Day: December 28, 2024

New Syllabus Chemistry Chapter 7: Acid-Base Chemistry – Lahore Board Guide

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Explore Chapter 7 of the new Lahore Board Chemistry syllabus, dedicated to Acid-Base Chemistry. This comprehensive guide simplifies key concepts such as the Arrhenius, Bronsted–Lowry, and Lewis theories of acids and bases, pH scale, neutralization reactions, titration techniques, and industrial applications. Ideal for students preparing for exams, this resource includes easy explanations, solved examples, MCQs, short questions, and detailed practice problems aligned with the latest syllabus. Boost your understanding of acid-base reactions and excel in your exams with this in-depth content!

Question (i):

Which acid is not used as a food or mixed with food?
Options:
(a) Tartaric acid
(b) Ascorbic acid
(c) Citric acid
(d) Formic acid
Correct Answer: (d) Formic acid
Explanation: Tartaric acid, ascorbic acid, and citric acid are commonly used in food products as preservatives or flavoring agents, whereas formic acid is not used in food due to its toxic and corrosive properties.

Question (ii):

While baking, which gas is responsible for raising the bread and making it soft?
Options:
(a) Oxygen
(b) Carbon dioxide
(c) Nitrogen
(d) Carbon monoxide
Correct Answer: (b) Carbon dioxide
Explanation: Baking powder or yeast releases carbon dioxide gas during baking, which causes the dough to rise and become soft.

Question (iii):

Predict the main characteristics of the reactions of metals with acids.
Options:
(a) Metals are dissolved
(b) Metals are converted into salts
(c) Hydrogen gas is evolved
(d) All the above-mentioned characteristics are true
Correct Answer: (d) All the above-mentioned characteristics are true
Explanation: When metals react with acids, they dissolve to form metal salts, and hydrogen gas is released as a by-product.

Question (iv):

How many hydroxide ions will calcium hydroxide release in water?
Options:
(a) 1
(b) 2
(c) Zero
(d) 3
Correct Answer: (b) 2
Explanation: Calcium hydroxide (Ca(OH)2) dissociates in water to release two hydroxide ions (OH⁻) per formula unit.

Question (v):

In a neutralization reaction between KOH and H3PO4, how many molecules of KOH will react with one molecule of H3PO4?
Options:
(a) 2
(b) 1
(c) 3
(d) 4
Correct Answer: (c) 3
Explanation: H3PO4 is a triprotic acid and can donate three hydrogen ions, requiring three KOH molecules to fully neutralize it.

Question (vi):

Which acid is used in the preparation of soap?
Options:
(a) Tartaric acid
(b) Citric acid
(c) Stearic acid
(d) Oxalic acid
Correct Answer: (c) Stearic acid
Explanation: Stearic acid is used in soap manufacturing because it reacts with a base (e.g., NaOH) to form soap through saponification.

Question (vii):

Which compound is formed when SO2 is dissolved in water?
Options:
(a) SO3
(b) H2SO3
(c) H2SO4
(d) H2S2O7
Correct Answer: (b) H2SO3
Explanation: When SO2 (sulfur dioxide) dissolves in water, it forms sulfurous acid (H2SO3).

Question (viii):

Which of the following contains oxalic acid?
Options:
(a) Tomato
(b) Orange
(c) Tamarind
(d) Sour milk
Correct Answer: (a) Tomato
Explanation: Tomatoes contain oxalic acid naturally, which contributes to their slightly acidic taste.

Question (ix):

Which compound in the following reaction is behaving as a conjugate base?
CH3COOH+H2O⇋CH3COO−+H3O+
Options:
(a) CH3COOH
(b) H2O
(c) CH3COO−
(d) H3O+
Correct Answer: (c) CH3COO−
Explanation: In the reaction, CH3COOH donates a proton to water, forming CH3COO−, which is its conjugate base.

Question (x):

When a chemical reaction is carried out with a substance Z, a gas is produced which turns red litmus paper blue. What is the reaction?
Options:
(a) Reaction of an acid with a metal carbonate
(b) Reaction of an acid with a base
(c) Reaction of an acid with a metal oxide
(d) Reaction of an acid with ammonium salt
Correct Answer: (b) Reaction of an acid with a base
Explanation: The reaction between an acid and a base produces ammonia gas (NH3 or similar compounds, which turn red litmus blue due to their basic nature.

2. Questions for Short Answers

(i) Choose Arrhenius Acids among the following compounds:

HF, NH3, H2SO4, SO2, H2S, H2O
Answer: HF, H2SO4, H2S, and H2O.
Explanation: According to the Arrhenius concept, acids are substances that increase H+concentration in water. HF, H2SO4, and H2S release H+ ions when dissolved in water, making them Arrhenius acids. Water (H2O) can also act as a weak acid in certain reactions.

(ii) How does calcium metal react with dilute H2SO4?

Answer: Calcium reacts with dilute sulfuric acid to produce hydrogen gas and calcium sulfate:
Ca+H2SO4→CaSO4+H2
Explanation: Calcium is a reactive metal that displaces hydrogen from sulfuric acid, forming an ionic salt (CaSO4) and hydrogen gas.

(iii) Which salt is formed when HCl reacts with BaCO3?

Answer: Barium chloride (BaCl2) is formed.
Reaction:
BaCO3+2HCl→BaCl2+CO2+H2O
Explanation: Hydrochloric acid reacts with barium carbonate to form barium chloride, releasing carbon dioxide gas and water as by-products.

(iv) How will you justify that HSO4−is a Bronsted–Lowry acid?

Answer: HSO4−can donate a proton (H+) to form SO42−:
HSO4−⇋SO42−+H+
Explanation: According to the Bronsted–Lowry concept, acids are proton donors. HSO4– donates a proton in this reaction, making it a Bronsted–Lowry acid.

(v) What chemical name will you give to soap as a compound?

Answer: Soap is chemically called sodium stearate or potassium stearate, depending on the alkali used.
Explanation: Soaps are salts of fatty acids (like stearic acid) formed by the reaction of a base (e.g., NaOH) with a fat or oil in the process of saponification. For example:
C17H35COOH+NaOH→C17H35COONa+H2O
This reaction forms sodium stearate, which is the chemical name for soap.

4. Descriptive Questions

(i) Explain Arrhenius concept of acids and bases.

The Arrhenius concept defines acids and bases based on their behavior in water:

  • Acids: Substances that increase the concentration of hydrogen ions (H+) or protons in an aqueous solution. For example, HCl dissociates in water to produce H+ ions:
    HCl→H++Cl−
  • Bases: Substances that increase the concentration of hydroxide ions (OH−) in an aqueous solution. For example, NaOH dissociates in water to produce OH−:
    NaOH→Na++OH−

The Arrhenius concept is simple but limited to aqueous solutions. It does not explain the behavior of substances like NH3, which acts as a base but does not contain OH− ions.

(ii) Compare Arrhenius and Bronsted–Lowry concepts of acids and bases.

  • Arrhenius Concept:
    • Acids are substances that increase H+ ion concentration in water.
    • Bases are substances that increase OH− ion concentration in water.
    • Limited to aqueous solutions.
    • Example: HCl is an acid, and NaOH is a base.
  • Bronsted–Lowry Concept:
    • Acids are proton (H+) donors.
    • Bases are proton (H+) acceptors.
    • Not restricted to aqueous solutions.
    • Example: NH3 accepts a proton to form NH4+, acting as a base.

The Bronsted–Lowry concept is more general and can describe acid-base behavior in non-aqueous systems.

(iii) How does sulphuric acid react with the following compounds?

  1. NH4Cl:
    H2SO4 reacts with NH4Cl to release hydrogen chloride gas:
    NH4Cl+H2SO4→NH4HSO4+HCl
    This is an acid-base reaction where H2SO4 acts as a stronger acid.
  2. NH3:
    H2SO4 reacts with NH3 to form ammonium sulfate:
    2NH3+H2SO4→(NH4)2SO4
    The reaction neutralizes H2SO4, producing a salt.
  3. MgO:
    H2SO4 reacts with MgO to form magnesium sulfate and water:
    MgO+H2SO4→MgSO4+H2O
    This reaction shows that magnesium oxide acts as a basic oxide.
  4. MgCO3:
    H2SO4 reacts with MgCO3 to produce magnesium sulfate, carbon dioxide, and water:
    MgCO3+H2SO4→MgSO4+CO2+H2O
    This is a typical reaction of acids with carbonates.

(iv) What happens when a base reacts with a non-metallic oxide? What do you infer about the nature of non-metallic oxide?

When a base reacts with a non-metallic oxide, a salt and water are formed. For example:
2NaOH+CO2→Na2CO3+H2O
This reaction indicates that non-metallic oxides are acidic in nature, as they react with bases to form salts and water.

(v) State the reason for showing acidic character by both dry HCl gas and HCl solution in water.

  • Dry HCl Gas: Dry HCl does not ionize because there is no water to dissociate it into H+ and Cl−. It does not show acidic properties in the absence of water.
  • HCl in Water: In water, HCl ionizes completely to form H+ ions, which are responsible for its acidic character:
    HCl→H++Cl−

Thus, the presence of water is essential for HCl to exhibit acidic properties.

(vi) Differentiate between an acid and its conjugate base.

  • Acid: A substance that donates a proton (H+) in a chemical reaction.
  • Conjugate Base: The species that remains after the acid has donated its proton.

Example:
In the reaction:
CH3COOH⇋CH3COO−+H+

  • CH3COOH is the acid.
  • CH3COO− is its conjugate base.

Acids and their conjugate bases differ by one proton.

5. Investigative Questions

(i) Why is significant CO2 passing through limewater on this statement?

When CO2 gas is passed through limewater (Ca(OH)2, it initially reacts to form insoluble calcium carbonate (CaCO3), which turns the solution milky:
CO2+Ca(OH)2→CaCO3+H2O
If more CO2CO_2 is passed, the calcium carbonate reacts further to form soluble calcium bicarbonate, causing the milkiness to disappear:
CaCO3+CO2+H2O→Ca(HCO3)2
This demonstrates that the reaction depends on the concentration of CO2 and the solubility of the products.

(ii) What is observed when CO2 is passed through limewater for a short duration (a) and for a long duration (b)?

  • (a) Short Duration: Limewater turns milky due to the formation of insoluble calcium carbonate:
    CO2+Ca(OH)2→CaCO3+H2O
  • (b) Long Duration: The milkiness disappears because calcium carbonate reacts with excess CO2 and water to form soluble calcium bicarbonate:
    CaCO3+CO2+H2O→Ca(HCO3)2

This reaction is used as a test for the presence of CO2 gas.

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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