Melting point, enthalpy of vaporisation and viscosity data of $\mathrm{H}_2 \mathrm{O}$ and $\mathrm{D}_2 \mathrm{O}$ is given below
| H$$_2$$O | D$$_2$$O | |
|---|---|---|
| Melting point/K | 373.0 | 374.4 |
| Enthalpy of vaporisation at (373 K)/kJ mol$$^{-1}$$ | 40.66 | 41.61 |
| Viscosity/centipoise | 0.8903 | 1.107 |
On the basis of this data explain in which of these liquids intermolecular forces are stronger?
Given that,
| H$$_2$$O | D$$_2$$O | |
|---|---|---|
| Melting point/K | 373.0 | 374.4 |
| Enthalpy of vaporisation at (373 K)/kJ mol$$^{-1}$$ | 40.66 | 41.61 |
| Viscosity/centipoise | 0.8903 | 1.107 |
From this data, it is concluded that the values of melting point, enthalpy of vaporisation and viscosity depend upon the intermolecular forces of attraction. Since, their values are higher for $\mathrm{D}_2 \mathrm{O}$ as compared to those of $\mathrm{H}_2 \mathrm{O}$, therefore, intermolecular forces of attraction are stronger in $\mathrm{D}_2 \mathrm{O}$ than in $\mathrm{H}_2 \mathrm{O}$.
Dihydrogen reacts with dioxygen $\left(\mathrm{O}_2\right)$ to form water. Write the name and formula of the product when the isotope of hydrogen which has one proton and one neutron in its nucleus is treated with oxygen. Will the reactivity of both the isotopes be the same towards oxygen? Justify your answer.
The isotope of hydrogen which contains one proton and one neutron is deuterium (D). Therefore, when dideuterium reacts with dioxygen, heavy water $\left(\mathrm{D}_2 \mathrm{O}\right)$ is produced.
$\underset{\text { Dideuterium }}{2 \mathrm{D}_2(g)}+\underset{\text { Dioxygen }}{\mathrm{O}_2(g)} \xrightarrow{\text { Heat }} \underset{\begin{array}{c}\text { Deuterium oxide } \\ \text { (Heavy water) }\end{array}}{2 \mathrm{D}_2 \mathrm{O}}$
The reactivity of $H_2$ and $D_2$ towards oxygen will be different. Since, the $D-D$ bond is stronger than $\mathrm{H}-\mathrm{H}$ bond, therefore, $\mathrm{H}_2$ is more reactive than $\mathrm{D}_2$.
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.
$$2 \mathrm{H}_2 \mathrm{O}_2(l) \xrightarrow{h \nu} 2 \mathrm{H}_2 \mathrm{O}(l)+\mathrm{O}_2(g) $$