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Sulfuric Acid - Contact Process

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A series of free IGCSE Chemistry Activities and Experiments (Cambridge IGCSE Chemistry).

The contact process, for making sulfuric acid, is a process which involves a reversible reaction.
The raw materials needed to make sulfuric acid are: sulfur, air, water.

Stage 1 - making sulfur dioxide
In the first stage of the contact process, sulfur is burned in air to make sulfur dioxide:
Equation: sulfur + oxygen → sulfur dioxide
Equation: S(l) + O2(g) → SO2(g) This is not a reversible reaction.
Sulfur dioxide must not be released as it contributes to acid rain.

Stage 2 - making sulfur trioxide
In the second stage, sulfur dioxide reacts with more oxygen to make sulfur trioxide:
Equation: sulfur dioxide + oxygen ⇄ sulfur trioxide
Equation: 2SO2(g) + O2(g) ⇄ 2SO3(g)
This reaction is reversible. The conditions needed for it are:
• a catalyst of vanadium(V) oxide, V2O5
• a temperature of around 450°C
• a pressure of approximately 2 atmospheres

Stage 3 - making sulfuric acid
In the final stage, sulfur trioxide reacts with water to make sulfuric acid:
Equation: H2O(l) + SO3(g) → H2SO4(aq)
This is not a reversible reaction, just like the first stage.



What is sulfur?
Sulfur is a non metal element in group 6 of the periodic table. In room temperature, Sulfur is a yellow, brittle solid which doesn’t conduct electricity as it is a non-metal. Sulfur is insoluble in water. It is able to react with both metals and non-metals.

Sources of Sulfur:
sulfur is found in many places in the world in different forms. It usually exists in volcanic regions could also be obtained from some metal ores like Copper pyrites (CuFeS2) and Blende (ZnS).

Sulfur Dioxide:
Sulfur dioxide is the product of combustion of sulfur or sulfur-containing fuels.
It is an air pollutant as it causes acid rain. However, sulfur dioxide has important uses too.

Uses of sulfur dioxide:
• Bleaching wood pulp for the manufacturing of paper.
• It is used as a food preservative as it kills bacteria.
• It is a dehydrating agent. This means it eliminates water from compounds.

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