Sulfur trioxide: structure and main reactions
Hexavalent sulfur forms stable compounds. Interestingly, sulfur(IV) compounds are thermodynamically unstable and tend to convert into sulfur(VI) upon contact with any oxidizing agents, including atmospheric oxygen.
In the gaseous state, sulfur trioxide (sulfuric anhydride, $SO_3$) consists of planar molecules. However, upon cooling, the substance tends to polymerize, aggregating into oligomers (e.g., cyclic trimer). This is a chemically extremely active compound that readily undergoes addition reactions:
- With water: forms sulfuric acid ($H_2SO_4$).
- With anhydrous sulfuric acid: synthesizes disulfuric acid ($H_2S_2O_7$).
- With hydrogen chloride: yields chlorosulfuric acid.
Industrial production of the anhydride requires the oxidation of sulfur dioxide ($SO_2$) with oxygen. The process proceeds only at temperatures not exceeding 500 °C in the presence of a special catalyst—vanadium pentoxide ($V_2O_5$). The initial gas mixture must be thoroughly purified from impurities that poison the catalyst.
Industrial synthesis of sulfuric acid
Although the reaction of $SO_3$ with water looks trivial, direct hydration is not used in practice. Upon contact of the gas with water, a dense and persistent acidic fog of microscopic droplets instantly forms, which is technologically very difficult to collect into a liquid.
Instead, the absorption method is employed:
- Sulfur trioxide is absorbed by concentrated sulfuric acid.
- As a result, oleum is formed—a solution of anhydride and disulfuric acid in sulfuric acid.
- The resulting oleum is carefully mixed with water. In the process, the dissolved $SO_3$ reacts with moisture, converting into an additional portion of $H_2SO_4$.
Sulfuric acid is produced in quantities of tens of millions of tons per year, making it one of the primary products of the global chemical industry.
Properties of concentrated solution
Pure concentrated Acidum sulfuricum is a heavy, colorless, oily liquid, historically known as oil of vitriol.
Its hallmark is extreme hygroscopicity. In closed vessels (desiccators), the acid is used for drying air and substances. It is capable of extracting water even from crystalline hydrates: for example, blue copper sulfate next to it loses its water of crystallization and turns into white anhydrous copper sulfate. Organic compounds upon contact with the acid undergo instantaneous dehydration and charring (sugar turns into pure carbon).
The concentrated solution also exhibits moderate oxidizing properties due to sulfur. It is capable of dissolving copper with the release of $SO_2$ and displacing free iodine from salts (e.g., from potassium iodide with the release of hydrogen sulfide).
Dilute acid and safety protocols
Dilute acid (with a mass fraction of less than 70%) behaves differently. It completely dissociates into ions, and the oxidizing agent is no longer sulfur, but the hydrogen ion ($H^+$). Such an acid reacts only with metals that precede hydrogen in the reactivity series.
Safety protocols:
- The acid causes severe, deep chemical burns.
- Main rule: concentrated acid must be slowly poured into water, but it is strictly forbidden to pour water into acid! Water is lighter, remains on the surface of the heavy "oil," and boils instantly from the sudden heat release, splashing the aggressive liquid.
- First aid: in the laboratory, the burn is immediately washed with a large volume of cold water. However, in the event of a massive industrial spill, the acid must first be carefully removed with dry rags (fabric) and only then washed with water to avoid thermal shock from the reaction on the skin.
Calculations involving oleum
Diluting oleum requires precise calculations. Oleum is treated as a binary mixture of $SO_3$ and anhydrous $H_2SO_4$. The calculation algorithm is as follows:
- The mass of pure $SO_3$ in the initial oleum is calculated.
- The mass of the new sulfuric acid produced by the reaction of this $SO_3$ with the added water is calculated.
- An equation is set up where the final mass of the acid is divided by the total mass of the solution (mass of oleum plus mass of added water), which allows finding the required volume of solvent.
Briefly: answers to topical questions
Which metals are passivated by concentrated sulfuric acid in the cold?
Under normal conditions, concentrated sulfuric acid passivates the surface of iron, so the reaction does not proceed. Upon heating, iron begins to react with concentrated sulfuric acid to form iron(III) sulfate, sulfur dioxide, and water: $$2Fe + 6H_2SO_4 \xrightarrow{t} Fe_2(SO_4)_3 + 3SO_2\uparrow + 6H_2O$$
What products are formed when copper dissolves in concentrated sulfuric acid?
When copper dissolves in concentrated sulfuric acid upon heating, sulfur dioxide (sulfur(IV) oxide), copper(II) sulfate, and water are formed. In this reaction, the concentrated acid exhibits moderately strong oxidizing properties. The chemical equation for the process is as follows: $$Cu + 2H_2SO_4 \xrightarrow{t} SO_2\uparrow + CuSO_4 + 2H_2O$$ The evolving sulfur dioxide is a colorless gas with an unpleasant, suffocating odor.
What is the biological role of sulfate ions and active sulfate (PAPS) in the human body?
Active sulfate PAPS (3'-phosphoadenosine-5'-phosphosulfate) participates in conjugation reactions in phase II of liver metabolism, including sulfation. For example, indoxyl reacts with PAPS in the presence of the enzyme sulfotransferase to form indoxyl sulfuric acid; the end product is the potassium salt of indoxyl sulfuric acid, animal indican.
Sulfation is characteristic of glycosaminoglycans: GAGs serve as the prosthetic group of proteoglycans and are characterized by a high degree of sulfation and a negative charge.
What is the chemical equation for the dehydration of sucrose by concentrated sulfuric acid?
The chemical equation for the dehydration of sugar by concentrated sulfuric acid is: $$C_{12}H_{22}O_{11} \xrightarrow{H_2SO_4} 12C + 11H_2O$$ During this process, organic substances are charred. Concentrated sulfuric acid exhibits a strong dehydrating effect: it removes hydrogen and oxygen atoms from the carbohydrate molecule, and the water released as a result of the reaction binds with the acid itself, forming hydrates.
"First the water, then the acid — otherwise disaster will be drastic!"
Class and exam questions
Why is the gas not dissolved directly in water during the production of sulfuric acid?
The direct reaction of sulfur trioxide with water causes the formation of a persistent acid fog that cannot be effectively condensed. Therefore, the gas is dissolved in concentrated acid to form oleum.
What happens if you pour water into concentrated sulfuric acid?
Water has a lower density and remains on the surface. Due to the massive heat release upon mixing, the water will instantly boil, leading to dangerous splashing of the hot concentrated acid.
How to prove the presence of sulfate ions in a solution?
To detect sulfates, a confirmatory test using soluble barium salts (e.g., barium chloride) is used. Upon their interaction, a white precipitate of barium sulfate instantly drops out, insoluble in acids.
What else to study on this topic
- Mechanism of the catalytic action of vanadium oxide during the oxidation of sulfur dioxide
- Structure and polymerization of sulfur trioxide in various state phases
- Mathematical algorithms for solving oleum dilution problems
- Redox reactions of concentrated sulfuric acid with halides
- Effect of hygroscopicity on the shift of chemical equilibrium in crystalline hydrates
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