Hydrogen


1. What are isotopes? Write the names of isotopes of hydrogen.

Isotopes are atoms of the same element having the same atomic number but different mass numbers.

The isotopes of hydrogen are:

  1. Protium (¹H)

  2. Deuterium (²H or D)

  3. Tritium (³H or T)

2. Give the uses of heavy water.

Heavy water (D₂O) is used:

  1. As a moderator in nuclear reactors to slow down neutrons.

  2. As a tracer in chemical and biological studies.

  3. In the preparation of deuterium and its compounds.

3. Explain the exchange reactions of deuterium.

Deuterium undergoes exchange reactions in which hydrogen atoms are replaced by deuterium.

For example:

H₂O + D₂ ⇌ 2HDO

Similarly, ammonia can undergo exchange with heavy water:

NH₃ + D₂O ⇌ NH₂D + HDO

Thus, deuterium can replace hydrogen atoms in compounds containing hydrogen.

4. What is ortho and para hydrogen?

Hydrogen exists in two forms depending on the relative spins of the two hydrogen nuclei.

  • Ortho hydrogen: The nuclear spins are parallel.

  • Para hydrogen: The nuclear spins are antiparallel.

At room temperature, hydrogen contains approximately 75% ortho hydrogen and 25% para hydrogen.

5. How do you convert para hydrogen into ortho hydrogen?

Para hydrogen can be converted into ortho hydrogen by:

  1. Heating hydrogen to a high temperature.

  2. Passing hydrogen over suitable catalysts such as platinum or iron.

  3. The conversion can also occur in the presence of paramagnetic substances.

Thus, the equilibrium between the two forms can be shifted towards ortho hydrogen.

6. Mention the uses of deuterium.

Deuterium is used:

  1. In the preparation of heavy water (D₂O).

  2. As a tracer in chemical and biological reactions.

  3. In nuclear reactions.

  4. In the preparation of deuterated compounds used in scientific research.

7. Explain preparation of hydrogen using electrolysis.

Hydrogen can be prepared by the electrolysis of water containing a small amount of dilute sulphuric acid.

At the cathode:
2H⁺ + 2e⁻ → H₂

At the anode:
2H₂O → O₂ + 4H⁺ + 4e⁻

Overall reaction:
2H₂O → 2H₂ + O₂

Thus, hydrogen is liberated at the cathode and oxygen at the anode.

8. What is water-gas shift reaction?

The reaction of water vapour with carbon monoxide to produce carbon dioxide and hydrogen is called the water-gas shift reaction.

CO + H₂O(g) ⇌ CO₂ + H₂

It is an important industrial method for increasing the hydrogen content of water gas.

9. What are the differences between ortho and para hydrogen?

Ortho hydrogen

Para hydrogen

Nuclear spins are parallel.

Nuclear spins are antiparallel.

It is the higher-energy form.

It is the lower-energy form.

About 75% at room temperature.

About 25% at room temperature.

More stable at ordinary temperature.

More stable at low temperature.

10. Discuss the three types of covalent hydrides.

Covalent hydrides are classified into three types:

1. Electron-precise hydrides:
They contain the exact number of electrons required for normal covalent bonds.
Examples: CH₄, SiH₄.

2. Electron-deficient hydrides:
They contain fewer electrons than required for normal covalent bonding.
Example: B₂H₆.

3. Electron-rich hydrides:
They contain excess electrons and have lone pairs of electrons.
Examples: NH₃, H₂O and HF.

11. H₂O₂ is not stored in glass bottles. Why?

Hydrogen peroxide is unstable and decomposes into water and oxygen.

2H₂O₂ → 2H₂O + O₂

The decomposition is accelerated by light, heat and impurities. Glass surfaces can contain impurities that catalyse its decomposition.

Therefore, H₂O₂ is stored in dark-coloured containers, usually with suitable stabilizers.

12. Explain the preparation of hydrogen peroxide.

Hydrogen peroxide is prepared industrially by the anthraquinone process.

  1. An anthraquinone derivative is first hydrogenated to form anthrahydroquinone.

  2. The anthrahydroquinone is then oxidised by air.

  3. During oxidation, H₂O₂ is formed and the original anthraquinone compound is regenerated.

  4. H₂O₂ is then extracted from the aqueous phase.

13. Hydrogen peroxide acts both as an oxidising and reducing agent. Explain.

Hydrogen peroxide can undergo both reduction and oxidation depending on the reacting substance.

As an oxidising agent:
It accepts electrons and gets reduced to water.

H₂O₂ + 2H⁺ + 2e⁻ → 2H₂O

As a reducing agent:
It loses electrons and gets oxidised to oxygen.

H₂O₂ → O₂ + 2H⁺ + 2e⁻

Hence, H₂O₂ acts as both an oxidising and reducing agent.

14. Draw the structures of H₂O and H₂O₂.

H₂O:

 ..
H–O– H
 ..

Oxygen has two lone pairs.

H₂O₂:

      ..  ..
  H - O – O – H
      ..  ..

Each oxygen atom has two lone pairs.

15. How is tritium prepared?

Tritium is prepared by the bombardment of lithium-6 with neutrons.

The nuclear reaction is:

⁶Li + ¹n → ³H + ⁴He

Thus, lithium-6 produces tritium (³H) and helium when bombarded with neutrons.

Memory fragment:
Li-6 + neutron → Tritium + Helium.


Bro, these are very manageable 5-markers. Keep them like this—short, structured, and scoring.



Long answers




1. Uses of Hydrogen

  • Fuel: Used as a clean fuel; liquid hydrogen is used in rockets.

  • Manufacture of ammonia: Used in the Haber process.

  • Hydrogenation: Used to convert vegetable oils into vanaspati.

  • Welding: Oxy-hydrogen flame is used for welding/cutting metals.

  • Petroleum industry: Used in hydrocracking and desulphurisation.

  • Reducing agent: Used to reduce metal oxides to metals.

2. Classification of Hydrides

Hydrides are compounds of hydrogen with other elements. They are classified into three types:

1. Ionic (saline) hydrides

  • Formed by hydrogen with highly electropositive metals.

  • Contain H⁻ (hydride ion).

  • Examples: NaH, CaH₂

2. Covalent (molecular) hydrides

  • Formed by hydrogen with non-metals.

  • Hydrogen and the other element share electrons.

  • Examples: CH₄, NH₃, H₂O

3. Metallic (interstitial) hydrides

  • Formed mainly by transition metals.

  • Hydrogen occupies interstitial spaces in the metal lattice.

  • Examples: PdHₓ, TiH₂

3. Types of Hydrogen Bonding

Hydrogen bonding occurs when H attached to a highly electronegative atom (F, O or N) is attracted to another electronegative atom.

1. Intermolecular hydrogen bonding

  • Occurs between different molecules.

  • Examples: H₂O, HF, NH₃

  • Example: H₂O molecules are linked by hydrogen bonds.

2. Intramolecular hydrogen bonding

  • Occurs within the same molecule.

  • Example: o-nitrophenol

  • Hydrogen bond forms between the –OH group and the nearby –NO₂ group.

4. Hardness of Water

Hardness is mainly caused by dissolved Ca²⁺ and Mg²⁺ salts.

Temporary hardness

Caused by bicarbonates:

  • Ca(HCO₃)₂

  • Mg(HCO₃)₂

Removal:
By boiling:

  • Ca(HCO₃)₂ → CaCO₃↓ + CO₂ + H₂O

Permanent hardness

Caused by chlorides and sulphates:

  • CaCl₂, MgCl₂

  • CaSO₄, MgSO₄

Removal:

  • Washing soda method: Ca²⁺/Mg²⁺ ions are precipitated as carbonates.

  • Ion-exchange method: Ca²⁺ and Mg²⁺ are exchanged with Na⁺/H⁺ ions.

Memory:
Temporary = bicarbonates = boiling
Permanent = chlorides/sulphates = washing soda/ion exchange