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.
50 What is the importance of heavy water?
Following are the importance of heavy water
(i) It is extensively used as a moderator in nuclear reactors.
(ii) It is used as a tracer compound in the study of reaction mechanism.
(iii) It is used for the preparation of other deuterium compounds such as $\mathrm{CD}_4, \mathrm{D}_2 \mathrm{SO}_4$, etc.