Properties of Dihydrogen — Revision Notes

⚡ 30-Second Revision

Physical Properties — Colorless, odorless, tasteless gas. Lightest element. Low M.P./B.P. ($13.99,\text{K}$, $20.28,\text{K}$). Sparingly soluble in water.
Bond Dissociation Enthalpy — High ($435.88,\text{kJ/mol}$) $implies$ inert at room temp.
Reactivity Order (Halogens) — $F_2 > Cl_2 > Br_2 > I_2$.

* $H_2 + F_2 \rightarrow 2HF$ (explosive, dark) * $H_2 + Cl_2 xrightarrow{\text{light/heat}} 2HCl$

Reaction with Oxygen — $2H_2 + O_2 \rightarrow 2H_2O$ (explosive, 'pop' sound).
Haber Process — $N_2 + 3H_2 xrightleftharpoons{Fe, \text{high P, T}} 2NH_3$.
Reducing Agent — Reduces metal oxides ($CuO + H_2 \rightarrow Cu + H_2O$).
Hydrogenation — Adds to unsaturated compounds ($C=C$, $C equiv C$) with Ni/Pd/Pt catalysts.
Ortho/Para Hydrogen — Nuclear spin isomers. Identical chemical, different physical properties. Para-H$_2$ more stable at low T.

2-Minute Revision

Dihydrogen ($H_2$) is a fundamental molecule with distinct properties. Physically, it's a colorless, odorless, tasteless, and the lightest gas, characterized by extremely low melting and boiling points due to weak intermolecular forces, and low solubility in water. Chemically, its high bond dissociation enthalpy ($435.88,\text{kJ/mol}$) makes it relatively inert at room temperature, requiring activation (heat, light, catalysts) to react.

Its key chemical reactions include explosive combination with fluorine, light-induced reaction with chlorine, and slower reactions with bromine and iodine. With oxygen, it forms water explosively, famously producing a 'pop' sound.

Industrially, it's crucial in the Haber process for ammonia synthesis ($N_2 + 3H_2 xrightarrow{Fe} 2NH_3$) and as a powerful reducing agent. It reduces metal oxides (e.g., $CuO + H_2 \rightarrow Cu + H_2O$) and hydrogenates unsaturated organic compounds (e.

g., vegetable oils to fats) in the presence of catalysts like Ni, Pd, or Pt. Remember the existence of ortho and para spin isomers, which differ in nuclear spin orientation and physical properties, with para-hydrogen being more stable at low temperatures.

5-Minute Revision

Let's consolidate the properties of dihydrogen ($H_2$) for NEET.

Physical Properties: Dihydrogen is the lightest known gas, making it less dense than air. It's completely transparent, odorless, and tasteless. Its molecular structure, a nonpolar diatomic molecule, results in very weak London dispersion forces between molecules. This explains its extremely low melting point ($13.99,\text{K}$) and boiling point ($20.28,\text{K}$), as minimal energy is needed to overcome these forces. Consequently, it's also sparingly soluble in water, a polar solvent.

Chemical Properties: The cornerstone of dihydrogen's chemical behavior is its high bond dissociation enthalpy ($435.88,\text{kJ/mol}$). This strong H-H bond means it's quite unreactive at room temperature, requiring significant activation energy to initiate reactions. However, once activated, it's highly reactive.

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Reactions with Halogens — Reactivity decreases down the group: $F_2$ (explosive, even in dark) > $Cl_2$ (light/heat) > $Br_2$ (heat, catalyst) > $I_2$ (high T, catalyst, reversible). Example: $H_2(g) + Cl_2(g) xrightarrow{\text{light}} 2HCl(g)$.
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Reaction with Oxygen — Highly exothermic and explosive, forming water. This is the basis of the 'pop' sound test: $2H_2(g) + O_2(g) xrightarrow{\text{ignition}} 2H_2O(l)$.
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Reaction with Nitrogen (Haber Process) — A crucial industrial process to synthesize ammonia. Requires specific conditions: $N_2(g) + 3H_2(g) xrightleftharpoons[Fe \text{ catalyst}]{450-500,^circ\text{C}, 200,\text{atm}} 2NH_3(g)$.
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Reducing Agent — Dihydrogen is a potent reducing agent, especially at higher temperatures. It reduces metal oxides to their respective metals, for example: $CuO(s) + H_2(g) xrightarrow{\text{heat}} Cu(s) + H_2O(g)$. It also reduces unsaturated organic compounds (alkenes, alkynes, aldehydes, ketones) in a process called hydrogenation, typically using catalysts like Ni, Pd, or Pt. Example: $CH_2=CH_2 + H_2 xrightarrow{Ni} CH_3-CH_3$.
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Hydride Formation — Reacts with electropositive metals to form ionic hydrides (e.g., $2Na + H_2 \rightarrow 2NaH$).

Ortho and Para Hydrogen: These are nuclear spin isomers. Ortho-hydrogen has parallel nuclear spins, while para-hydrogen has antiparallel spins. They have identical chemical properties but differ slightly in physical properties. Para-hydrogen is more stable at very low temperatures, and normal hydrogen is a 75% ortho, 25% para mixture at room temperature.

Key Takeaway: Focus on the reasons behind the properties (e.g., high bond enthalpy for inertness), specific catalysts, and the industrial applications.

Prelims Revision Notes

For NEET, a quick recall of dihydrogen properties is essential:

I. Physical Properties:

State, Color, Odor, Taste — Colorless, odorless, tasteless gas.
Density — Lightest known gas ($2.016,\text{g/mol}$). Less dense than air.
Melting Point (M.P.) & Boiling Point (B.P.) — Extremely low (M.P. $13.99,\text{K}$, B.P. $20.28,\text{K}$) due to weak London dispersion forces.
Solubility — Sparingly soluble in water (nonpolar molecule in polar solvent).
Thermal Conductivity — High.
Ortho & Para Hydrogen — Nuclear spin isomers. Ortho (parallel spins), Para (antiparallel spins). Chemically identical, physically different. Para-H$_2$ is more stable at low temperatures. Normal H$_2$ is ~75% ortho, ~25% para at room temp.

II. Chemical Properties:

Bond Dissociation Enthalpy — Very high ($435.88,\text{kJ/mol}$). This makes $H_2$ relatively inert at room temperature, requiring high activation energy.
Reactivity with Halogens ($X_2$) — $H_2 + X_2 \rightarrow 2HX$. Reactivity order: $F_2 > Cl_2 > Br_2 > I_2$.

* $F_2$: Explosive, even in dark. * $Cl_2$: Requires light or heat. * $Br_2$: Requires heat and catalyst. * $I_2$: Requires high temperature and catalyst, reversible.

Reactivity with Oxygen ($O_2$) — $2H_2(g) + O_2(g) xrightarrow{\text{ignition}} 2H_2O(l)$. Highly exothermic, explosive ('pop' sound test).
Reactivity with Nitrogen ($N_2$) - Haber Process — $N_2(g) + 3H_2(g) xrightleftharpoons[Fe \text{ catalyst}]{450-500,^circ\text{C}, 200,\text{atm}} 2NH_3(g)$.
Reducing Nature — Dihydrogen is a powerful reducing agent, especially at elevated temperatures.

* Reduction of Metal Oxides: $CuO(s) + H_2(g) xrightarrow{\text{heat}} Cu(s) + H_2O(g)$. * Hydrogenation of Unsaturated Hydrocarbons: Addition across $C=C$ or $C equiv C$ bonds. Catalysts: Ni, Pd, Pt. E.g., $R-CH=CH-R' + H_2 xrightarrow{Ni} R-CH_2-CH_2-R'$. Used in vegetable oil hardening. * Reduction of Aldehydes/Ketones: To alcohols.

Reactivity with Metals (Hydride Formation) — Forms ionic hydrides with s-block metals (e.g., $2Na + H_2 \rightarrow 2NaH$).

III. Key Concepts to Remember:

High bond enthalpy $implies$ inertness at room temp.
Role of catalysts in lowering activation energy.
Dihydrogen as a reducing agent (gets oxidized from 0 to +1).
Specific catalysts for specific reactions (Ni for hydrogenation, Fe for Haber).
Ortho/para hydrogen are spin isomers, not chemical isomers.

Vyyuha Quick Recall

To remember Dihydrogen's reactivity with Halogens: Fast Cats Bite Iguanas.

Fast: Fluorine (Fast, explosive)
Cats: Chlorine (Catalyzed by light/heat)
Bite: Bromine (Requires heat, catalyst)
Iguanas: Iodine (Inert, slow, high T, catalyst, reversible)