Bases and Salts – SS1 Chemistry Lesson Note (Second Term)

A base is a substance that neutralises an acid to form a salt and water only.

• Alkali: a basic hydroxide that dissolves in water.
• Salt: a compound formed when all or part of the replaceable hydrogen in an acid is replaced by a metal or ammonium ion.

Behavioural objectives

By the end of the lesson, students should be able to:

1. define a base, an alkali and a salt;
2. distinguish between strong and weak bases;
3. state the properties and uses of alkalis;
4. define neutralisation in three ways and write equations for it;
5. name the five types of salts and give examples of each;
6. explain hydrolysis of salts, efflorescence, deliquescence and hygroscopy.

What are bases and alkalis?

A base is a substance that neutralises an acid to give a salt and water only.

• Base: most metal oxides and hydroxides are bases, e.g. Na₂O, K₂O, MgO, NaOH, KOH.
• Alkali: a basic hydroxide that is soluble in water, e.g. NaOH, KOH, Ca(OH)₂.
• Basic oxide or hydroxide: a metal oxide or hydroxide containing O^2- or OH^– ions, which reacts with an acid to form a salt and water only.
• Exception: lead (IV) oxide reacts with hydrochloric acid to give a salt, water and chlorine gas, so PbO₂ is not a base.
• Equation: PbO₂(s) + 4HCl(aq) → PbCl₂(aq) + 2H₂O(l) + Cl₂(g)

How do metal hydroxides vary in the electrochemical series?

The nature of a metal hydroxide depends on the position of the metal in the electrochemical series.

• Amphoteric hydroxide: a hydroxide that reacts with both acids and alkalis.
• With an acid: Zn(OH)₂(s) + 2HCl(aq) → ZnCl₂(aq) + 2H₂O(l)
• With an alkali: Zn(OH)₂(s) + 2NaOH(aq) → Na₂Zn(OH)₄(aq)

What is the strength of a base?

The strength of a base is how completely it ionises in water, and it can be weak or strong.

What is a weak base?

A weak base is a base that ionises only slightly in water to give positive ions and hydroxide ions.

• Example: ammonia, NH₃.
• Equation: NH₃(g) + H₂O(l) ⇌ NH₄⁺(aq) + OH^–(aq)

What is a strong base?

A strong base is a base that ionises completely in water to give metal ions and hydroxide ions.

• Examples: NaOH and KOH, formed when Na₂O and K₂O react with water.
• Na₂O(s) + H₂O(l) → 2NaOH(aq)
• K₂O(s) + H₂O(l) → 2KOH(aq)
• NaOH(aq) → Na⁺(aq) + OH^–(aq)
• KOH(aq) → K⁺(aq) + OH^–(aq)

What are the properties of alkalis?

Alkalis have typical physical properties and react with acids and ammonium salts.

What are the physical properties of alkalis?

1. Taste: alkalis taste bitter (never taste chemicals in the laboratory).
2. Feel: alkalis are soapy to the touch.
3. Litmus: alkalis turn red litmus blue.
4. Corrosiveness: concentrated caustic alkalis, NaOH and KOH, are corrosive.

What are the chemical properties of alkalis?

1. Reaction with acids

All bases react with acids to form a salt and water only.

• NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)
• MgO(s) + 2HNO₃(aq) → Mg(NO₃)₂(aq) + H₂O(l)

2. Displacement of ammonia from ammonium salts

A non-volatile alkali displaces volatile ammonia from an ammonium salt.

• Warm the ammonium salt with the alkali in the presence of water, and ammonia gas is given off.
• NaOH(aq) + NH₄Cl(aq) → NaCl(aq) + H₂O(l) + NH₃(g)
• Ca(OH)₂(aq) + (NH₄)₂SO₄(aq) → CaSO₄(aq) + 2H₂O(l) + 2NH₃(g)

What are the uses of alkalis and bases?

Alkalis and bases are used in industry, farming and medicine.

• NaOH: manufacture of soap, sodium salts and plastic; petrol refining.
• KOH: manufacture of liquid soap; dyeing and electroplating.
• Ca(OH)₂: manufacture of mortar, cement and plaster; neutralising acidic soil.
• Mg(OH)₂: manufacture of toothpaste; used as a laxative.
• Aqueous NH₃: bleaching cloth; used as a detergent.

What is neutralisation?

Neutralisation is the reaction in which an acid reacts completely with an alkali or base to form a salt and water.

It can be defined in three ways:

1. By acid and base: an acid and a base react to form a salt and water.
2. By ions: hydrogen ions (H⁺) combine with hydroxide ions (OH^–) to form water molecules.
3. By oxonium ions: oxonium ions (H₃O⁺) combine with hydroxide ions (OH^–) to form water molecules.

What are the equations for neutralisation?

• Acid and base: HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
• Ionic equation: H⁺(aq) + OH^–(aq) → H₂O(l)
• With oxonium ions: H₃OCl(aq) + KOH(aq) → KCl(aq) + 2H₂O(l)
• With oxonium ions: (H₃O)₂SO₄(aq) + 2NaOH(aq) → Na₂SO₄(aq) + 4H₂O(l)
• Ionic equation: H₃O⁺(aq) + OH^–(aq) → 2H₂O(l)

A salt is formed at the same time in each case.

What is an oxonium ion?

An oxonium ion is the ion H₃O⁺, formed when a hydrogen ion joins a water molecule.

• Formation: in aqueous solution, H⁺ + H₂O → H₃O⁺
• Bond type: this is a coordinate covalent combination.
• In neutralisation: the oxonium ion behaves as a hydrogen ion and reacts with OH^– to form water.

What is a salt?

A salt is a compound formed when all or part of the ionisable hydrogen in an acid is replaced by a metal or ammonium ion.

• HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
• H₂SO₄(aq) + KOH(aq) → KHSO₄(aq) + H₂O(l)

What are the types of salts?

There are five types of salts: normal, acid, basic, double and complex salts.

What are normal salts?

Normal salts are formed when all the replaceable hydrogen ions in an acid are replaced by a metal or ammonium ion.

• Examples: NaCl, K₂SO₄, Na₃PO₄, NaNO₃.
• Litmus: NaCl and K₂SO₄ are neutral to litmus. Some normal salts, such as Na₂CO₃, are not, because of hydrolysis.
• HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
• H₂SO₄(aq) + 2KOH(aq) → K₂SO₄(aq) + 2H₂O(l)

What are acid salts?

Acid salts are formed when the replaceable hydrogen ions in an acid are only partly replaced by a metal.

• Examples: NaHSO₄, Na₂HPO₄, NaH₂PO₄, NaHCO₃.
• Source: they come from acids with more than one replaceable hydrogen ion.
• Acid with two replaceable hydrogens: forms only one acid salt.
• Acid with three replaceable hydrogens: forms two different acid salts.
• Litmus: acid salts such as NaHSO₄ turn blue litmus red. NaHCO₃ is an exception, as its solution is weakly alkaline.
• Conversion: an acid salt becomes a normal salt when the remaining replaceable hydrogen is replaced by a metal ion.
• H₂SO₄(aq) + NaOH(aq) → NaHSO₄(aq) + H₂O(l)
• H₃PO₄(aq) + NaOH(aq) → NaH₂PO₄(aq) + H₂O(l)
• H₃PO₄(aq) + 2NaOH(aq) → Na₂HPO₄(aq) + 2H₂O(l)
• Both together: 2H₃PO₄(aq) + 3NaOH(aq) → NaH₂PO₄(aq) + Na₂HPO₄(aq) + 3H₂O(l)
• KHSO₄(aq) + KOH(aq) → K₂SO₄(aq) + H₂O(l)

What are basic salts?

Basic salts are formed when only part of the hydroxide ions of a base is replaced by the negative ions of an acid.

• Cause: there is not enough acid to neutralise the base completely.
• Examples: Zn(OH)Cl, Mg(OH)Cl, Mg(OH)NO₃, Bi(OH)₂NO₃.
• Litmus: they contain hydroxide ions, so they turn red litmus blue.
• With excess acid: they form a normal salt and water only.
• Zn(OH)₂(s) + HCl(aq) → Zn(OH)Cl(aq) + H₂O(l)
• Mg(OH)NO₃(aq) + HNO₃(aq) → Mg(NO₃)₂(aq) + H₂O(l)

What are double salts?

Double salts are salts that ionise to give three different types of ions in solution.

• Ions: usually two positive ions (metal or NH₄⁺) and one negative ion.
• (NH₄)₂Fe(SO₄)₂.6H₂O: ammonium iron (II) tetraoxosulphate (VI) hexahydrate.
• KAl(SO₄)₂.12H₂O: aluminium potassium tetraoxosulphate (VI) dodecahydrate (potash alum).
• KCr(SO₄)₂.12H₂O: chromium (III) potassium tetraoxosulphate (VI) dodecahydrate (chrome alum).

What are complex salts?

Complex salts are salts that contain complex ions, which are charged groups of atoms.

• Na₂Zn(OH)₄: sodium tetrahydroxozincate (II).
• K₄Fe(CN)₆: potassium hexacyanoferrate (II).
• NaAl(OH)₄: sodium tetrahydroxoaluminate (III).
• Na₂Zn(OH)₄ → 2Na⁺ + [Zn(OH)₄]^2-
• K₄Fe(CN)₆ → 4K⁺ + [Fe(CN)₆]^4-

What is hydrolysis of salts?

Hydrolysis of a salt is the reaction of the salt with water, which can make the solution acidic or alkaline.

• Examples of salts that hydrolyse: Na₂CO₃, NaHCO₃, AlCl₃, Na₂S, NH₄Cl, CH₃COONa.
• Na₂CO₃ + H₂O ⇌ NaOH + H₂CO₃
• AlCl₃ + H₂O ⇌ Al(OH)₃ + HCl
• Na₂S + H₂O ⇌ NaOH + H₂S
• The change in the pH of the solution is due to hydrolysis.

What are the uses of salts?

Salts have many uses in the home, industry, farming and medicine.

1. NH₄Cl: electrolyte in dry cells (Leclanché cell).
2. CaCO₃: medicine to neutralise acidity in the stomach.
3. CaCl₂: antifreeze; fused CaCl₂ is a drying agent and desiccator.
4. CaSO₄: making plaster of Paris.
5. CuSO₄: dyeing and calico printing.
6. MgSO₄: laxative.
7. KNO₃: making gunpowder, matches and soil fertiliser.
8. NaCl: preserving food and glazing pottery.
9. ZnCl₂: petroleum refining.

What are efflorescent, deliquescent and hygroscopic substances?

These are compounds that lose water of crystallisation to the air or take in moisture from it.

What are efflorescent substances?

Efflorescent substances lose some or all of their water of crystallisation when exposed to air, and the process is called efflorescence.

• Mass: there is a loss of mass.
• Example: Na₂CO₃.10H₂O → Na₂CO₃.H₂O + 9H₂O
• Others: Na₂SO₄.10H₂O, MgSO₄.7H₂O and CuSO₄.5H₂O.

What are deliquescent substances?

Deliquescent substances absorb so much water from the air that they dissolve and form a solution.

• Mass: there is a gain in mass.
• Examples: NaOH, CaCl₂, FeCl₃, MgCl₂, KOH and P₄O₁₀.

What are hygroscopic substances?

Hygroscopic substances absorb moisture from the air without forming a solution.

• Solids: no solution is formed.
• Liquids: the liquid becomes diluted.
• Mass: there is little or no change in mass.
• Examples: conc. H₂SO₄, CaO and anhydrous Na₂CO₃.

How do the three types compare?

What are drying agents?

Drying agents are substances with a high affinity for water or moisture, and they are either deliquescent or hygroscopic.

• Action: they remove water molecules, which is a physical change.
• Dehydrating agents: these remove the elements of water, i.e. hydrogen and oxygen atoms, or intra-molecular water.
• Rule: a drying agent that reacts with a gas is not used to dry that gas.
• Example: conc. H₂SO₄ is not used to dry NH₃ and H₂S gases.
• 2NH₃(g) + H₂SO₄(aq) → (NH₄)₂SO₄(aq)
• H₂S(g) + H₂SO₄(aq) → 2H₂O(l) + SO₂(g) + S(s)

Evaluation

1. Define a base and an alkali.
2. Distinguish between a strong base and a weak base. Give one example of each.
3. Write an equation for the neutralisation of hydrochloric acid by sodium hydroxide.
4. Name the five types of salts and give one example of each.
5. Distinguish between efflorescent, deliquescent and hygroscopic substances, with one example of each.

Assignment

Answer all parts:

1. Write equations to show the reaction of H₂SO₄ with KOH to form (a) an acid salt and (b) a normal salt.
2. State two uses each of NaOH and Ca(OH)₂.
3. Explain why conc. H₂SO₄ is not used to dry NH₃ gas.