Reactivity Series
8 marks · HardIron and steel are commonly used in outdoor structures such as bridges and ships. They must be protected from rusting to ensure a long lifespan.
Describe the conditions required for iron to rust. (2 marks)
Explain how galvanising prevents rusting, even if the zinc coating is scratched. (2 marks)
Large steel ships are often protected from rust by attaching blocks of a different metal to their hulls.
State the name given to these blocks and explain their function. (2 marks)
Give one example of a metal commonly used for this purpose and explain why it is suitable. (2 marks)
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) Both oxygen (from air) and water must be present for iron to rust. · must show Oxygen, Water
(b) Galvanising prevents rusting by acting as a barrier to air and water; if the zinc layer is scratched, zinc is more reactive than iron and corrodes first, protecting the iron. · must show Barrier action, Sacrificial action of zinc
(c(i)) They are called sacrificial anodes. They are blocks of a more reactive metal that corrode instead of the iron/steel, preventing rust. · must show Sacrificial anode (name), Explanation of protection
(c(ii)) Magnesium (or zinc/aluminium) is commonly used because it is more reactive than iron and will be oxidised first, thus protecting the iron from rusting. · must show Example metal (magnesium, zinc, aluminium), Reason (more reactive than iron)
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Identifies oxygen (air) is required for rusting.
B2 (a) · 1 mark · Identifies water is required for rusting.
M1 (b) · 1 mark · Describes how zinc layer acts as a barrier.
M2 (b) · 1 mark · Explains that zinc protects iron even when scratched due to being more reactive.
B3 (c(i)) · 1 mark · Names sacrificial anode.
M3 (c(i)) · 1 mark · Explains that they are more reactive metals that corrode instead of iron/steel.
B4 (c(ii)) · 1 mark · Gives an example: magnesium, zinc, or aluminium.
M4 (c(ii)) · 1 mark · States that the chosen metal is more reactive than iron and thus corrodes preferentially.
Worked solution
a) Iron rusts when both oxygen (air) and water are present.
b) The zinc layer in galvanising acts as a barrier, preventing air and water from reaching the iron. If the zinc is scratched, the zinc still protects the iron because it is more reactive and will corrode in preference to the iron, preventing rusting.
c) (i) These blocks are called sacrificial anodes. They work by corroding (oxidising) in place of the iron, as they are more reactive metals.
(ii) Magnesium is an example of a metal used as a sacrificial anode. Magnesium is more reactive than iron, so it gets oxidised instead of the iron, protecting the steel from rusting.