2021 O Level Chem P1 Q26 — Redox / Reduction of Metal Oxides Solution
Question
Copper(II) oxide is reduced to copper as shown. Dry hydrogen is passed over heated copper(II) oxide, with the excess hydrogen burning at the end of the tube. The burner is then turned off but the hydrogen is kept flowing until the tube is cold. What is the main reason why the hydrogen is kept flowing?
Correct answer: C
Video Solution
Explanation
Answer: C. The reaction is CuO + H₂ → Cu + H₂O. The copper produced is hot and reactive. If the hydrogen flow were stopped while the tube is still hot, air could enter and the hot copper would be re-oxidised back to copper(II) oxide by oxygen: 2Cu + O₂ → 2CuO.
Keeping the hydrogen flowing fills the tube with hydrogen and excludes air until the copper has cooled and is no longer reactive enough to be oxidised. So the main reason is to prevent the copper from reacting with the air.
Option A is wrong (the explosion risk is reduced by stopping hydrogen, not continuing it); the reaction is already complete (B), and removing water (D) is not the purpose.
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