Question

Deuterium differs from hydrogen in:
(A) Chemical properties
(B) Physical properties
(C) Both physical and chemical properties
(D) Radioactive properties

Answer 1

Hint: Isotopes are atoms of a given element which have the same atomic number but differ in their mass number. The chemical properties of all the isotopes of an element are the same but their physical properties are different. Physical properties include melting point, boiling point, density, enthalpy of fusion, ionic radius, ionization enthalpy and electron gain enthalpy etc.
- Hydrogen shows isotopic effect (the effect which can change the physical and chemical properties of isotopes). Isotopic effect is found only in hydrogen isotopes, because there is a large difference in the mass of isotopes.
- Tritium is the only radioactive isotope of hydrogen.

Complete Solution :
- Protium, deuterium and tritium are three isotopes of hydrogen. All of them have one proton and one electron each. Protium has no neutron, deuterium has one neutron and tritium has two neutrons. Due to isotopic effect maximum changes occur in the physical properties like melting point, boiling point, bond energy and rate of diffusion while minimum changes occur in the chemical properties of the isotopes. Heavy water$\text{(}{{\text{D}}_{\text{2}}}\text{O)}$ is chemically similar to the ordinary water$\text{(}{{\text{H}}_{\text{2}}}\text{O})$.
- Melting point of deuterium gas is $\text{18}\text{.73K}$, which is higher than melting point of hydrogen gas $\text{(13}\text{.96K)}$ and boiling point of deuterium is $\text{23}\text{.67K}$ which is higher than the boiling point of hydrogen $\text{20}\text{.39K}$, due to higher mass of deuterium.
Rate of diffusion of deuterium gas $\text{(}{{\text{D}}_{\text{2}}})$ is higher than rate of diffusion of hydrogen gas $\text{(}{{\text{H}}_{\text{2}}})$.
- Density of deuterium gas $\text{(}{{\text{D}}_{\text{2}}})$ is $0.18{\scriptstyle{}^{\text{g}}/{}_{\text{L}}}$, while density of hydrogen gas $\text{(}{{\text{H}}_{\text{2}}})$ is $0.09{\scriptstyle{}^{\text{g}}/{}_{\text{L}}}$.
- Enthalpy of bond dissociation of deuterium gas $\text{(}{{\text{D}}_{\text{2}}})$ is $443.35{\scriptstyle{}^{\text{KJ}}/{}_{\text{mol}}}$, while enthalpy of bond dissociation of hydrogen gas $\text{(}{{\text{H}}_{\text{2}}})$ is $435.88{\scriptstyle{}^{\text{KJ}}/{}_{\text{mol}}}$.

Additional information-
So, the correct answer is “Option B”.


Note: Difference in the atomic mass of hydrogen and deuterium affects the chemical properties of the compounds of these elements, due to this $\text{C-H}$ bond energy is less in comparison to the $\text{C-D}$ bond energy.
Heavy water$\text{(}{{\text{D}}_{\text{2}}}\text{O)}$ reacts more slowly than ordinary $\text{(}{{\text{H}}_{\text{2}}}\text{O})$ in chemical reactions.