Chapter 7: Acid Base ChemistryLong Questions with Answers

Background

Although the earlier definitions of acids and bases describe some distinctive features, new and broader definitions were needed to explain their chemical behaviour on the molecular level. Svante Arrhenius, a Swedish chemist, suggested that acids and bases may be classified in terms of their behaviour in water.

Arrhenius acid

An acid is that substance which dissociates in water to give proton (H⁺) or hydroxonium ion (H₃O⁺). Typical Arrhenius acids are HCl, HNO₃, H₂SO₄ and HCN.

Arrhenius base

A base is that substance which dissociates in water to give hydroxyl ions (OH⁻). Typical Arrhenius bases are NaOH, KOH and Ba(OH)₂.

Role of water

Water has an essential role. Whenever an acid or a base dissociates, its molecules surround the resultant H⁺ and OH⁻. Since a proton is very small and its charge density is very high, it forms a strong bond with the lone pair of a water molecule to give H₃O⁺. Up to four water molecules may surround the proton; hydroxyl ions are also surrounded by water molecules.

Neutralisation

When a strong acid and a strong base are dissolved in water, they completely dissociate into ions. The hydroxonium ion and the hydroxyl ion then react to form water with the evolution of heat. The salt NaCl does not exist as solid crystals; it remains in solution as hydrated spectator Na⁺ and Cl⁻ ions.

Limitations

Arrhenius concept

An acid dissociates in water to give H⁺ (H₃O⁺); a base dissociates in water to give OH⁻.

Bronsted-Lowry concept

An acid is a substance that donates a proton; a base is a substance that accepts a proton. An acid-base reaction is one in which a proton is transferred from a proton donor to its acceptor; it may take place in the gas phase or in any solvent.

Comparison

Relation

All Arrhenius acids are Bronsted-Lowry acids, and all Arrhenius bases are Bronsted-Lowry bases. But all Bronsted-Lowry acids and bases are not Arrhenius acids and bases: NH₄⁺ is not an Arrhenius acid and NH₃ is not an Arrhenius base.

1. With ammonium chloride

Concentrated sulphuric acid is a non-volatile acid; on warming it drives out the more volatile acid from its salt. Hydrogen chloride gas is given off (it fumes in moist air):

2. With ammonium nitrate

In the same way, heating with concentrated H₂SO₄ gives nitric acid vapour and ammonium sulphate:

3. With magnesium oxide

Acids with metal oxides form salts and water (neutralisation):

4. With magnesium carbonate

Acids decompose metal carbonates evolving carbon dioxide gas:

Reaction

A base reacts with a non-metal oxide to give a salt and water, just as it does with an acid:

Inference

Since they neutralise bases, non-metallic oxides are acidic in nature (acidic oxides). With water they form acids:

Compare with metal oxides

Metal oxides (Na₂O, CaO, MgO) are basic: they react with acids. This is why SO₂ and NO₂ in the air give acid rain, and why lime is used to remove SO₂ from power-station smoke.

The reason: HCl is a proton donor

According to Bronsted-Lowry, an acid is a substance that donates a proton. This definition requires that to behave as an acid a compound must have a proton to donate. The condition of the presence of water was eliminated. HCl has a proton to donate in both cases.

HCl in water

HCl donates its proton to water, which acts as a base. The H₃O⁺ ions give the solution all its acid properties: sour, turns blue litmus red, reacts with metals and carbonates. This is acidic by both Arrhenius and Bronsted-Lowry.

Dry HCl gas

With no water at all, HCl gas still donates a proton to ammonia gas, giving ammonium chloride. The reaction may take place in the gas phase, so dry HCl is an acid by Bronsted-Lowry.

Note

Dry HCl does not turn dry blue litmus red and does not attack dry metals: by Arrhenius it is not acidic until water is present. Its acid character in the dry state is explained only by the Bronsted-Lowry concept.

Definitions

An acid (Bronsted-Lowry) is a proton donor. When it donates its proton, the species that remains is its conjugate base, which can accept the proton back. The two differ by just one proton (H⁺) and are called a conjugate acid-base pair.

Examples

Differences

Hydrochloric acid

Our stomach produces hydrochloric acid to digest the food that we eat. It provides the acidic conditions in which the enzyme pepsin breaks down proteins, and it kills most germs swallowed with food.

Amino acids

Proteins are made of amino acids; they build muscles, enzymes and hormones.

Nucleic acids

DNA and RNA (deoxyribonucleic and ribonucleic acid) carry our genetic information and control the making of proteins.

Fatty acids

Fats are made from fatty acids; they store energy and form cell membranes.

Lactic acid

During hard exercise with little oxygen, muscles form lactic acid, which causes muscle fatigue.

Ascorbic acid

Vitamin C (ascorbic acid, from amla, guava, citrus) keeps skin, gums and blood vessels healthy and helps fight infection.

Carbonic acid

CO₂ from respiration forms carbonic acid in blood; with hydrogen carbonate ions it keeps the blood pH steady at about 7.4.

(i) Short duration: milky

Lime water is a solution of calcium hydroxide, an alkali. CO₂ is an acidic oxide, so they react to form insoluble calcium carbonate, a white precipitate, and the lime water turns milky:

(ii) Long duration: clear again

With excess CO₂ the calcium carbonate reacts further with CO₂ and water to form calcium hydrogen carbonate, which is soluble, so the milkiness disappears:

Uses of this test

The milkiness is the standard test for carbon dioxide. The second reaction is how limestone caves form and why hard water contains calcium hydrogen carbonate; boiling such water reverses it and deposits CaCO₃ (scale in kettles).

Acids

Acids have been known for their sour taste like the taste of lemon and their ability to change the colour of litmus paper from blue to red. Curd, tamarind, lemon and lime contain acids. Common acids: acetic, hydrochloric, nitric, sulphuric and tartaric acid.

Bases and alkalis

Bases are known for their bitter taste, slippery touch and ability to change red litmus to blue. An alkali is a base that dissolves in water: NaOH, KOH, Ca(OH)₂, NH₄OH. Metal oxides are also basic because they react with acids to form salts and water (Na₂O, CaO, ZnO, MgO).

Reactions of acids

Reactions of bases

Need for a new concept

Arrhenius concept is simple but has limitations: Na₂CO₃, K₂CO₃ and NH₃ have no OH group but behave as bases; and water must be present as a solvent.

Definitions

An acid is a substance that donates a proton. A base is a substance that accepts a proton. The presence of water is not required. An acid-base reaction is a reaction in which a proton is transferred from a proton donor to its acceptor, in the gas phase or in any solvent.

Example 1: HCl and water

HCl gas acts as an acid because it donates its proton to water, which acts as a base.

Example 2: ammonia and water

When ammonia dissolves in water, a proton is transferred from water to ammonia and the ammonium ion is formed. Ammonia is a base while water is an acid in this reaction.

Conjugate pairs

In the reverse reaction OH⁻ is a base because it accepts a proton donated by the acid NH₄⁺. To differentiate, OH⁻ is called the conjugate base and NH₄⁺ the conjugate acid.

Water is amphoteric

Water has the ability to act both as an acid or a base depending upon the other compound with which it reacts. It is therefore called an amphoteric compound.

Strength

The strength of an acid depends upon the extent to which it is ionised in water.

Hydrochloric acid (strong)

It ionises in water completely, giving a large amount of hydroxonium ions.

Sulphuric acid (strong, diprotic)

Nitric acid (strong)

Ethanoic (acetic) acid (weak)

Organic acids ionise up to a very limited extent and hence they are weak acids. Acetic acid is 0.132 % ionised: out of 1000 molecules only about 1.32 dissociate. Formic acid (1.50 M) is 1.06 % ionised.

Bases

Definition

When rain water has pH between 4.2 and 4.4, it is called acid rain.

Formation

Burning of fossil fuels releases harmful gases into the air. These gases, SO₂ and NO₂, when mixed with moisture present in air, form acid droplets. These droplets then fall on the ground as acid rain.

Effects on soil and plants

Acid rain makes soil more acidic. It dissolves and washes away nutrients present in the soil which are needed by plants. Many plants cannot live or grow in an acidic soil; it can damage vegetation. (Acids react with basic minerals and dissolve them.)

Effects on aquatic life

It can make water bodies too acidic for aquatic animals to live in. Many lakes, rivers, ponds and streams no longer have fish.

Effects on buildings

Acid rain and dry deposition of acidic particles damage buildings, statues, automobiles and other structures made of stone and metal. Marble and limestone (CaCO₃) react like any carbonate with acid: CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂. Iron reacts like a metal with acid: Fe + H₂SO₄ → FeSO₄ + H₂.

On the basis of occurrence

Acids are divided into two types on the basis of their occurrence: natural acids and mineral acids. Acids obtained from natural sources are called natural or organic acids. Mineral or man-made acids are prepared from minerals like sodium chloride or sodium nitrate.

On the basis of strength

Strong acids (ionise completely: HCl, H₂SO₄, HNO₃) and weak acids (ionise partially: CH₃COOH, HCOOH).

On the basis of protons

Monoprotic (HCl, HNO₃, CH₃COOH), diprotic (H₂SO₄) and triprotic (H₃PO₄).

Uses of mineral acids