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# Specific acids and bases

## Sulphur dioxide

Sulfur dioxide ($\ce{SO2}$) is an acidic oxide that forms sulfurous acid in water.

$$\ce{SO2 {(g)} + H2O {(l)} -> H2SO3 {(aq)}}$$

It is often used as a bleach. It is also used in the manufacture of wood pulp and as a food preservative.

Sulfur dioxide is commonly used to preserve dried fruits.

Sulfur dioxide is used to produce sulfuric acid ($\ce{H2SO4}$) in the contact process.

The contact process starts with the production of sulfur dioxide, which is used in the eventual production of sulfuric acid.

## Acidic and basic oxides

An oxide is a compound that contains oxygen and one other element.

Oxides that contain a metal tend to be basic. This is because metal oxides react with water to form compounds containing hydroxide ions.

$$\ce{MgO {(s)} + H2O {(l)} -> Mg(OH)2 {(s)}}$$

Metal oxides can neutralise acids because of their basic behaviour.

$$\ce{MgO {(s)} + 2HCl {(aq)} -> MgCl2 {(aq)} + H2O {(l)}}$$

Oxides that contain a non-metal tend to be acidic because they form compounds that release $\ce{H^+}$ ions after reacting with water.

$$\ce{CO2 {(g)} + H2O {(l)} -> H2CO3 {(aq)}}$$

Non-metal oxides can neutralise bases because of their acidic behaviour.

$$\ce{CO2 {(g)} + 2NaOH {(aq)} -> Na2CO3 {(aq)} + H2O {(l)}}$$

Sand consists of silicon dioxide ($\ce{SiO2}$), an acidic oxide.

## Amphoteric and neutral oxides

Amphoteric oxides can behave as either acids or bases.

The term amphoteric is used to describe all substances that can behave as acids or bases.

Oxides that contain elements that are near the dividing line between metals and non-metals on the periodic table tend to be amphoteric.

Aluminium oxide ($\ce{Al2O3}$) is amphoteric. It neutralises acids, and therefore acts as a base under acidic conditions.

$\ce{Al2O3 {(s)} + 6HCl {(aq)} -> 2AlCl3 {(aq)} + 3H2O {(l)}}$

Aluminium oxide neutralises bases, and so can act as an acid.

$\ce{Al2O3 {(s)} + 2NaOH {(aq)} + 3H2O {(l)} -> 2NaAl(OH)4 {(aq)}}$

Powdered form of aluminium oxide. It does not dissolve in water but dissolves in acids and bases when it reacts to form soluble products.

Neutral oxides do not behave as either acids or bases and hence do not participate in neutralisation reactions.

Carbon monoxide ($\ce{CO}$), nitrous oxide ($\ce{N2O}$) and nitric oxide ($\ce{NO}$) are neutral oxides.

## Sulphuric acid

Sulfuric acid ($\ce{H2SO4}$) is an acid that dissociates twice.

$$\ce{H2SO4 {(aq)} -> H^+ {(aq)} + HSO4^- {(aq)} <=> 2H^+ {(aq)} + SO4^2- {(aq)}}$$

$\ce{H2SO4}$ is a strong acid as its dissociation is complete. $\ce{HSO4^-}$ is a weak acid because it partially dissociates.

Sulfuric acid is used to produce fertilisers (particularly phosphate fertilisers). Sulfuric acid is also used as an industrial cleaning liquid and as a catalyst.

Additionally, sulfuric acid is an electrolyte in lead acid batteries (found in cars).

Sulfuric acid is produced industrially using the contact process.

Drain cleaners contain sulfuric acid. One must be careful when handling this as sulfuric acid is highly corrosive.