Colourful Solutions > Proton transfer reactions > Neutralisation reactions

The Mad Science Lab

Standard level

To neutralise something means to nullify or cancel its properties. In chemical terms, this means to remove either the acidic or basic properties of a substance rendering it neutral, i.e. not acidic or basic.

Syllabus ref: R3.1.7

Reactivity 3.1.7 - Acids react with bases in neutralization reactions.

  • Formulate equations for the reactions between acids and metal oxides, metal hydroxides, hydrogencarbonates and carbonates.

Guidance

  • Identify the parent acid and base of different salts.
  • Bases should include ammonia, amines, soluble carbonates and hydrogencarbonates; acids should include organic acids.

Tools and links

  • Tool 1, Structure 1.1 - How can the salts formed in neutralization reactions be separated?
  • Reactivity 1.1 - Neutralization reactions are exothermic. How can this be explained in terms of bond enthalpies?
  • Reactivity 3.2 - How could we classify the reaction that occurs when hydrogen gas is released from the reaction between an acid and a metal?

Neutralisation

Acids react with bases in neutralisation reactions. These are fast, stoichiometric reactions. Bases are the chemical opposites of acids. This means that they contain ions that can neutralise (react with and cancel out) the H+ ions of the acids.

H+ + OH- → H2O

The following types of compounds are classified as bases:

Base
examples
formulae
Metal oxides
calcium oxide, magnesium oxide, zinc oxide, iron(II) oxide
CaO, MgO, ZnO, FeO
Metal hydroxides
calcium hydroxide, magnesium hydroxide, sodium hydroxide, potassium hydroxide
Ca(OH)2, Mg(OH)2, NaOH, KOH
Metal carbonates
calcium carbonate, magnesium carbonate, sodium carbonate, potassium carbonate
CaCO3, Na2CO3, K2CO3
Metal hydrogencarbonates
sodium hydrogencarbonate, potassium hydrogencarbonate
NaHCO3, KHCO3

^ top

Neutralisation reactions

Below is a list of neutralisation reactions showing the behaviour of acids with different types of bases. Notice that water is always produced in neutralisation.

sulfuric acid + magnesium oxide → magnesium sulfate + water    
H2SO4   MgO   MgSO4   H2O    
                 
hydrochloric acid + calcium oxide → calcium chloride + water    
2HCl   CaO   CaCl2   H2O    
                 
sulfuric acid + sodium hydroxide → sodium sulfate + water    
H2SO4   2NaOH   Na2SO4   2H2O    
                 
hydrochloric acid + calcium hydroxide → calcium chloride + water    
2HCl   Ca(OH)2   CaCl2   2H2O    
                 
sulfuric acid + zinc carbonate → zinc sulfate + water + carbon dioxide
H2SO4   ZnCO3   ZnSO4   H2O   CO2
                 
hydrochloric acid + potassium carbonate → potassium chloride + water + carbon dioxide
2HCl   K2CO3   2KCl   H2O   CO2
                 
hydrochloric acid + sodium hydrogen carbonate → sodium chloride + water + carbon dioxide
HCl   NaHCO3   NaCl   H2O   CO2

Note: sulfuric acid always makes salts called sulfates, hydrochloric acid always makes salts called chlorides ( nitric acid makes nitrates, ethanoic acid makes ethanoates - etc.)

^ top

Reaction with metals

The reaction of metals with acids is often called neutralisation, as the acid gets used up. However, it is nothing of the sort; it is a redox reaction (reduction oxidation). The metal loses its outer electrons and the hydrogen ions from the acid gain electrons to become hydrogen gas. The overall result is a transfer of electrons from the metal to the hydrogen.

M(s) + 2H+(aq) → M2+(aq) + H2(g)

This reaction can only take place if the metal is higher in the reactivity series than hydrogen. In other words, the reaction does not proceed with metals less reactive than lead, such as copper and silver.

Example: The reaction of magnesium with dilute hydrochloric acid:

Mg + 2HCl → MgCl2 + H2

Metals high in the series react very violently with acids and this reaction must not be performed.

Care must be taken with nitric acid, as it does not behave like a typical acid in its reactions with metals. Nitric acid is a strong oxidising agent and preferentially gets reduced to oxides of nitrogen. It is able to react in this way with most metals, it does not depend on any reactivity series.

Example: The reaction of copper with dilute nitric acid:

3Cu + 8HNO3 → 3Cu(NO3)2 + 2NO + 4H2O

It is easier for the metal to reduce the nitrogen atom than to reduce the hydrogen atom in nitric acid. Some metals, such as iron, become 'passive' when treated with nitric acid. This means that there is an initial reaction which soon ceases as the surface of the metal develops an impervious layer.

Finally, magnesium can cause nitric acid to behave as a normal acid (cf: behaviour of nitric acid with most metals). If the acid is very dilute, hydrogen is evolved.

Quick Check 1 - Reactions of acids
 
 
Your answer:

^ top

Worked examples

Q812-01 Which of the following is/are formed when a metal oxide reacts with a dilute acid?
  1. I.   A metal salt
  2. II.  Water
  3. III. Hydrogen gas
  1. I only
  2. I and II only
  3. II and III only
  4. I, II and III
Answer

Metal oxide + acid → metal salt + water

Therefore the correct response is I and II only = B

Q812-02 Sodium hydrogen carbonate is sometimes used as a test for the presence of an acid. Explain how this might work.
Answer
Sodium hydrogen carbonate releases carbon dioxide gas when reacting with acids. This can be observed as gas bubbles.
Q812-03 Restaurants often provide slices of lemon to for guests to remove the smell of fish from hands after eating shellfish. Suggest why lemon juice is goood at removing fishy odours.
Answer

Lemon juice contains acids (ascorbic acid and citric acid). The acids neutralise the bases that are responsible for the fishy odour. These bases are organic compounds, called amines, that are turned to non-volatile salts by the acids in the lemon. It is a neutralisation reaction.
Q812-04 Which type of acid is needed to prepare a sulfate?
Answer

Each different acid produces different types of salts according to the negative ions that it contains.

To prepare sulfates, sulfuric acid is needed.
Q812-05 Name a compound that could be used to react with hydrochloric acid to make sodium chloride.
Answer

The compound must be a base containing soldium ions, therefore either sodium hydroxide, or sodium carbonate would be suitable.
Q812-06 Why can't nitric acid and iron metal be used to prepare a sample of iron(II) nitrate?
Answer

Iron metal forms a passive (unreactive) layer on the surface when treated with nitric acid, which resists further reaction.
Q812-07 Suggest two suitable compounds that could be used to make a sample of magnesium nitrate.
Answer

Magnesium ions must be supplied using a suitable base containing magnesium, such as magnesium oxide or magnesium carbonate.

Either of these could be used to react with nitric acid.

MgO + HNO3 → Mg(NO3)2 + H2O
Q812-08 Copper metal is low down in the reactivity series and does not react with hydrogen ions in solution (acids). Why does copper successfully react with nitric acid?
Answer

Nitric acid behaves as an oxidising agent with metals, rather than an acid. The metals get oxidised to ions and the nitric acid gets reduced to oxides of nitrogen:

3Cu + 8HNO3 → 3Cu(NO3)2 + 2NO + H2O
Q812-09 Aluminium oxide is described as an amphoteric oxide. Which of the following is the correct definition of an amphoteric substance.
  1. A substance that doesn't react with either acids or bases.
  2. An insoluble base that reacts with acids.
  3. A substance that neutralises both acids and bases.
  4. An insoluble acid that neutralises bases.
Answer

Amphoteric substances neutralise both acids and bases.
Q812-10 The reaction between sulfuric acid and marble chips (calcium carbonate) is unsuitable for preparing calcium sulfate. Suggest a reason why this might be the case.
Answer

Although at first sight it may seem to be an appropriate reaction between an acid and a base, the low solubility of calcium sulfate prevents the reaction proceeding.

After addition of sulfuric acid, the marble chips develop an insoluble layer of calcium sulfate on the surface that prevents further reaction.

^ top