Contact Process

It is said that, if one could estimate the amount of sulfuric acid produced in a country, then that information is more than enough to estimate the industrial growth of that particular country. This is because sulfuric acid is an essential raw material for almost everything that is industrially made. It is commonly used in fertilizer manufacturing, oil refining, mineral processing and it is even used in wastewater processing. Some other uses include domestic acidic drain cleaners, the electrolyte in lead-acid batteries, as a dehydrating agent, etc.

Having said that, one of the most popular or common methods to manufacture sulphuric acid is via the contact process. This process was invented by a British merchant named Peregrine Phillips. It was patented in the year 1831. Apart from being an economical process for manufacturing sulfuric acid, sulfur trioxide and oleum are also obtained from this process. We will be learning more about the contact process in this lesson.

Sulfuric Acid (H2SO4)

In a nutshell, sulfuric acid is a strong mineral acid characterized by its strong dehydrating and oxidizing nature. It has a chemical formula H2SO4. This acid is colourless with a pungent smell. It is soluble in water and releases heat on contact. On top of that, this acid is corrosive to metals and most other organic matters like tissue, wood, etc. On contact with such organic substances, sulfuric acid instantly dehydrates them causing char to form. Sulfuric acid has a density of 1.83 g / cm3.

The molecular structure of sulfuric acid is represented below in the form of a line-wedge-dash structure.

Sulphuric Acid

Properties of sulphuric acid are listed below:

Chemical formula H2SO4
Molar mass 98.079 g/mol
Appearance Clear, colourless liquid
Density 1.8302 g/cm3
Melting point and boiling point 50.560 F and 6390F
Solubility in water Exothermic, miscible
Viscosity 26.7 cP

Manufacture of Sulfuric Acid By Contact Process

Generally, there are several ways to manufacture sulfuric acid. Each of them varies in effort, cost, and purity of the sulfuric acid that is produced. However, the most common process among these is the contact process. Let us discuss this process in detail below.

The manufacture of sulfuric acid using contact process involves four steps. These include;

  • Extraction of sulfur.
  • Preparation of sulfur dioxide.
  • Conversion of sulfur dioxide to sulfur trioxide.
  • Conversion of sulfur trioxide to sulfuric acid.

Extraction of Sulfur

Pure sulfur is required for the production of sulfur dioxide gas. There are many sources for extracting sulfur. The most important one among these is the recovery from natural gas and oil. The organic or mineral parts of these are removed to obtain sulfur.

Apart from the extraction of pure sulfur, there are few ways by which sulfur dioxide can be extracted. Metal refining is one of them. Many metal ores occur as sulfides in the soil. Later they are roasted to obtain their respective oxide and sulfur dioxide.

Consider the extraction process of lead:

2PbS + 3O2 → 2PbO + 2SO2

Likewise, sulfur dioxide can be obtained from the metallurgy of copper, nickel, zink, etc. In China, most of the sulfur dioxide is extracted from pyrite, an iron sulfide ore.

Preparation of Sulfur Dioxide

Sulfur dioxide can be prepared by burning pure sulfur in the presence of excess air. This causes the formation of dioxide of sulfur. The process of sulfur dioxide preparation is as follows:

  • Molten sulfur is pumped into the stationary atomizer. This causes the formation of atomized sulfur. This atomized sulfur is applied to the hot furnace. Air is preheated and dried using a sulfuric acid dehydrator and is also applied to the hot furnace.
  • Air and atomized molten sulfur are kept at one end of the hot furnace. When a reaction occurs sulfur dioxide is produced at the other end.

Also Read: Oxides of Sulfur

The chemical reaction involved in this process is shown below:

  • Melting and atomization.
S(s)445oCS(G)S_{(s)} \overset{445^{o}C}{\rightarrow} S_{(G)}
  • Production of sulfur dioxide using the hot furnace.

S(g) + O2 → SO2(g) + Δ

Conversion of Sulfur Dioxide to Sulfur Trioxide

This is a really delicate step in the contact process. This is partly because it produces corrosive sulfur trioxide. And also this process is a reversible reaction and hence involves setting different reaction parameters to obtain maximum output. The reaction equation is represented below:

2SO2(g) + O2(g) ⇌ SO3 (g) ΔH = -196 KJ mol-1

The detailed steps involved are pictured below:

Conversion of Sulfur Dioxide to Sulfur Trioxide

Sulfur dioxide and oxygen are combined at a ratio of 1:1 inside a lead chamber. The temperature is set to 400 – 450 degrees Celcius with 1 to 2 atm. The process is catalyzed by the vanadium pentoxide catalyst.

Conversion of Sulfur Trioxide to Sulfuric Acid

Dilution of sulfur trioxide in water can yield sulfuric acid. This is however really dangerous and hence not followed. The addition of sulfur trioxide in water is highly exothermic and causes the fuming of sulfuric acid. The fumes prevent the further dissolution of sulfur trioxide in water.

The accepted method is to dilute the sulfur trioxide in sulfuric acid. This produces oleum.

SO3 (g) + H2SO4 → H2S2O7 (1)

Oleum can be further diluted in water to obtain concentrated sulfuric acid.

H2S2O7 (1) + H2O(1) → 2H2SO4(1)

These are the steps involved in producing sulfuric acid via the contact process.

The preparation of sulfuric acid by the contact process can be explained in a single diagram as shown below

Contact Process

Essential Conditions for Obtaining Maximum Sulfuric Acid Yield

Concentration Consideration

A 1:1 proportion of sulfur dioxide and oxygen is used. From the equation, we can see that there is always an excess in oxygen if we use this ratio. According to Le’ Chatelier’s principle, if the amount of reactant becomes high, the equilibrium will tip towards the product side. Since the amount of oxygen is always higher in the reactant side, the reaction will yield sulfur trioxide.

Temperature Consideration

This reaction is exothermic in nature as shown:

2SO2(g) + O2(g) ⇌ SO3 (g) ΔH = -196 KJ mol-1

Hence, according to Le’ Chateliers’ principle, a lower temperature yields more sulfur trioxide. However, it is not a feasible way to reduce the temperature too low. Some heat is always required for keeping the rate of reaction to the required level. Hence, most often an optimum temperature of 400 – 450 degrees celsius is selected.

Pressure Consideration

From the equation, we can see that on the reactant side, there are three reactant molecules and on the product side only two. That means the application of excess pressure favours the production of more sulfur trioxide. That is, the equilibrium will tip towards the product side.

According to Le Chataliers’ principle, the excess pressure causes the system to yield more products so that finally the pressure can subside by reducing the number of molecules from three to two. However, a low value of pressure generally 1 – 2 atm is preferred due to safety and economic concerns.

The Catalyst

The catalyst used in the contact process is vanadium pentoxide. The action of catalyst is not to yield more product but to make the reaction faster by forcing the reaction to reach equilibrium quickly. Nonetheless, the plant is designed in a manner to not let the reaction stay in equilibrium.


  • The contact process is a present method of producing concentrated sulphuric acid which is required for industrial use.
  • The catalyst used during the process is vanadium oxide.
  • The contact process is divided into three stages:

1 Stage: Sulfur dioxide preparation and purification.

2 stage: Conversion of sulfur dioxide to sulfur trioxide by catalytic oxidation.

3 Stage: Sulfur trioxide is converted into Sulfuric acid.