it under normal conditions because the products are too stable or have left the system (e.g., as gas).Reversible to Irreversible:
Yes, conditionally. A reversible reaction can be made to behave irreversibly if:One of the products is removed continuously (e.g., by distillation, precipitation, or gas escape).
Conditions like temperature or pressure are changed to stop the reverse reaction.
Example:
In the reaction:
CH3COOH+C2H5OH⇌CH3COOC2H5+H2O
(Esterification is reversible)
If water is removed, the reaction is forced to complete, behaving like an irreversible reaction.
✅ Conclusion:
You cannot reverse truly irreversible reactions, but can control reversible ones to favor one direction.
iv. How do you know if a reaction is reversible or irreversible?
Detailed Answer:
To determine the nature of a reaction:
Reversible Reactions:
Take place in closed systems.
Have a dynamic equilibrium, where forward and reverse reactions occur at the same rate.
The concentrations of reactants and products remain constant at equilibrium.
Can be represented by a double arrow (⇌).
Usually slow and sensitive to temperature, pressure, and concentration changes.
Irreversible Reactions:
Proceed in one direction only.
Often involve gas evolution, precipitate formation, or energy release.
The products cannot convert back into reactants easily.
Represented by a single arrow (→).
✅ Conclusion:
You can identify reversibility by reaction conditions, system closure, and product stability.
v. Are the phase changes in water (solid to liquid, liquid to vapour) reversible or irreversible?
Detailed Answer:
Phase changes in water are physical changes, not chemical ones. They are reversible under the right conditions:
Melting: Ice → Water
Freezing: Water → Ice
Evaporation: Water → Steam
Condensation: Steam → Water
Sublimation/Deposition: Ice ⇌ Water vapour (in special conditions)
✅ These changes do not alter the chemical structure (H₂O) of the substance, so water remains water — only the state changes.
✅ Conclusion:
All phase changes of water (solid ⇌ liquid ⇌ gas) are reversible, and can be repeated by changing temperature or pressure.
Descriptive Questions
i. How can you drive a reversible reaction at equilibrium?
(a) In the forward direction
(b) In the backward direction
Answer:
To control the direction of a reversible reaction, we apply Le Chatelier’s Principle, which says that if conditions are changed, the equilibrium will shift to oppose that change.
(a) Driving the reaction in the forward direction:
You can favor the forward direction by:
Increasing the concentration of reactants
Removing products as they are formed
Changing temperature: For an endothermic reaction, increase temperature
Increasing pressure (if forward reaction has fewer gas molecules)
Example:
N2+3H2⇌2NH3+Heat
To favor forward reaction: use high pressure, low temperature, and remove ammonia.
(b) Driving the reaction in the backward direction:
You can favor the reverse reaction by:
Adding products
Removing reactants
For exothermic reactions, increase temperature
Lower pressure (if reverse reaction produces more gas molecules)
ii. Explain how the rates of forward and backward reactions change when the system approaches equilibrium.
Answer:
At the beginning of a reversible reaction:
Forward reaction is fast (high concentration of reactants)
Reverse reaction is slow (few or no products)
As products form:
Reactants decrease → forward reaction slows down
Products increase → reverse reaction speeds up
Eventually:
Rate of forward = Rate of reverse
Dynamic equilibrium is established
✅ At equilibrium, reactions continue, but concentrations remain constant.
iii. Describe the effect of a catalyst on the reversible reaction.
Answer:
A catalyst:
Speeds up both forward and backward reactions equally
Does NOT affect the position of equilibrium
Lowers the activation energy for both directions
Its role is to help the system reach equilibrium faster, not to change the amount of products or reactants at equilibrium.
Example:
In the Haber process, iron catalyst is used to accelerate the production of ammonia without shifting the equilibrium itself.
iv. How can a reversible reaction be forced to go to completion?
Answer:
A reversible reaction can be forced to go in one direction (toward completion) by:
Removing one or more products (e.g., distilling water, filtering a precipitate, or allowing gas to escape)
Using excess reactants
Applying temperature and pressure conditions that heavily favor one direction
Changing solvent or concentration to disrupt reverse reaction
Example:
In esterification:
CH3COOH+C2H5OH⇌CH3COOC2H5+H2O
Removing water shifts the reaction to product side, forcing completion.
v. How does change in temperature affect the reaction at equilibrium?
Answer:
The effect of temperature on a reaction at equilibrium depends on whether the reaction is exothermic or endothermic:
Exothermic Reaction (releases heat):
A+B⇌C+D+Heat
Increasing temperature shifts equilibrium backward
Decreasing temperature shifts equilibrium forward
Endothermic Reaction (absorbs heat):
A+B+Heat⇌C+D
Increasing temperature shifts equilibrium forward
Decreasing temperature shifts equilibrium backward
✅ Conclusion:
Changing temperature affects both the position of equilibrium and the rate of reaction.
Investigate
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 a bright blue crystalline solid.
Why it is coloured:
The blue color is due to the presence of water of crystallization.
Water molecules are coordinated with Cu²⁺ ions, which cause electronic transitions that absorb certain wavelengths of light, making the salt appear blue.
Effect of heat:
When heated:
CuSO₄\cdotp5H₂O (blue)→heatCuSO₄ (white)+5H2O(g)
The water of crystallization is lost.
The salt turns white, indicating the formation of anhydrous copper sulphate.
✅ Conclusion:
The hydrated salt is blue because of water molecules attached to copper ions.
On heating, water is removed, the structure changes, and the color changes to white.
🔁 Note: If water is added back, the blue color returns, showing this is a reversible physical change.
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:
Ammonia is synthesized by the Haber process, which is a reversible reaction:
N2(g)+3H2(g)⇌2NH3(g)+Heat
To get maximum yield of ammonia, the following conditions are applied:
✅ 1. High Pressure (~200 atm):
The forward reaction produces fewer gas molecules (4 → 2).
According to Le Chatelier’s Principle, high pressure shifts the equilibrium toward ammonia.
✅ 2. Moderately Low Temperature (~450–500°C):
The reaction is exothermic.
Lower temperature favors forward reaction, but too low temperature makes the reaction too slow.
So, a compromise temperature is used to balance yield and rate.
✅ 3. Catalyst (Iron with potassium and aluminum oxide promoters):
Increases the rate of reaction without affecting the position of equilibrium.
Helps the system reach equilibrium faster.
✅ 4. Removal of Ammonia Continuously:
As ammonia forms, it is condensed and removed.
This shifts the equilibrium forward, producing more NH₃.
Conclusion:
To get the maximum yield of ammonia in industry:
Use high pressure
Use moderately low temperature
Use an efficient catalyst
Remove ammonia as it forms
This ensures a high production rate suitable for fertilizer (urea) manufacturing.