Whenever a reaction between an oxidising agent and a reducing agent is carried out, a compound of lower oxidation state is formed if the reducing agent is in excess and a compound of higher oxidation state is formed if the oxidising agent is in excess.

Justify this statement giving three illustrations.

How do you count for the following observations ?

(a) Though alkaline potassium permanganate and acidic potassium permanganate both are used as oxidants, yet in the manufacture of benzoic acid from toluene we use alcoholic potassium permanganate as an oxidant. Why ? Write a balanced redox equation for the reactions.

How do you count for the following observations?

When concentrated sulphuric acid is added to an inorganic mixture containing chloride, we get colourless pungent smelling gas $\mathrm{HCl}$, but if the mixture contains bromide then we get red vapour of bromine. Why?

Identify the substance oxidised, reduced, oxidising agent and reducing agent for each of the following reactions:

(a) $2{\mathrm{AgBr}}_{\left(\mathrm{s}\right)} + {\mathrm{C}}_6{\mathrm{H}}_6{\mathrm{O}}_{2\left(\mathrm{aq}\right)} \to 2{\mathrm{Ag}}_{9\left(\mathrm{s}\right)} + 2{\mathrm{HBr}}_{\left(\mathrm{aq}\right)} + {\mathrm{C}}_6{\mathrm{H}}_4{\mathrm{O}}_{2\left(\mathrm{aq}\right)}$

Identify the substance oxidised, reduced, oxidising agent and reducing agent for each of the following reactions:

${\mathrm{HCHO}}_{\left(\mathrm{l}\right)} + 2 {{\mathrm{Cu}}^{2+}}_{\left(\mathrm{aq}\right)} + 5 {\mathrm{OH}}_{\left(\mathrm{aq}\right)} \to {\mathrm{Cu}}_2{\mathrm{O}}_{\left(\mathrm{s}\right)} + {{\mathrm{HCOO}}^{-}}_{\left(\mathrm{aq}\right)} + 3{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{l}\right)}$

Identify the substance oxidised, reduced, oxidising agent and reducing agent for the following reaction:

${\mathrm{N}}_2{\mathrm{H}}_{4\left(\mathrm{l}\right)} + 2{\mathrm{H}}_2{\mathrm{O}}_{2\left(\mathrm{l}\right)} \to {\mathrm{N}}_{2\left(\mathrm{g}\right)} + 4{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{l}\right)}$.

Identified the substance oxidized, reduced, oxidizing agents, and reducing agent for the following reaction:

$\mathrm{Pb}\left(\mathrm{s}\right) + {\mathrm{PbO}}_2\left(\mathrm{s}\right)+ 2{\mathrm{H}}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right) \to 2{\mathrm{PbSO}}_4\left(\mathrm{s}\right) +2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$

Consider the reactions:

$$\begin{gathered} 2 {\mathrm{S}}_2{\mathrm{O}}_3{ }^{2-}{ }_{\left(\mathrm{aq}\right)} + {\mathrm{I}}_{2\left(\mathrm{s}\right)}\to {\mathrm{S}}_4{\mathrm{O}}_6{ }^{2-}{ }_{\left(\mathrm{aq}\right)} + 2{{\mathrm{I}}^{-}}_{\left(\mathrm{aq}\right)} \\ {\mathrm{S}}_2{\mathrm{O}}_3{{ }^{2- }}_{\left(\mathrm{aq}\right)} + 2{\mathrm{Br}}_{2\left(\mathrm{l}\right)} + 5 {\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{l}\right)} \to 2{{{\mathrm{SO}}_4}^{ 2-}}_{\left(\mathrm{aq}\right)} + 4{{\mathrm{Br}}^{-}}_{\left(\mathrm{aq}\right)} + 10{{\mathrm{H}}^{+}}_{ \left(\mathrm{aq}\right)} \end{gathered}$$

Why does the same reductant, thiosulphate react differently with iodine and bromine ?