Sulphuric Acid

Sulphuric Acid

Sulphuric acid ranks among the most important industrial chemicals produced anywhere in the world — a nation's industrial strength is sometimes judged by how much sulphuric acid it manufactures and consumes.

Manufacture — The Contact Process

Sulphuric acid is manufactured by the Contact Process, which proceeds in three stages:

1. Sulphur, or a sulphide ore, is burnt in air to generate sulphur dioxide.
2. The $SO_2$ produced is converted to $SO_3$ by reaction with oxygen in the presence of a catalyst.
3. The $SO_3$ is absorbed in concentrated sulphuric acid to give oleum ($H_2S_2O_7$).

The overall plant flow, shown in the figure, begins at the sulphur burner, where sulphur burns in a stream of air, with a dust precipitator at its base removing solid particles; the resulting impure $SO_2 + O_2$ gas mixture then passes through a washing and cooling tower (sprayed with water) and a drying tower (sprayed with concentrated $H_2SO_4$) to remove moisture, giving dry $SO_2 + O_2$. This dry gas is further purified by passing it through an arsenic purifier packed with gelatinous hydrated ferric oxide (which removes arsenic compounds and other catalyst poisons) and a preheater, before entering the catalytic converter — a column packed with beds of the $V_2O_5$ catalyst. The $SO_3$ gas leaving the converter finally passes into an absorption tower packed with quartz, where it is absorbed into concentrated $H_2SO_4$ sprayed from the top, with oleum drawn off at the base.

The key step — the catalytic oxidation of $SO_2$ to $SO_3$ — is:

This reaction is exothermic and reversible, and the forward reaction proceeds with a decrease in gas volume. Low temperature and high pressure therefore favour a maximum yield of $SO_3$ — but the temperature cannot be lowered indefinitely, since the rate of reaction would then become too slow to be useful. In practice, industrial plants operate at a compromise of about 2 bar pressure and 720 K.

The $SO_3$ produced is absorbed into concentrated $H_2SO_4$ to form oleum:

Diluting the oleum with the calculated amount of water then gives sulphuric acid at whatever concentration is desired. Industrially, the $SO_2 \rightarrow SO_3$ conversion and the $SO_3$-absorption step are carried out simultaneously and continuously, both to keep the process running without interruption and to reduce cost. Sulphuric acid produced by the Contact Process is typically 96–98% pure.

Physical Properties

Pure sulphuric acid is a heavy, oily, colourless liquid carrying a specific gravity of 1.84 at 298 K. It freezes at 283 K and boils at 611 K. Dissolving it in water releases a large quantity of heat, which is why concentrated sulphuric acid must always be added slowly into water with constant stirring when preparing the dilute acid — never the reverse — to avoid dangerous local boiling and spattering.

Chemical Properties

The characteristic reactions of sulphuric acid arise from four features: its low volatility, its strong acidic character, its strong affinity for water, and its ability to act as an oxidising agent.

Acidic ionisation. In aqueous solution, $H_2SO_4$ ionises in two successive steps:

Since a larger dissociation constant signifies a stronger acid, the very large $K_{a1}$ shows that the first ionisation is essentially complete, while the much smaller $K_{a2}$ shows that the second ionisation is only partial. The acid accordingly forms two series of salts: normal sulphates (e.g. sodium sulphate, copper sulphate) and acid sulphates (e.g. sodium hydrogen sulphate).

Low volatility. Because sulphuric acid is far less volatile than many other acids, it can displace more volatile acids from their salts on heating:

Dehydrating action. Concentrated sulphuric acid is a powerful dehydrating agent. Many moist gases that do not themselves react with the acid can be dried by bubbling them through it, provided they are otherwise unreactive towards it. It can even strip water directly out of organic compounds — its charring action on sugar (a carbohydrate) vividly demonstrates this dehydrating power:

$C_{12}H_{22}O_{11} \xrightarrow{H_2SO_4} 12C + 11H_2O$ …