Neutralisation of Acids and Salt Production (AQA GCSE Chemistry: Combined Science)

• When an acid reacts with a base, a neutralisation reaction occurs
• A base is a chemical that neutralises acids
• Examples of bases include:
  • Metal oxides, e.g. copper(II) oxide, Cu₂O
  • Metal hydroxides, e.g. sodium hydroxide, NaOH
  • Metal carbonates, e.g. calcium carbonate, CaCO₃ 
• Bases have pH values above 7
• Many bases are insoluble in water
• Some bases dissolve in water and are called alkalis because they form an alkaline solution
  • Examples of alkalis are soluble metal hydroxides such as sodium hydroxide, NaOH (aq), and calcium hydroxide, Ca(OH)₂ (aq)

Neutralisation reactions

• In all acid-base neutralisation reactions, salt and water are produced:

acid + base ⟶ salt + water

• If the base is a metal carbonate, carbon dioxide is also produced:

acid + base ⟶ salt + water + carbon dioxide

• The identity of the salt produced depends on the acid used and the positive ions in the base
  • Hydrochloric acid produces chlorides
  • Sulfuric acid produces sulfate
  • Nitric acid produces nitrates

Table salt / sodium chloride

Sodium chloride is commonly known as table salt and can be formed from a neutralisation reaction

Photo by Chemical Safety Facts

Reactions of acids with metal oxides

• Metal oxides also act as bases by reacting with acids in a neutralisation reaction to produce the corresponding metal salt and water
• The following are some specific examples of reactions between acids and metal oxides:

2HCl + CuO ⟶ CuCl₂ + H₂O

H₂SO₄ + Na₂O⟶ Na₂SO₄ + H₂O

2HNO₃ + MgO ⟶ Mg(NO₃)₂ + H₂O

Reactions of acids with metal hydroxides

• Metal hydroxides also act as bases by reacting with acids in a neutralisation reaction to produce the corresponding metal salt and water
• The following are some specific examples of reactions between acids and metal hydroxides:

2HCl + 2LiOH ⟶ 2LiCl + H₂O

H₂SO₄ + 2NaOH ⟶ Na₂SO₄ + 2H₂O

HNO₃ + KOH ⟶ KNO₃ + H₂O

Reactions of acids with metal carbonates

• Metal carbonates act as bases by reacting with acids in a neutralisation reaction to form the corresponding metal salt, carbon dioxide and water
• These reactions are easily distinguishable from acid – metal oxide/hydroxide reactions due to the effervescence caused by the release of carbon dioxide gas
  • We can test for the presence of carbon dioxide gas by bubbling it through limewater
  • If the limewater turns milky or cloudy carbon dioxide is present
• The following are some specific examples of reactions between acids and metal carbonates:

2HCl + Na₂CO₃ ⟶ 2NaCl + H₂O + CO₂

H₂SO₄ + CaCO₃⟶ CaSO₄ + H₂O + CO₂

2HNO₃ + MgCO₃ → Mg(NO₃)₂ + H₂O + CO₂

• The name of a salt has two parts
  • The first part comes from the metal, metal oxide or metal carbonate used in the reaction
  • The second part comes from the acid
• Hydrochloric acid always produces salts that end in chloride and contain the chloride ion, Cl^–
• Sulfuric acid always produces salts that end in chloride and contain the sulfate ion, SO₄^2–
• Nitric acid always produces salts that end in chloride and contain the nitrate ion, NO₃^–

Naming salts examples

Acid	Base	Name of Salt	Formula of Salt
Sulfuric acid, H2SO4	Calcium carbonate, CaCO3	Calcium sulfate	CaSO4
Hydrochloric acid, HCl	Magnesium oxide, MgO	Magnesium chloride	MgCl2
Nitric acid, HNO3	Potassium hydroxide, KOH	Potassium nitrate	KNO3

• Salts have no overall charge since the sum of the charges on the ions is equal to zero
• If you know the ions present in a salt you can identify the formula from balancing the charges

Naming salts using ions table

Ion Formula Group 1 metals M+, e.g. Na+ Group 2 metals M2+, e.g. Mg2+ Iron(II) Fe2+ Iron(III) Fe3+ Copper(II) Cu2+ Aluminium Al3+ Chromium Cr3+ Ammonium NH4+

Ion Formula Group 7 halides X–, e.g. Br– Sulfate SO42– Nitrate  NO3– Phosphate  PO43– Carbonate CO32– Hydroxide OH– Hydrogen carbonate HCO3–