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Practise 26 exam-style questions on Corrosion and its prevention (chemistry only) for AQA GCSE Chemistry. Write your answer as you would in the exam and get it marked in seconds against the mark scheme, point by point, with an AI tutor on hand if you get stuck.
Here are 4 of the questions in this topic, each with a model answer, an explanation of why it earns the marks and the mark scheme an examiner would use.
Name the process of coating iron with a layer of zinc to prevent rusting.
Galvanising.
Galvanised iron is coated with zinc. The zinc acts as a barrier and also gives sacrificial protection because zinc is more reactive than iron.
galvanising
allow galvanisation; do not accept electroplating
Underground steel pipes are protected by attaching blocks of magnesium to them. The blocks have to be replaced every few years.
Explain why.
The magnesium corrodes instead of the iron in the steel, so the block is gradually used up and must be replaced before it is gone.
The word 'sacrificial' means the magnesium is sacrificed: it reacts in place of the iron. Once it has all reacted, the steel is no longer protected.
the magnesium reacts / corrodes / is oxidised instead of the iron
so the magnesium is gradually used up
allow so there is no magnesium left to protect the pipe
The list shows part of the reactivity series, from most reactive to least reactive:
magnesium
zinc
iron
tin
copper
Which of the metals magnesium, zinc, tin and copper could be used to give sacrificial protection to iron? Give a reason for your answer.
Magnesium and zinc, because they are more reactive than iron, so they would react and corrode instead of the iron. Tin and copper are less reactive than iron, so they would not protect it.
Sacrificial protection only works if the protecting metal is above iron in the reactivity series. A less reactive metal in contact with iron makes the iron corrode instead.
magnesium
if tin or copper is also given, max 2
zinc
they are more reactive than iron, so they corrode instead of the iron
allow tin and copper are less reactive than iron so would not protect it
Iron can be galvanised to prevent it from rusting.
Explain how galvanising protects iron, even when the coating is scratched.
Galvanising means coating the iron with a layer of zinc. The zinc acts as a barrier, keeping air and water away from the iron. Zinc is also more reactive than iron, so if the coating is scratched, the zinc reacts with air and water instead of the iron. This is called sacrificial protection.
Galvanising protects in two ways: as a barrier and as sacrificial protection. Because zinc is more reactive, it corrodes in preference to the iron, so exposed iron still does not rust.
galvanising is coating the iron with zinc
the zinc acts as a barrier to air and water
zinc is more reactive than iron
so if the coating is scratched, the zinc reacts / corrodes instead of the iron (sacrificial protection)
allow zinc is oxidised instead of iron
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