Acids, Bases and Salts notes

Basic Meaning of Acids and Bases

An acid is a substance that produces H+ ions in aqueous solution. In water, a free H+ ion is strongly associated with a water molecule and forms H3O+, but H+ is commonly written for simplicity. Acids change blue litmus red and react with many metals, carbonates and bases.

A base produces OH− ions in aqueous solution or accepts a proton, depending on the definition being used. Bases change red litmus blue. A soluble base is called an alkali. Metallic oxides generally form bases when they react with water, although some oxides are amphoteric or acidic.

• An acid gives H+ in water according to the Arrhenius concept.
• A base gives OH− in water according to the Arrhenius concept.
• HCl + H2O → H3O+ + Cl−.
• NaOH → Na+ + OH− in aqueous solution.
• CO2 dissolving in water forms carbonic acid, so the pH of water decreases.
• A metallic oxide such as Na2O reacts with water to form a base: Na2O + H2O → 2NaOH.
• Dilute ethanoic acid, CH3COOH, is called vinegar.
• Acidic taste and corrosiveness are not safe tests for identifying an acid.

Bronsted-Lowry Concept and Conjugate Pairs

According to the Bronsted-Lowry concept, an acid is a proton donor and a base is a proton acceptor. An acid and the species formed after it loses H+ are called a conjugate acid-base pair. Similarly, a base and the species formed after it accepts H+ form another conjugate pair.

A substance that can donate or accept a proton is amphiprotic. Water is amphiprotic because it accepts H+ from HCl and donates H+ to NH3. The strength of an acid is related to the stability of its conjugate base. A more stable conjugate base corresponds to a stronger acid.

• HCl + H2O → H3O+ + Cl−. HCl is the acid and H2O is the base.
• NH3 + H2O ⇌ NH4+ + OH−. NH3 is the base and H2O is the acid.
• The conjugate base of HCN is CN−.
• The conjugate acid of NH3 is NH4+.
• HI has a very stable conjugate base, I−, in aqueous solution and is stronger than HF.
• An acid becomes weaker when its conjugate base is less stable.
• For a conjugate pair, stronger acid means weaker conjugate base.
• Water can act as both an acid and a base.

Lewis Concept, Protonation and Other Concepts

According to the Lewis concept, an acid accepts an electron pair and a base donates an electron pair. A Lewis acid usually has a vacant orbital or an electron-deficient centre. A Lewis base has a lone pair that can be donated to form a coordinate covalent bond.

In the reaction between NH3 and BF3, ammonia donates the lone pair on nitrogen to boron. Boron in BF3 has a vacant 2p orbital and accepts this electron pair. The product contains a coordinate covalent bond, often represented as H3N→BF3.

Protonation means addition of H+ to a molecule or ion. In a carboxylic acid, protonation generally occurs at the carbonyl oxygen because the oxygen has lone pairs. Protonation increases the electron deficiency of the carbonyl carbon and can facilitate further reaction.

The Lux-Flood concept defines a base as an oxide ion donor and an acid as an oxide ion acceptor. SiO2 is not a Lux-Flood base because it does not donate O2−; it behaves as a Lux-Flood acid.

• Lewis acid: electron-pair acceptor, for example BF3, AlCl3 and H+.
• Lewis base: electron-pair donor, for example NH3, H2O and OH−.
• NH3 donates its lone pair to the vacant 2p orbital of boron in BF3.
• A coordinate covalent bond contains both bonding electrons from one atom.
• In carboxylic acid protonation, the carbonyl oxygen is the usual protonation site.
• Lux-Flood acid: oxide ion acceptor.
• Lux-Flood base: oxide ion donor.
• SiO2 is not a base according to the Lux-Flood concept.

Acid and Base Strength, Ka and Kb

The ionisation constant of an acid is Ka. A larger Ka means that the acid ionises more extensively and is stronger. The ionisation constant of a base is Kb. A larger Kb means that the base reacts more extensively with water and is stronger.

For a conjugate acid-base pair at 25 °C, Ka × Kb = Kw = 1.0 × 10−14. Therefore, pKa + pKb = 14. This relation applies to a conjugate acid and its conjugate base, not to two unrelated substances.

The order of base strength can be understood from the stability of the negative charge and the availability of the electron pair. In the stated order, ethyl anion is stronger than amide ion, which is stronger than acetylide ion, which is stronger than hydroxide ion. Aniline is a weak base because the nitrogen lone pair is delocalised into the benzene ring.

• At 25 °C, Ka × Kb = 1.0 × 10−14 for a conjugate pair.
• If Ka = 10−6, then Kb = 10−8.
• If pKa = 3.7, the pKb of the conjugate base is 10.3.
• If pKa of carbonic acid is 6.4, pKb of its conjugate base is 7.6.
• Smaller pKa means a stronger acid.
• Smaller pKb means a stronger base.
• Acid strength order for the given values is HCl > H2SO4 > HF > CH3COOH.
• Aniline is weaker than many aliphatic amines because of resonance delocalisation of its lone pair.
• A stable conjugate base makes the parent acid stronger.

pH, pOH and Ionic Product of Water

The pH of a solution is the negative logarithm of its hydrogen ion concentration: pH = −log[H+]. The pOH is the negative logarithm of hydroxide ion concentration: pOH = −log[OH−]. At 25 °C, pH + pOH = 14.

A neutral solution at 25 °C has [H+] = [OH−] = 1.0 × 10−7 mol dm−3, so its pH and pOH are both 7. Acidic solutions have pH less than 7, while alkaline solutions have pH greater than 7 at this temperature.

The ionic product of water is Kw = [H+][OH−]. Kw changes with temperature. At 100 °C, water ionises more than it does at 25 °C, so Kw is greater and pKw is less than 14.

• pH = −log[H+].
• pOH = −log[OH−].
• At 25 °C, pH + pOH = 14.
• At 25 °C, pKw = 14.
• At 25 °C, Kw = 1.0 × 10−14.
• Acidic solution: [H+] > [OH−] and pH < 7 at 25 °C.
• Basic solution: [OH−] > [H+] and pH > 7 at 25 °C.
• On dissolving CO2 in water, carbonic acid forms and pH decreases.
• At 100 °C, pKw is less than 14.

Neutralisation and Normality Calculations

Neutralisation is the reaction between an acid and a base to form salt and water. In equivalent terms, equivalents of acid equal equivalents of base at the neutralisation point. The relation N1V1 = N2V2 is useful when the reacting solutions and their volumes are known.

Normality is the number of gram equivalents of solute per litre of solution. The equivalent mass of an acid depends on the number of replaceable H+ ions involved in the reaction. For H3BO3 in its neutralisation reaction, one mole supplies one equivalent because boric acid behaves as a monobasic acid in water.

When several acids and bases are mixed, calculate the acid and base equivalents separately. Excess equivalents determine whether the final solution is acidic or alkaline.

• Neutralisation produces salt and water in the usual acid-base reaction.
• Equivalent of solute = normality × volume in litres.
• For 100 mL of N/5 NaOH, equivalents of NaOH = 0.2 × 0.100 = 0.020.
• The equivalent mass of H3BO3 for neutralisation is 61.84 g per equivalent.
• Mass neutralised = equivalents × equivalent mass, so 0.020 equivalent H3BO3 requires 1.2368 g.
• In a mixture of 100 mL 0.5 N NaOH, 100 mL 0.2 N HCl and 100 mL 0.1 N H2SO4, base equivalents are 0.050 and acid equivalents are 0.030.
• The mixture in the stated calculation has excess OH− equivalents and is alkaline.
• Normality must be multiplied by volume in litres, not millilitres.

Salts and Their Formation

A salt is an ionic compound formed when the replaceable hydrogen of an acid is partly or completely replaced by a metal ion or ammonium ion. Salts are commonly produced by neutralisation, reaction of acids with metals, reaction of acids with carbonates, or direct combination of suitable substances.

The nature of an aqueous salt solution depends on the strengths of the parent acid and base. A salt of a strong acid and strong base is generally neutral. A salt of a strong acid and weak base is acidic. A salt of a weak acid and strong base is basic. A salt of a weak acid and weak base depends on the relative values of Ka and Kb.

• HCl + NaOH → NaCl + H2O. NaCl is from a strong acid and strong base and is approximately neutral.
• NH4Cl is from a strong acid and weak base, so its solution is acidic.
• CH3COONa is from a weak acid and strong base, so its solution is basic.
• CH3COONH4 is formed from weak acid CH3COOH and weak base NH3.
• NH4NO2 is a salt of weak acid HNO2 and weak base NH3.
• A salt formed from a weak acid and weak base may be acidic, basic or nearly neutral.
• The pH of a salt solution is determined after hydrolysis and not only by the formula of the salt.
• CuCl2 gives an acidic aqueous solution because Cu2+ undergoes hydrolysis and produces H+ ions.

Hydrolysis of Salts and Common Salts

Hydrolysis is the reaction of salt ions with water. It is responsible for the acidic or basic nature of many salt solutions. Ions of strong acids and strong bases usually do not hydrolyse significantly, while ions that are conjugates of weak acids or weak bases hydrolyse appreciably.

The ammonium ion is the conjugate acid of the weak base ammonia. It reacts with water to produce H3O+, making salts such as NH4Cl and NH4NO3 acidic. The acetate ion is the conjugate base of weak ethanoic acid and reacts with water to produce OH−, making sodium acetate basic.

Gypsum is calcium sulfate dihydrate, CaSO4·2H2O. It is added to cement to control setting and prevent rapid hardening. Other common salts have important household and industrial uses.

• NH4+ + H2O ⇌ NH3 + H3O+, so NH4+ makes solution acidic.
• CH3COO− + H2O ⇌ CH3COOH + OH−, so acetate makes solution basic.
• Cu2+ hydrolysis is responsible for the acidic nature of aqueous CuCl2.
• NH4NO3 gives an acidic aqueous solution because NH4+ hydrolyses.
• A salt of weak acid and weak base undergoes hydrolysis through both ions.
• Gypsum is CaSO4·2H2O.
• Adding about 2% gypsum to cement prevents rapid hardening.
• Vinegar contains dilute CH3COOH.
• Hydrolysis is different from simple dissolution. Dissolution separates ions, while hydrolysis involves reaction of ions with water.

Strength of Acid Mixtures and Solvent Levelling

The strength of an acid mixture can be expressed in normality or grams per litre. For a chloride-containing mixture, AgNO3 precipitates chloride ions as AgCl. The mass of AgCl can therefore be used to find the amount of HCl present, provided HCl is the source of chloride ions.

The reaction is Ag+ + Cl− → AgCl. One mole of AgCl corresponds to one mole of Cl− and therefore one mole of HCl. After finding the HCl equivalents, the remaining acid strength in the mixture can be assigned to H2SO4.

In a protophilic solvent, strong acids can transfer protons readily to the solvent. The solvent makes different strong acids appear to have similar effective strength because they are converted to the same solvated proton species. This is called the levelling effect.

• AgNO3 precipitates chloride as white AgCl.
• Ag+ + Cl− → AgCl.
• Molar mass of AgCl is approximately 143.5 g mol−1.
• A mass of 0.1435 g AgCl represents 0.001 mol AgCl and 0.001 mol chloride.
• The chloride amount identifies the HCl contribution in an acid mixture.
• The total normality of the stated HCl and H2SO4 mixture is 0.1 N.
• The calculated strength of H2SO4 in the stated mixture is 2.45 g L−1.
• Strong acids show a levelling effect in a protophilic solvent.
• The solvent used can affect the apparent strength of an acid.