Explain why HCl is a gas and HF is a liquid?
F is smaller and more electronegative than Cl, so it forms stronger H -bonds as compared to Cl . As the consequence, more energy is needed to break the H -bonds in HF than HCl and hence the boiling point of HF is higher than that of HCl .
That's why HF is liquid and HCl is a gas.
When the first element of the periodic table is treated with dioxygen, it gives a compound whose solid state floats on its liquid state. This compound has an ability to act as an acid as well as a base. What products will be formed when this compound undergoes autoionisation?
The first element of the periodic table is hydrogen and its molecular form is dihydrogen $\left(\mathrm{H}_2\right)$. When $\mathrm{H}_2$ reacts with $\mathrm{O}_2$, water is formed. Water is a liquid at room temperature. When liquid water freezes, it expands to form ice.
Density of ice is lower than that of liquid water and hence ice floats over water. Water is amphoteric in nature. It acts as a base in presence of strong acids and as an acid in presence of strong bases.
$$\mathop {{H_2}O(l)}\limits_{Bas{e_1}} + \mathop {{H_2}S(aq)}\limits_{Aci{d_2}} \to \mathop {{H_3}{O^ + }(aq)}\limits_{Aci{d_1}} + \mathop {H{S^ - }(aq)}\limits_{Bas{e_2}} $$
$$\mathop {{H_2}O(l)}\limits_{Aci{d_1}} + \mathop {N{H_3}(aq)}\limits_{Bas{e_2}} \to \mathop {NH_4^ + (aq)}\limits_{Aci{d_2}} + \mathop {O{H^ - }(aq)}\limits_{Bas{e_1}} $$
Due to amphoteric character, water undergoes self ionisation as shown below
$$\mathop {{H_2}O(l)}\limits_{Aci{d_1}} + \mathop {{H_2}O(aq)}\limits_{Bas{e_2}} \rightleftharpoons\mathop {{H_3}{O^ + }(aq)}\limits_{Aci{d_1}\,(Conjugate\,acid)} + \mathop {O{H^ - }(aq)}\limits_{Bas{e_1}\,(Conjugate\,base)}$$
This self ionisation of water is called auto-protolysis or autoionisation.
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.