WAEC Chemistry 2017 Paper

Concept:Oxidation number is assigned to an element by treating shared electrons as belonging to the more electronegative atom.Explanation:For $\mathrm{NO}$, oxygen has an oxidation number of $-2$.Let the oxidation number of nitrogen be $x$.For the neutral molecule, $x + (-2) = 0$.Therefore, $x = +2$.So nitrogen in $\mathrm{NO}$ is $+2$.For $\mathrm{NO}_2$, there are two oxygen atoms, each with oxidation number $-2$.Total contribution of oxygen $= 2 \times (-2) = -4$.For the neutral molecule, $y + (-4) = 0$.Therefore, $y = +4$.So nitrogen in $\mathrm{NO}_2$ is $+4$.In the reaction $2\mathrm{NO} + \mathrm{O}_2 \to 2\mathrm{NO}_2$, nitrogen changes from $+2$ to $+4$.Thus, the oxidation number increases by $2$.Answer:The change in oxidation number of nitrogen is from $+2$ to $+4$.So the correct option is C.