NEET Hydrogen Previous Year Questions with Solutions

Dihydrogen is used in metallurgy to reduce oxides of metals less active than iron (such as tungsten, molybdenum, copper) to their respective elemental metals at elevated temperatures. Therefore, Statement A is correct.

Heavy water (D₂O) is extensively employed as an isotopic tracer to elucidate organic and biochemical reaction mechanisms by tracking deuterium exchange. Therefore, Statement B is correct.

Hydrogenation of polyunsaturated vegetable oils using finely divided nickel catalyst at ~473 K yields saturated edible fats (vanaspati ghee). Therefore, Statement C is correct.

Owing to the small size of hydrogen atoms and the effective overlap of 1s orbitals, the H–H bond dissociation enthalpy is 435.88 kJ mol⁻¹, which is the HIGHEST for a single bond between two atoms of any element. Statement D falsely claims it is the lowest, making Statement D INCORRECT.

Hydrogen can reduce only the oxides of metals that are less electropositive than iron in the activity series (e.g., CuO, PbO, Fe₂O₃ under vigorous conditions). It CANNOT reduce oxides of metals more active than iron (such as Na, K, Ca, Mg, Al). Therefore, Statement E is INCORRECT.

Hydrogen has three naturally occurring isotopes: Protium ($^1_1H$), Deuterium ($^2_1H$ or $D$), and Tritium ($^3_1H$ or $T$). Protium and deuterium are non-radioactive stable isotopes, whereas tritium is unstable.

Tritium contains 1 proton and 2 neutrons in its nucleus. The high neutron-to-proton ratio ($n/p = 2$) places it above the nuclear stability belt. Stability is achieved by beta-minus ($\beta^-$) decay, in which a nuclear neutron transforms into a proton, an electron (beta particle), and an electron antineutrino:

The emitted $\beta^-$ particle has very low maximum kinetic energy ($E_{\text{max}} \approx 18.6\text{ keV}$), and tritium has a half-life of $t_{1/2} = 12.33\text{ years}$.

The equimolar mixture of $$\text{CO}$$ and $$\text{H}_2$$ is called water gas or synthesis gas ('syngas') because it is widely used in the synthesis of methanol and a number of hydrocarbons. Thus, (a) matches (iii).

Temporary hardness of water is caused by the presence of soluble bicarbonates (hydrogen carbonates) of calcium and magnesium, namely $$\text{Ca(HCO}_3)_2$$ and $$\text{Mg(HCO}_3)_2$$, which precipitate as carbonates on boiling. Thus, (b) matches (i).

Diborane ($$\text{B}_2\text{H}_6$$) has 12 valence electrons, insufficient to form conventional 2-center 2-electron bonds for all atoms, requiring two 3-center 2-electron ($$3c-2e$$) B-H-B banana bonds. It is classified as an electron-deficient hydride. Thus, (c) matches (ii).

Hydrogen peroxide has a non-planar structure resembling an open book, with the two O-H bonds located on different intersecting planes with a dihedral angle of $$111.5^\circ$$ in the gas phase and $$90.2^\circ$$ in crystalline phase. Thus, (d) matches (iv).