Sulfuric acid

Sulfuric acid is the strong, diprotic industrial acid with the formula H₂SO₄, manufactured by the Contact process and one of the most heavily produced chemicals in the world. Malay: asid sulfurik · Chinese: 硫酸.

Because it is diprotic, one molecule can release two hydrogen ions, so dilute sulfuric acid shows the full set of acidic reactions. It reacts with reactive metals to give a salt and hydrogen, with bases and alkalis in neutralisation to give a salt and water, and with carbonates to give a salt, water and carbon dioxide. Its salts are the sulfates. Concentrated sulfuric acid behaves differently: it is a powerful dehydrating agent and can char sugar, and it must always be diluted by adding the acid slowly to water, never the reverse.

Example. Sulfuric acid is used to make fertilisers such as ammonium sulfate, to make detergents and paints, to process metals, and as the electrolyte in a lead–acid car accumulator. In the laboratory, dilute sulfuric acid with zinc gives zinc sulfate and hydrogen: Zn + H₂SO₄ → ZnSO₄ + H₂.

Confusion to avoid. Do not confuse sulfuric acid, H₂SO₄, with sulfurous acid, H₂SO₃, which is the weaker acid formed when sulfur dioxide dissolves in water. Keep the dilute acid, which acts by its hydrogen ions, separate from the concentrated acid, which acts as a dehydrating agent.

A frequent exam point is to explain why the acid is so useful: it is a strong acid, it is cheap because the Contact process makes it in bulk, and its two acidic hydrogens let it form both normal and acid salts. When answering, always name the specific salt produced and remember that concentrated and dilute sulfuric acid are treated as two very different reagents.

Sulfuric acid is the headline product of this chapter, linking the Contact process to the acid and salt chemistry you already know, and it is common in SPM Chemistry.