Answer

A lead storage battery consists of a lead anode, a grid of lead packed with lead oxide $\left({PbO_2}\right)$ as the cathode, and a $38 \%$ solution of sulphuric acid $\left({H_2} {SO_4}\right)$ as an electrolyte.

When the battery is in use, the following cell reactions take place:

At anode : $\quad{Pb}(s)+{SO}_4^{2-}(a q) \longrightarrow {PbSO}_4(s)+2 e^{-} \quad\quad\quad\quad\quad (i)$

At cathode : ${PbO}_2(s)+{SO}_4^{2-}(a q)+4 {H}^{+}(a q)+2 e^{-} \longrightarrow {PbSO}_4(s)+2 {H}_2 {O}(l)\quad\quad\quad\quad\quad (ii)$

The overall cell reaction is given by,

$\quad\quad\quad\quad\quad {Pb}(s)+{PbO}_2({~s})+2 {H}_2 {SO}_4(a q) \longrightarrow 2 {PbSO}_4(s)+2 {H}_2 {O}(l)$

On charging the battery, the reverse reaction takes place, i.e. , ${PbSO}_4$ deposited on the electrodes is converted back into Pb and ${PbO}_2$ and ${H}_2 {SO}_4$ is regenerated.

Reverse of reaction (i) will be reduction and hence will take place at cathode. Reverse of reaction (ii) will be oxidation and hence will take place at anode.

Answer

Methane and methanol can be used as fuels in fuel cells.

Answer

The water layer present on the surface of iron (especially in the rainy season) dissolves acidic oxides of the air like ${CO}_2, {SO}_2$ etc. to form acids which dissociate to give ${H}^{+}$ ions

$\hspace{2cm} {H}_2 {O}+{CO}_2 \longrightarrow {H}_2 {CO}_3 \rightleftharpoons 2 {H}^{+}+{CO}_3^{2-} $

In the presence of ${H}^{+}$ ions, iron starts losing electrons at some spot to form ferrous ions, i.e., its oxidation takes place. Hence, this spot acts as the anode. The electrons thus released move through the metal to reach another spot where ${H}^{+}$ ions and the dissolved oxygen take up these electrons and reduction reaction takes place. Hence, this spot acts as the cathode :

$ \hspace{2cm} \text{ At anode :} \quad\hspace{1.5mm}{Fe}(s) \longrightarrow {Fe}^{2+}(a q)+2 e^{-} $

$ \hspace{2cm} \text{At cathode :} \quad {O}_2(g)+4 {H}^{+}(a q)+4 e^{-} \longrightarrow 2 {H}_2 {O}(l) $

The overall reaction is : $\quad 2 {Fe}(s)+{O}_2(g)+4 {H}^{+}(a q) \longrightarrow 2 {Fe}^{2+}(a q)+2 {H}_2 {O}(l)$

Thus, an electrochemical cell is set up on the surface. Ferrous ions are further oxidized by the atmospheric oxygen to ferric ions which combine with water molecules to form hydrated ferric oxide, ${Fe}_2 {O}_3 .x {H}_2 {O}$, which is rust.