Question

Which of the following reactions is an example of autoreduction?

• A. $Fe_3O_4 + 4CO \to 3Fe + 4CO_2$
• B. $Cu_2O + C \to 2Cu + CO$
• C. $Cu^{2+}_{(aq)} + Fe_{(s)} \to Cu_{(s)} + Fe^{2+}_{(aq)}$
• D. $Cu_{2}O + \frac{1}{2} Cu_{2}S \to 3Cu + \frac{1}{2} SO_{2}$

Answer 1

Answer: (D)

The sulphide ores of certain less electropositive metals like those of $Hg$, $Pb$, $Cu$, etc. when heated in air bring about the conversion of a part of the sulphide ore to oxide or sulphate which then reacts with the remaining sulphide to give the metal and $SO_2$. No external reducing agent is used in this process.
$2Cu_2S + 3O_2 \to 2Cu_2O + 2SO_2$
$2Cu_2O + Cu_2S \to 6Cu + SO_2$