Manufacture of sulphuric acid

The whole process is a three-stage relay, and each stage hands a purified, more oxidised product to the next.

Stage 1 — burn sulphur to sulphur dioxide. Sulphur is burnt in air (or, if elemental sulphur is scarce, a sulphide ore such as iron pyrites, FeS₂, is roasted in air instead):

This is an ordinary combustion — cheap, fast, and it gets the sulphur into a gaseous, reactive oxide straight away.

Stage 2 — catalytic oxidation of sulphur dioxide to sulphur trioxide. The sulphur dioxide, now purified of dust and dried, is mixed with excess air and passed over a vanadium pentoxide (V₂O₅) catalyst held at about 450 degrees Celsius:

This is the step the whole plant is built around: it is slow without a catalyst, so V₂O₅ and a controlled temperature are what make it industrially fast.

Stage 3 — absorption and dilution. Sulphur trioxide is never bubbled straight into water. Instead it is absorbed in concentrated sulphuric acid to give oleum (fuming sulphuric acid):

Oleum is then diluted with a measured amount of water to give sulphuric acid of the required strength:

Two housekeeping facts sit alongside the three stages. First, the SO₂ feeding Stage 2 must be dust-free and dry, because solid impurities and moisture coat and deactivate the vanadium pentoxide catalyst, slowing or stopping the SO₂-to-SO₃ conversion. Second, the choice of sulphur as the starting material is deliberate: it is cheap, abundant, and burns readily in air, so it gets the plant to SO₂ — the common entry point of the whole process — as economically as possible.

The Contact Process is a purely descriptive, step-by-step manufacturing sequence rather than a formula-driven derivation, so the reasoning below explains why each design choice — the catalyst, the temperature, the two-step absorption — solves a real practical problem instead of deriving a numeric result.

Step 1. Start from the raw-material choice. Sulphur is picked because it is cheap, plentiful, and combines with oxygen the moment it is lit, so a single combustion step gets the entire process to its common gateway compound, SO₂. Sulphide ores like iron pyrites achieve the same gateway when elemental sulphur is not locally available, simply by roasting instead of burning — the destination, SO₂, is identical either way.

Step 2. Consider why Stage 2 needs help at all. Sulphur dioxide reacting with oxygen to give sulphur trioxide is a real reaction, but left alone at ordinary conditions it proceeds far too slowly to be useful on an industrial scale. A catalyst is needed to speed the reaction up without being consumed by it, and vanadium pentoxide is chosen because it does this job efficiently, is chemically robust at the operating temperature, and is far cheaper than the platinum used for the analogous step in the Ostwald process for nitric acid.

Step 3. Ask why the SO₂ feed must be purified and dried before it reaches that catalyst. A solid catalyst like V₂O₅ works at its surface — gas molecules must be able to reach and bind to that surface to react. Dust particles and moisture coat the catalyst surface, physically blocking access and "poisoning" it, so the conversion of SO₂ to SO₃ falls even though the catalyst is technically still present. Cleaning and drying the gas beforehand protects the plant's most expensive, most delicate component.

Step 4. Finally, work out why sulphur trioxide cannot simply be piped into water, which is the seemingly obvious way to make an acid from an acidic oxide. SO₃ reacting with water is intensely exothermic; passed directly into water, the heat released flashes much of the water into vapour instantly, producing a dense, corrosive fog of tiny sulphuric acid droplets suspended in the gas stream rather than a clean liquid. That mist is hazardous and almost impossible to collect efficiently. Absorbing SO₃ in already-concentrated sulphuric acid instead avoids the flash-boiling problem entirely, because the reaction with a small amount of liquid acid is far gentler and produces a stable liquid product, oleum, which can then be diluted with water in a slow, controlled, deliberately metered way to reach the exact strength wanted.

Problem: A student is given the raw material sulphur and asked to write the complete equation sequence of the Contact Process, name the catalyst and its operating temperature, and state why the final absorption step uses concentrated sulphuric acid rather than water.

Step 1. Combustion of sulphur in air:

Step 2. Catalytic oxidation of the sulphur dioxide, using vanadium pentoxide (V₂O₅) as catalyst at about 450 degrees Celsius:

Step 3. Absorption of the sulphur trioxide in concentrated sulphuric acid, forming oleum:

Step 4. Dilution of the oleum with a calculated amount of water:

Step 5. Reasoning for the last part: SO₃ reacts with water so violently and exothermically that direct absorption flashes the water into a dense, corrosive, hard-to-collect acid mist rather than a clean liquid. Absorbing in concentrated acid first avoids this, and the resulting oleum can then be diluted with water safely and under control.

Answer: the four equations above, in that order; catalyst = vanadium pentoxide (V₂O₅) at about 450 degrees Celsius; SO₃ is absorbed in concentrated H₂SO₄, not water, to avoid an uncontrollable acid mist.

Competitive stretch

If asked to identify the catalyst and approximate temperature for the analogous oxidation step in Ostwald's process (ammonia to nitric acid) for comparison, the answer is platinum (or a platinum-rhodium gauze) at about 800 degrees Celsius — a useful contrast that also helps keep the three industrial catalysts (Fe for Haber's, Pt for Ostwald's, V₂O₅ for Contact) from being mixed up.

• Writing that SO₃ is passed directly into water — the industrial process specifically avoids this because it forms an uncontrollable, corrosive acid mist; oleum is the required intermediate.
• Naming the wrong catalyst for the Contact Process — it is vanadium pentoxide (V₂O₅), not platinum (used in Ostwald's process) or iron (used in Haber's Process).
• Forgetting the SO₂ gas must be purified and dried before the catalytic stage, since dust or moisture would poison/deactivate the V₂O₅ catalyst and cut the conversion efficiency.
• Trying to justify the catalyst or temperature choice using equilibrium or reaction-rate theory — at this level the syllabus wants the steps and conditions stated descriptively, not derived from equilibrium reasoning.

Ask the tutor: want a quick way to keep the three industrial catalysts — Fe, Pt, and V₂O₅ — straight across Haber's, Ostwald's, and the Contact Process? Ask the AI tutor about "Manufacture of sulphuric acid."