Oleum

Key takeaways

Oleum belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Oleum to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Oleum from memory before moving on to harder problems.

Reference excerpt

Oleum (Latin oleum, meaning oil), or fuming sulfuric acid, is a term referring to solutions of various compositions of sulfur trioxide in sulfuric acid, or sometimes more specifically to disulfuric acid (also known as pyrosulfuric acid). Oleums can be described by the formula ySO3·H2O where y is the total molar mass of sulfur trioxide content. The value of y can be varied, to include different oleums. They can also be described by the formula H2SO4·xSO3 where x is now defined as the molar free sulfur trioxide content. Oleum is generally assessed according to the free SO3 content by mass. It can also be expressed as a percentage of sulfuric acid strength; for oleum concentrations, that would be over 100%. For example, 10% oleum can also be expressed as H2SO4·0.13611SO3, 1.13611SO3·H2O or 102.25% sulfuric acid. The conversion between % acid and % oleum is:

% acid = 100 + 18 80 × % oleum {\displaystyle \%\,{\text{acid}}=100+{\frac {18}{80}}\times \%\,{\text{oleum}}}

For x = 1 and y = 2 the empirical formula H2S2O7 for disulfuric (pyrosulfuric) acid is obtained. Pure disulfuric acid is a solid at room temperature, melting at 36 °C and rarely used either in the laboratory or industrial processes — although some research indicates that pure disulfuric acid has never been isolated yet.

Production Oleum is produced in the contact process, where sulfur is oxidized to sulfur trioxide which is subsequently dissolved in concentrated sulfuric acid. Sulfuric acid itself is regenerated by dilution of part of the oleum. The lead chamber process for sulfuric acid production was abandoned, partly because it could not produce sulfur trioxide or concentrated sulfuric acid directly due to corrosion of the lead, and absorption of NO2 gas. Until this process was made obsolete by the contact process, oleum had to be obtained through indirect methods. Historically, the biggest production of oleum came from the distillation of iron sulfates at Nordhausen, from which the historical name Nordhausen sulfuric acid is derived.

Applications

Sulfuric acid production Oleum is an important intermediate in the manufacture of sulfuric acid due to its high enthalpy of hydration. When SO3 is added to water, rather than dissolving, it tends to form a fine mist of sulfuric acid, which is difficult to manage. However, SO3 added to concentrated sulfuric acid readily dissolves, forming oleum which can then be diluted with water to produce additional concentrated sulfuric acid. Typically, above concentrations of 98.3%, sulfuric acid will undergo a spontaneous decomposition into sulfur trioxide and water

H2SO4 ⇌ SO3 + H2O This means that sulfuric acid above said concentration will readily degenerate until it reaches 98.3%; this is impractical in some applications such as synthesis where anhydrous conditions are preferred (like alcohol eliminations). Adding sulfur trioxide alters the chemical equilibrium, allowing concentration to be increased beyond 98.3%.

As an intermediate for transportation Oleum is a useful form for transporting sulfuric acid compounds, typically in rail tank cars, between oil refineries, which produce various sulfur compounds as a byproduct of refining, and industrial consumers. Certain compositions of oleum are solid at room temperature, and thus are safer to ship than as a liquid. Solid oleum can be converted into liquid at the destination by steam heating or dilution or concentration. This requires care to prevent overheating and evaporation of sulfur trioxide. To extract it from a tank car requires careful heating using steam conduits inside the tank car. Great care must be taken to avoid overheating, as this can increase the pressure in the tank car beyond the tank's safety valve limit. In addition, oleum lacks free water to attack surfaces, making it less corrosive to metals. Because of that, sulfuric acid is sometimes concentrated to oleum for in-plant pipelines and then diluted back to acid for use in industrial reactions. The freezing point of oleum varies strongly and non-monotonically with concentration, and so small shifts in concentration (up or down) can lead to oleum unexpectedly freezing in pipes as temperatures as high as 35 C, or remaining liquid at temperatures as low as 0 C. In Richmond, California in 1993 a tank car of oleum overheated and released sulfur trioxide, creating a mist of micrometre-sized sulfuric acid particles that spread over a wide area.

Organic chemistry research Oleum is a harsh reagent, and is highly corrosive. One important use of oleum as a reagent is the secondary nitration of nitrobenzene. The first nitration can occur with nitric acid in sulfuric acid, but this deactivates the ring towards further electrophilic substitution. A stronger reagent, oleum, is needed to introduce the second nitro group onto the aromatic ring.

Explosives manufacture Oleum is used in the manufacture of many explosives with the notable exception of nitrocellulose. (In modern manufacturing of nitrocellulose, the H2SO4 concentration is often adjusted using oleum.) The chemical requirements for explosives manufacture often require anhydrous mixtures containing nitric acid and sulfuric acid. Ordinary commercial grade nitric acid consists of the constant boiling azeotrope of nitric acid and water, and contains 68% nitric acid. Mixtures of ordinary nitric acid in sulfuric acid therefore contain substantial amounts of water and are unsuitable for processes such as those that occur in the manufacture of trinitrotoluene. The synthesis of RDX and certain other explosives does not require oleum. Anhydrous nitric acid, referred to as white fuming nitric acid, can be used to prepare water-free nitration mixtures, and this method is used in laboratory scale operations where the cost of material is not of primary importance. Fuming nitric acid is hazardous to handle and transport, because it is extremely corrosive and volatile. For industrial use, such strong nitration mixtures are prepared by mixing oleum with ordinary commercial nitric acid so that the free sulfur trioxide in the oleum consumes the water in the nitric acid.

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Oleum

Start with the simplest possible case. Write down what Oleum claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Oleum before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Oleum ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Oleum

In research: Oleum appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Oleum in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Oleum is common in secondary-school and first-year university syllabi. It links to neighbouring topics Acids, Sulfates, Sulfur oxoacids, so understanding it makes those chapters shorter.
In everyday life: Look for Oleum outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Oleum in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Oleum means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Oleum out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Oleum in simple terms?

Oleum (Latin oleum, meaning oil), or fuming sulfuric acid, is a term referring to solutions of various compositions of sulfur trioxide in sulfuric acid, or sometimes more specifically to disulfuric acid (also known as pyrosulfuric acid). Oleums can be described by the formula ySO3·H2O where y is th…

Why does Oleum matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Oleum?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Oleum.