Neutralisation

Acids react wth bases in a neutralisation reaction. 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:

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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.)

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Amphoteric behaviour

An amphoteric substance can neutralise both acids and bases. Such substances are exemplified by oxides of 'poor' metals, i.e. metals from the centre of the periodic table, such as aluminium oxide and zinc oxide.

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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 work with metals less reactive than lead, such as copper and silver.

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.

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.

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Indicators

Indicators are substances that change colour in the presence of an acid (or a base). The colour change depends on the type of indicator used. The most common indicators are universal indicator, litmus indicator and phenolphthalein.

The colour changes are:

The degree of acidity or basicity of a solution can be measured by the pH scale. This gives a value proportional to the concentration of H+ ions in the solution

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