4_8 Sulphuric Acid: Acid, Dehydrating Agent, Oxidizing Agent and Its Uses

Sulphuric acid,

\mathrm{H_2SO_4}

, behaves in three distinct ways depending on how concentrated and how hot it is. Dilute, it is a typical strong acid. Concentrated, it is a powerful dehydrating agent that tears the elements of water out of other compounds. Hot and concentrated, it is an oxidizing agent. Knowing which role it is playing explains almost all of its reactions.

Physical Properties and Safe Handling

Pure sulphuric acid is a colourless, odourless, dense, oily liquid (density about 1.83 g/cm³). It freezes at about 10 °C and boils at about 338 °C. Its high boiling point comes from extensive hydrogen bonding, and it makes sulphuric acid useful for displacing more volatile acids from their salts. The pure acid barely conducts electricity, but it becomes a good conductor once water is added and it ionizes. Mixing it with water releases a great deal of heat. Concentrated acid must always be added slowly to water, never the reverse, and it causes severe burns to skin.

As an Acid

Sulphuric acid is dibasic and ionizes in two steps. The first is complete, and the second is partial, because

\mathrm{HSO_4^-}

is a weaker acid:

\begin{aligned} \mathrm{H_2SO_4 + H_2O} &\rightarrow \mathrm{H_3O^+ + HSO_4^-} \\ \mathrm{HSO_4^- + H_2O} &\rightleftharpoons \mathrm{H_3O^+ + SO_4^{2-}} \end{aligned}

• With bases it forms two series of salts, hydrogensulphates such as

  \mathrm{NaHSO_4}

  and sulphates such as

  \mathrm{Na_2SO_4}

  .
• Carbonates and hydrogencarbonates release carbon dioxide.
• Dilute acid reacts with metals above hydrogen, such as Zn, Fe and Mg, to give hydrogen and the metal sulphate.
• Being non-volatile, concentrated acid displaces volatile acids from their salts:

  \mathrm{NaCl + H_2SO_4 \rightarrow NaHSO_4 + HCl}

  , and nitrates give nitric acid.
• It forms white precipitates with barium, lead(II) and strontium salts. Barium sulphate is the standard test for sulphate ions.

As a Dehydrating Agent

Concentrated sulphuric acid has such a strong attraction for water that it removes hydrogen and oxygen in a 2 : 1 ratio even from compounds that contain no water molecules. Sugar turns into a rising column of black carbon. Paper, wood and cloth char. Formic acid gives carbon monoxide, oxalic acid gives a mixture of CO and

\mathrm{CO_2}

, and ethanol gives ethene:

CCO>>C=C.O

\mathrm{C_{12}H_{22}O_{11}(s) \xrightarrow{\ conc.\ H_2SO_4\ } 12C(s) + 11H_2O}

The same property makes it useful for drying gases that do not react with it, such as chlorine and sulphur dioxide. It cannot be used to dry ammonia, which it neutralizes to ammonium sulphate.

As an Oxidizing Agent

Hot concentrated sulphuric acid is a fairly strong oxidizing agent, and it is reduced to sulphur dioxide. That is how it attacks copper, silver and mercury, which are below hydrogen and do not react with dilute acid:

\mathrm{Cu + 2H_2SO_4(conc) \xrightarrow{\ \Delta\ } CuSO_4 + SO_2 + 2H_2O}

It also oxidizes non-metals: carbon to

\mathrm{CO_2}

and sulphur to

\mathrm{SO_2}

. It oxidizes hydrogen sulphide to sulphur, and hydrogen bromide and hydrogen iodide to bromine and iodine. This is why concentrated sulphuric acid cannot be used to make pure HBr or HI from their salts.

Reactions with Organic Compounds

With benzene, concentrated or fuming sulphuric acid carries out an electrophilic substitution called sulphonation, giving benzenesulphonic acid. This is a key step in making detergents and dyes:

c1ccccc1.OS(=O)(=O)O>>OS(=O)(=O)c1ccccc1.O

Uses

• Fertilizers: the largest single use, making superphosphates, phosphoric acid and ammonium sulphate.
• Chemicals: making other acids (HCl, HF,

  \mathrm{H_3PO_4}

  ), sulphate salts, detergents, dyes, drugs, pigments and explosives.
• Petroleum refining and metal processing: removing impurities, and pickling steel to remove rust before coating.
• Lead–acid batteries: the electrolyte in car batteries.
• Industry and laboratories: a drying agent, a reagent and a catalyst. It is also used in textiles, paper, leather and plastics.

Tips for remembering

• Add Acid to water (A then W, alphabetical): pouring water into concentrated acid can make it boil and spit, so always add the acid slowly to water.
• Dilute = acid, concentrated + hot = oxidizer: dilute H₂SO₄ gives H₂ with zinc, while hot concentrated acid gives SO₂ even with copper.
• Thirsty acid: concentrated H₂SO₄ pulls the elements of water out of sugar, leaving a column of black carbon.