Rohan heard that instructions were given to the laboratory attendent to store a particular chemical, i.e., keep it in the dark room, add some urea in it, and keep it away from dust. This chemical acts as an oxidising as well as a reducing agent in both acidic and alkaline media. This chemical is important for use in the pollution control treatment of domestic and industrial effluents.
(i) Write the name of this compound.
(ii) Explain why such precautions are taken for storing this chemical.
(i) The name of the compound is hydrogen peroxide, $\mathrm{H}_2 \mathrm{O}_2$. It acts as an oxidising agent as well as reducing agent in both acidic and basic medium.
(ii) $\mathrm{H}_2 \mathrm{O}_2$ decomposes slowly on exposure to light and dust particles. In the presence of metal surfaces or traces of alkali present in glass containers, the decomposition of $\mathrm{H}_2 \mathrm{O}_2$ is catalysed.
It is, therefore, stored in wax lined glass or plastic vessels in dark. Urea is added as a negative catalyst or stabiliser to check its decomposition.
$$2 \mathrm{H}_2 \mathrm{O}_2(l) \xrightarrow{h \nu} 2 \mathrm{H}_2 \mathrm{O}(l)+\mathrm{O}_2(g) $$
Give reasons why hydrogen resembles alkali metals?
Hydrogen resembles alkali metals, i.e., $\mathrm{Li}, \mathrm{Na}, \mathrm{K}, \mathrm{Rb}, \mathrm{Cs}$ and Fr of group I of the periodic table in the following respects
(i) Like alkali metals, hydrogen also contain one electron in its outermost (valence) shell and exhibit +1 oxidation state.
(ii) Like alkali metals, hydrogen also loses its only electron to form hydrogen ion, i.e., $\mathrm{H}^{+}$ (proton).
(iii) Like alkali metals, hydrogen combines with electronegative elements (non-metals) such as oxygen, halogens and sulphur forming their oxides, halides and sulphides respectively.
(iv) Like alkali metals, hydrogen also acts as a strong reducing agent.
Hydrogen generally forms covalent compounds. Give reason.
Hydrogen has one electron which it can either lose or gain or share to acquire noble gas, i.e., helium gas configuration.
Therefore, in principle, it can form either ionic or covalent bonds. But the ionisation enthalpy of hydrogen is very high ( $1312 \mathrm{~kJ} \mathrm{~mol}^{-1}$ ) and its electron gain enthalpy is only slightly negative $(-73 \mathrm{~kJ} \mathrm{~mol}^{-1})$.
From this consequence, it does not have a high tendency to form ionic bonds but rather prefers to form only covalent bonds.
Why is the ionisation enthalpy of hydrogen higher than that of sodium?
The ionisation enthalpy of hydrogen higher than that of sodium. Both hydrogen and sodium have one electron in the valence shell. But the size of hydrogen is much smaller than that of sodium and hence, the ionisation enthalpy of hydrogen is much higher $\left(1312 \mathrm{~kJ} \mathrm{~mol}^{-1}\right)$ than that of sodium $\left(496 \mathrm{~kJ} \mathrm{~mol}^{-1}\right)$.
Basic principle of hydrogen economy is transportation and storage of energy in the form of liquid or gaseous hydrogen. Which property of hydrogen may be useful for this purpose? Support your answer with the chemical equation if required.
Basic principle of hydrogen economy is transportation and storage of energy in the form of liquid or gaseous hydrogen. Hydrogen is a gas at room temperature.
However, by cooling and applying high pressure, gaseous $\mathrm{H}_2$ can be converted into liquid $\mathrm{H}_2$ which has much smaller volume and hence can be transported easily. Thus, the basic property of hydrogen which is useful for hydrogen economy is that it can be converted into a liquid by cooling under high pressure.