$A$ sulphur molecule is converted into a sulphide ion when it:

In the reaction $8 \ Al + 3 \ Fe_3O_4 \to 4 \ Al_2O_3 + 9 \ Fe$,the number of moles of electrons transferred by $1 \ mol$ of reductant is:

When $Cu$ reacts with $AgNO_3$ solution,the reaction that takes place is:

Which of the following processes occurs during the reaction of $Cu$ with $AgNO_3$ solution?

Identify the redox couples involved in the following reactions:
$(a) \ Cu + Zn^{+2} \to Cu^{+2} + Zn$
$(b) \ Mg + Fe^{+2} \to Mg^{+2} + Fe$
$(c) \ Br_{2} + 2Cl^{-} \to Cl_{2} + 2Br^{-}$
$(d) \ Fe + Cd^{+2} \to Cd + Fe^{+2}$

When $Fe^{2+}$ changes to $Fe^{3+}$ in a reaction,