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Contact Process Questions

The contact process is one of the most popular or widely used methods of producing sulphuric acid. Peregrine Phillips, a British merchant, invented the contact process. It was first patented in 1831. Apart from being a cost-effective method of producing sulphuric acid, this method also yields sulphur trioxide and oleum.

Definition: The contact process is the current method of producing sulfuric acid in high concentrations required for industrial processes.

Contact Process Chemistry Questions with Solutions

Q-1:Which of the following catalysts is used in the contact process to produce sulphuric acid?

a) Iron

b) Vanadium pentoxide

c) Vanadium trioxide

d) Molybdenum

Answer: b) Vanadium pentoxide

Q-2: Which of the following is the key step in the manufacture of H2SO4 by contact process?

a) Burning of sulphur of sulphide ores in air

b) Absorption of SO3 in H2SO4 to give oleum

c) Conversion of SO2 to SO3

d) All of the above

Answer: c) Conversion of SO2 to SO3

Explanation: The key step in the manufacture of H2SO4 is the catalytic oxidation of SO2 with O2 to give SO3 in the presence of V2O5 (catalyst).

Q-3: Which of the following impurities are usually found during the production of SO2 in the contact process?

a) Hydrogen compounds

b) Arsenic compounds

c) Nitrogen compounds

d) All of the above

Answer: b) Arsenic compounds

Explanation: Purification of the SO2 produced includes the removal of dust and other impurities such as arsenic compounds.

Q-4: Which property of sulphuric acid makes it suitable for the production of more volatile acids from their salts?

Answer: Sulphuric acid, due to its low volatility, can be used to make more volatile acids from their salts. The following reaction takes place:

2 MX + H2SO4 → 2 HX + M2SO4 (X = F, Cl, NO3 and M= metal)

Q-5: What role does the catalyst play in the production of sulphuric acid during the contact process?

Answer: Vanadium pentoxide is the catalyst used in the contact process. The catalyst’s action is not to produce more product but to speed up the reaction by forcing it to reach equilibrium quickly.

Q-6: Conversion of SO2 to SO3 by the reaction with oxygen in the presence of a catalyst is a _________.

a) Endothermic Reaction

b) Exothermic Reaction

c) Precipitation reaction

d) Reduction Reaction

Answer: b) Exothermic Reaction

Explanation: Heat is produced during the conversion of SO2 to SO3 by the reaction with oxygen in the presence of a catalyst. Exothermic reactions are defined as those in which heat is produced.

Q-7: For the following exothermic reaction,

2SO2(g) + O2(g) ⇌ SO3(g)

Explain the effect of

a) Increase in Pressure

b) Decrease in Volume


a) According to Le Chatelier’s principle, increasing pressure causes the reaction to move in the direction with the fewest moles. We can see from the equation that there are three reactant molecules on the reactant side and only two on the product side. Therefore, applying excessive pressure promotes the formation of more sulphur trioxide.

b) We know that volume is directly proportional to the number of moles. As a result, a decrease in volume will shift the reaction in the direction where there are fewer moles. As a result, the reaction moves forward.

Q-8: What are the byproducts of the contact process?

Answer: The contact process produces sulphur trioxide and oleum as byproducts.

Q-9: Which of the following is the correct pressure and temperature conditions for plant operation during the contact process?

a) T= 740 K, P= 2 bar

b) T= 720 K, P= 2 bar

c) T= 760 K, P= 1 bar

d) T= 720 K, P= 1 bar

Answer: b) T= 720 K, P= 2 bar

Explanation: In practice, the plant is operated at a pressure of 2 bar and a temperature of 720 K.

Q-10: Sulphuric acid oxidises a variety of metals and nonmetals. Sulphuric acid is itself reduced to

a) SO3

b) SO2

c) S2-

d) None of the above

Answer: b) SO2

Explanation: Both metals and nonmetals are oxidised by concentrated sulphuric acid, which itself is reduced to SO2.

Q-11: How does sulphuric acid act as a dehydrating agent?

Answer: Concentrated sulphuric acid is a powerful dehydrating agent. Many wet gases get dried by passing them through sulphuric acid, provided the gases do not react with the acid. Sulphuric acid removes water from organic compounds, as evidenced by its charring action on carbohydrates.

Q-12: How is pure sulphur extracted for the production of sulphur dioxide in the contact process?

Answer: Sulphur dioxide gas cannot be produced without pure sulphur. Sulphur can be extracted from a variety of sources. The recovery of sulphur from natural gas and oil is the most important of these. To obtain sulphur, the organic or mineral components of them are removed.

Q-13: Sulphuric acid can be produced by dissolving sulphur trioxide in water. But this method is not used in the contact process. Why?

Answer: Sulphuric acid can be produced by diluting sulphur trioxide in water. This is, however, extremely dangerous and is not followed. The reaction of sulphur trioxide with water is highly exothermic, resulting in sulphuric acid fuming. The fumes prevent sulphur trioxide from dissolving further in water.

Q-14: Write the three steps involved in the manufacture of sulphuric acid by the contact process.

Answer: The contact process, which consists of three steps, used to produce sulphuric acid are:

(i) Combustion of sulphur or sulphide ores in the air to produce SO2.

(ii) SO2 to SO3 conversion via oxygen in the presence of a catalyst (V2O5).

(iii) SO3 absorption in H2SO4 to yield Oleum (H2S2O7).

Q-15: Where is sulphur trioxide gas present to produce oleum when absorbed in concentrated sulphuric acid?

a) Blast Furnace

b) Washing Tower

c) Catalytic Converter

d) Beaker

Answer: c) Catalytic Converter

Explanation: The SO3 gas from the catalytic converter is absorbed in concentrated H2SO4 to produce oleum.

Practice Questions on Contact Process

Q-1: What is the purity percentage of sulphuric acid obtained by the contact process?

a) 100%

b) 96-98%

c) 94-95 %

d) 90-91%

Q-2: Why is it necessary to remove the arsenic oxide impurity before passing the SO2 and air mixture through the catalytic chamber?

Q-3: Which of the following steps gives H2SO4 of desired concentration in the contact process?

a) Dilution of SO3 in water

b) Dilution of H2S2O7 in water

c) Dilution of SO2 in water

d) Absorption of SO3 in H2SO4

Q-4: Give the various uses of sulphuric acid.

Q-5: What happens when:

a) SO3 is passed through water.

b) SO3 is passed through H2SO4

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