GCSEChemistry: Combined ScienceWJECRevision NotesChemical Bonding, Application Of Chemical Reactions & Organic ChemistryAcids, Bases & SaltsNeutralisation of Acids

Neutralisation of Acids (WJEC GCSE Chemistry: Combined Science)

Revision Note

Philippa Platt

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Neutralisation of Acids

What happens when an acid reacts with a base?

  • When an acid reacts with a base, a neutralisation reaction occurs
  • Bases have pH values above 7
  • Many bases are insoluble in water but the ones that do dissolve in water are called alkalis
  • They thus form an alkaline solution
  • Examples of alkalis are soluble metal hydroxides such as NaOH and Ca(OH)2
  • 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 + carbonate ⟶ 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 salts and nitric acid produces nitrates

Table salt or sodium chloride

table-salt

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 and Metal Hydroxides

  • Metal oxides and metal hydroxides act as bases
  • When they react with acid, a neutralisation reaction occurs
  • In all acid-base neutralisation reactions, salt and water are produced
  • 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

  • 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

  • Acids will react with metal carbonates to form the corresponding metal salt, carbon dioxide and water
  • These reactions are easily distinguishable from acid – metal oxide/hydroxide reactions due to the presence of effervescence caused by the 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

Examiner Tip

  • For a neutralisation reaction to occur, water must be formed.
    • Therefore, the reaction between an acid and a metal is not a neutralisation reaction.
  • If an acid-base reaction effervesces (fizzes), then the base must be a metal carbonate as carbon dioxide gas is produced.

Explaining Neutralisation Using Ions

Higher Tier

  • Acids are a source of hydrogen ions, H+
  • Bases (or alkalis) are a sources of  hydroxide ions, OH– 
  • When they react together in a neutralisation reaction, the H+ ions react with the OH– ions to produce water
  • This is the net ionic equation of all acid-base neutralisations and is what leads to a neutral solution, since water has a pH of 7:

H+  (aq) + OH– (aq)⟶ H2O (l)

  • Not all reactions of acids are neutralisations
    • For example when a metal reacts with an acid, although a salt is produced there is no water formed so it does not fit the definition of neutralisation

  • Neutralisation is very important in the treatment of soils to raise the pH as some crops cannot tolerate pH levels below 7
  • This is achieved by adding bases to the soil such as limestone and quicklime

Examiner Tip

Make sure you learn this equation for neutralisation, H+ (aq) + OH– (aq) → H2O (l), including state symbols, as it is a common question that is asked in exams.

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    Philippa Platt

    Author: Philippa Platt

    Expertise: Chemistry

    Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener.