More Lessons for IGCSE Chemistry
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
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
(g) + O2
(g) ⇄ 2SO3
This reaction is reversible. The conditions needed for it are:
• a catalyst of vanadium(V) oxide, V2
• 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:
O(l) + SO3
(g) → H2
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 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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