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?
- 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. - NH3:
H2SO4 reacts with NH3 to form ammonium sulfate:
2NH3+H2SO4→(NH4)2SO4
The reaction neutralizes H2SO4, producing a salt. - 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. - 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.
