Oleum

Oleum is the substance with the formula H₂S₂O₇, formed in the Contact process when sulfur trioxide is dissolved in concentrated sulfuric acid. It is then diluted carefully with water to give more concentrated sulfuric acid. Malay: oleum · Chinese: 发烟硫酸.

Oleum exists because of a practical problem in the last stage of the Contact process. Sulfur trioxide cannot be added straight to water, because the reaction is so vigorous that it produces a thick, choking mist of sulfuric acid droplets that is hard to condense and collect. Instead the sulfur trioxide is absorbed into concentrated sulfuric acid, where it dissolves smoothly to form oleum, and the oleum is then diluted with the right amount of water in a controlled step to yield sulfuric acid.

Example. The two steps can be written as SO₃ + H₂SO₄ → H₂S₂O₇, forming oleum, followed by H₂S₂O₇ + H₂O → 2H₂SO₄, giving sulfuric acid. Notice that the overall effect is the same as adding sulfur trioxide to water, but carried out safely and in a controllable way.

Confusion to avoid. Do not confuse oleum with sulfuric acid itself or with sulfur trioxide. Oleum is the intermediate between them: it is richer in sulfur trioxide than ordinary concentrated acid, which is why it is also called fuming sulfuric acid.

It also helps to remember the order of the two steps. Sulfur trioxide is absorbed into concentrated acid first, forming oleum, and only then is water added to the oleum to give sulfuric acid. Writing the steps the wrong way round, or leaving oleum out of the description altogether, is a common way to lose marks when describing the last stage of the Contact process.

Knowing why the oleum route is used, rather than dissolving sulfur trioxide directly in water, is a standard point that SPM Chemistry rewards in Contact-process questions.