Chapter 7Acid Base Chemistry

An Arrhenius acid is a substance which dissociates in water to give protons (H⁺) or hydroxonium ions (H₃O⁺).

Arrhenius acids: HF, H₂SO₃ and H₂S.

Calcium is a reactive metal, so it dissolves in the acid, forms a salt and evolves hydrogen gas:

Calcium sulphate is only sparingly soluble, so it forms a coating on the calcium and the reaction soon slows down.

Barium chloride (BaCl₂). Acids decompose metal carbonates evolving carbon dioxide:

A Bronsted-Lowry acid is a substance that donates a proton. HSO₄⁻ still has one hydrogen it can give away:

It donates a proton to water, so it is an acid; SO₄²⁻ is its conjugate base. (HSO₄⁻ can also accept a proton to become H₂SO₄, so it is amphoteric, but in water it acts mainly as an acid.)

Hydrochloric acid is a strong mineral acid that ionises completely and is highly corrosive. Concentrated or even moderately dilute HCl would burn the mouth, throat and food pipe.

The stomach makes only a very dilute solution of HCl (about 0.1 M) for digestion, and its lining is protected by a layer of mucus that the acid cannot attack. The mouth and throat have no such protection. When the stomach makes too much acid, even this causes acidity and heartburn.

Lemon, orange, lime (kinu, mosambi), grapefruit and strawberries.

Acids turn blue litmus red; bases turn red litmus blue.

As hydrated chloride ions, Cl⁻(aq). Water molecules surround each Cl⁻ with their partially positive hydrogen ends pointing towards the ion, held by ion-dipole attraction.

NH₄OH is really ammonia dissolved in water. Ammonia is a weak base: only a small fraction of NH₃ molecules accept a proton from water, and the NH₄⁺ and OH⁻ formed readily recombine. So an equilibrium is set up with most of the ammonia un-ionised: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻.

NH₃ (accepts H⁺ to form NH₄⁺) and CO₃²⁻ (accepts H⁺ to form HCO₃⁻). CN⁻ and Cl⁻ are other examples: none of these contains OH⁻.

Zinc dissolves with fizzing and a colourless gas is given off. The gas burns with a pop sound at the match flame: it is hydrogen.

Vinegar (acetic acid) reacts with sodium bicarbonate, releasing bubbles of carbon dioxide that loosen the blockage; boiling water then flushes it out. NaHCO₃ + CH₃COOH → CH₃COONa + H₂O + CO₂. Caustic cleaners (NaOH) dissolve grease by turning it into soap.

A base is an oxide or hydroxide of a metal that neutralises an acid (CuO, Cu(OH)₂, MgO). An alkali is a base that dissolves in water (NaOH, KOH). All alkalis are bases, but all bases are not alkalis.

Na₂O contains the oxide ion O²⁻, which is a very strong base with a strong tendency to react with water to produce hydroxide ions: Na₂O + H₂O → 2NaOH. It also reacts with acids to form salt and water.

Acids and bases cancel the properties of each other when mixed in equal amounts. The reaction is called a neutralisation reaction; a salt and water are formed. HCl + NaOH → NaCl + H₂O.

Caustic soda: NaOH; caustic potash: KOH; lime water: Ca(OH)₂ solution.

Acid: a substance which dissociates in water to give H⁺ or H₃O⁺ (HCl, HNO₃, H₂SO₄, HCN). Base: a substance which dissociates in water to give OH⁻ (NaOH, KOH, Ba(OH)₂).

H₃O⁺. A proton is very small with a very high charge density, so it forms a strong bond with a lone pair of a water molecule: H⁺ + H₂O → H₃O⁺. Up to four water molecules may surround it.

A strong acid and a strong base dissolved in water completely dissociate into ions. The hydroxonium and hydroxyl ions then react to form water with the evolution of heat: H₃O⁺ + OH⁻ → 2H₂O. The salt stays as hydrated spectator ions.

Upon the extent to which it is ionised in water. A strong acid ionises completely (HCl); a weak acid ionises partially (CH₃COOH).

Organic acids ionise up to a very limited extent. Acetic acid is only 0.132 % ionised: out of 1000 molecules only about 1.32 dissociate.

It is diprotic: it has two protons to donate.

Strong: NaOH, KOH (ionise completely). Weak: NH₄OH, Al(OH)₃ (ionise partially).

Antacids are weak bases (magnesium hydroxide, aluminium hydroxide) that neutralise the extra stomach acid; their concentration of hydroxyl ions is too low to harm the throat or stomach. Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O.

Fatty and spicy foods cause the stomach lining to produce more hydrochloric acid than is needed. The person feels a burning sensation below the breastbone, a sour taste in the mouth and heartburn.

An acid is a substance that donates a proton. A base is a substance that accepts a proton. Water need not be present.

Every Arrhenius acid gives H⁺ in water, which means it donates a proton to water: HCl + H₂O → H₃O⁺ + Cl⁻. That is exactly the Bronsted-Lowry definition.

When an acid donates a proton, the species left is its conjugate base; when a base accepts a proton, it becomes its conjugate acid. In NH₃ + H₂O ⇌ NH₄⁺ + OH⁻, OH⁻ is the conjugate base (of water) and NH₄⁺ the conjugate acid (of ammonia).

HCl / Cl⁻ (acid and its conjugate base) and H₂O / H₃O⁺ (base and its conjugate acid).

It donates a proton to ammonia in the gas phase: HCl(g) + NH₃(g) → NH₄Cl(s), white fumes. By Bronsted-Lowry, a proton donor is an acid, whether water is present or not.

Acids with alkalis or metal oxides form salts and water.

Cu, Ag, Au and Pt. They are less reactive than hydrogen, so they cannot displace it from acids.

Alkalis react with ammonium salts and liberate ammonia gas, which has a sharp smell and turns red litmus blue.

It has a sharp, pungent smell; it turns moist red litmus blue; and it gives white fumes of NH₄Cl with a glass rod dipped in concentrated HCl.

The sodium or potassium salt of a long-chain fatty acid, e.g. sodium stearate C₁₇H₃₅COONa, made by heating fats or oils with NaOH (saponification).

When rain water has pH between 4.2 and 4.4 it is called acid rain. Burning fossil fuels releases SO₂ and NO₂; with moisture in the air they form acid droplets (H₂SO₄, HNO₃) that fall as acid rain.

It makes soil more acidic and washes away nutrients plants need; many plants cannot grow. It makes lakes, rivers and ponds too acidic for aquatic animals, so many no longer have fish.

Marble and limestone are calcium carbonate. Acids decompose carbonates: CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂. The stone slowly dissolves and crumbles, as at the Taj Mahal. Acid rain also corrodes iron and steel.

Burn less fossil fuel; remove sulphur from fuels; fit scrubbers in power stations (lime absorbs SO₂) and catalytic converters in vehicles (remove NOₓ); use clean energy such as solar and wind. Lime is added to acidic lakes and soils to neutralise them.

Soap is a salt: the sodium or potassium salt of a long-chain fatty acid. Common soap is sodium stearate, C₁₇H₃₅COONa, the salt of stearic acid (the acid named in MCQ vi).

Soaps are made by heating fats or oils with NaOH; potassium soaps are softer (liquid soaps). Because it is the salt of a weak acid and a strong base, soap solution is slightly basic and turns red litmus blue.

Both. Water gives H⁺ (an acid property) and OH⁻ (a base property) in equal amounts. In the Bronsted-Lowry sense it accepts a proton from acids (HCl + H₂O → H₃O⁺ + Cl⁻) and donates a proton to bases (NH₃ + H₂O ⇌ NH₄⁺ + OH⁻).

Water has the ability to act both as an acid or a base depending upon the other compound with which it reacts. Water is therefore called an amphoteric compound. Pure water is neutral because the H⁺ and OH⁻ are equal.

Na₂CO₃ has no OH group, but the carbonate ion CO₃²⁻ accepts a proton from water, producing hydroxide ions:

The extra OH⁻ makes the solution basic (it turns red litmus blue, pH about 11). So Na₂CO₃ is a Bronsted-Lowry base; this is one of the cases the Arrhenius concept could not explain.

NaHCO₃ is amphoteric (it can act as both), but in water it behaves as a weak base.

Its solution is slightly basic (pH about 8.3), and it neutralises acids with fizzing: NaHCO₃ + HCl → NaCl + H₂O + CO₂. That is why it is used as an antacid (baking soda) and in baking powder.

So "dilute HCl" is a dilute strong acid, and "concentrated acetic acid" is a concentrated weak acid.

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