IB Chemistry home > Syllabus 2016 > Acids and Bases > The properties of acids

Syllabus ref: 8.1

Acids react in a characteristic manner with several types of reagent.

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:

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, MgCO3, Na2CO3, K2CO3

Metal hydrogen carbonates sodium hydrogen carbonate, potassium hydrogen carbonate NaHCO3, KHCO3

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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.

Example: The reaction of aluminium oxide and sodium hydroxide:

Al2O3 + 2NaOH 2NaAlO2 + H2O

Aluminium oxide + sodium hydroxide sodium aluminate + water

Example: The reaction of aluminium oxide and sulfuric acid

2Al2O3 + 3H2SO4 2Al2(SO4)3 + 3H2O

aluminium oxide + sulfuric acid aluminium sulfate + water


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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.

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.


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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:

medium
Universal indicator
Litmus
Phenolphthalein
Acidic
red
red
colorless
Neutral
green
purple
pink
Basic
purple-blue
blue
red

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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