Physical and Chemical Properties — Revision Notes

⚡ 30-Second Revision

Physical Properties: — Colorless, odorless, tasteless gas. Lightest gas ($M_r=2$). Sparingly soluble in water. Very low MP/BP ($20,\text{K}$). High thermal conductivity. Rapid diffusion.
Chemical Properties: — Kinetically inert at room temp (High H-H bond enthalpy: $435.88,\text{kJ/mol}$). Reactive with activation.
Reactions:

- Halogens: $H_2 + X_2 \rightarrow 2HX$ (Reactivity $F_2 > Cl_2 > Br_2 > I_2$). $F_2$ explosive in dark. - Oxygen: $2H_2 + O_2 \rightarrow 2H_2O$ (explosive, exothermic). - Nitrogen: $N_2 + 3H_2 \rightleftharpoons 2NH_3$ (Haber-Bosch, Fe catalyst, high T, P).

- Metals: Forms ionic hydrides ($NaH$, $CaH_2$) with Group 1/2 metals. Forms interstitial hydrides with transition metals. - Reducing Agent: Reduces metal oxides ($CuO + H_2 \rightarrow Cu + H_2O$). Hydrogenation of oils ($C=C + H_2 \rightarrow C-C$, Ni/Pd/Pt catalyst).

Ortho/Para H2: — Nuclear spin isomers. Ortho (parallel spins), Para (anti-parallel spins, lower energy).

2-Minute Revision

Dihydrogen ($H_2$) is a colorless, odorless, tasteless, and the lightest known gas, characterized by extremely low melting and boiling points due to weak intermolecular forces. It's sparingly soluble in water but has high thermal conductivity.

Chemically, $H_2$ is kinetically inert at room temperature because of its very high H-H bond dissociation enthalpy ($435.88,\text{kJ/mol}$), requiring significant activation energy to react. However, under appropriate conditions (heat, light, catalyst), it becomes highly reactive.

It reacts explosively with fluorine, and with oxygen to form water. Industrially, it's crucial in the Haber-Bosch process to synthesize ammonia from nitrogen. Dihydrogen is a powerful reducing agent, used to reduce metal oxides to metals and in the hydrogenation of unsaturated vegetable oils to solid fats, typically with Ni, Pd, or Pt catalysts.

It forms ionic hydrides with highly electropositive metals and interstitial hydrides with transition metals. Remember the distinct ortho and para forms based on nuclear spin orientation.

5-Minute Revision

Dihydrogen ($H_2$) is the simplest diatomic molecule, exhibiting unique physical and chemical properties. Physically, it's a gas at STP, colorless, odorless, and tasteless. It's the lightest gas, with a molecular mass of $2,\text{g/mol}$, leading to rapid diffusion. Its non-polar nature results in weak London dispersion forces between molecules, hence extremely low melting (13.99 K) and boiling (20.28 K) points, and sparing solubility in water. It also possesses high thermal conductivity.

Chemically, the most significant aspect is its high H-H bond dissociation enthalpy ($435.88,\text{kJ/mol}$), which makes it kinetically inert at room temperature. This means it requires a substantial energy input (heat, light, or a catalyst) to initiate reactions. Once activated, it's highly reactive:

1
Reactions with Halogens: — Forms hydrogen halides ($HX$). Reactivity decreases down the group: $F_2$ reacts explosively even in the dark; $Cl_2$ needs light/heat; $Br_2$ needs heating; $I_2$ reacts reversibly with a catalyst.
2
Reaction with Oxygen: — Forms water explosively when ignited ($2H_2 + O_2 \rightarrow 2H_2O$). This is highly exothermic.
3
Reaction with Nitrogen: — Key to the Haber-Bosch process for ammonia synthesis ($N_2 + 3H_2 \rightleftharpoons 2NH_3$) under high T, P, and Fe catalyst.
4
Formation of Hydrides: — With highly electropositive metals (Group 1 & 2), it forms ionic (saline) hydrides ($NaH$, $CaH_2$) where hydrogen is $H^-$. With transition metals, it forms non-stoichiometric interstitial hydrides.
5
Reducing Agent: — A potent reducing agent, especially at high temperatures. It reduces metal oxides (e.g., $CuO + H_2 \rightarrow Cu + H_2O$) and is crucial in the hydrogenation of unsaturated vegetable oils to solid fats ($R-CH=CH-R' + H_2 xrightarrow{\text{Ni/Pd/Pt}} R-CH_2-CH_2-R'$). It also participates in the water gas shift reaction ($CO + H_2O \rightarrow CO_2 + H_2$) and Fischer-Tropsch synthesis.

Remember the existence of ortho and para hydrogen, nuclear spin isomers with slightly different physical properties, with para-hydrogen being more stable at low temperatures. Focus on the industrial applications and the underlying reasons for its reactivity (or inertness) for NEET.

Prelims Revision Notes

Dihydrogen ($H_2$) Properties: NEET Revision

I. Physical Properties:

State: — Gas at STP.
Appearance: — Colorless, odorless, tasteless.
Density: — Lightest known gas ($M_r = 2,\text{g/mol}$). Density $approx 0.08988,\text{g/L}$ at STP.
Solubility: — Sparingly soluble in water and organic solvents (non-polar nature).
Melting Point (MP) & Boiling Point (BP): — Extremely low (MP $approx 13.99,\text{K}$, BP $approx 20.28,\text{K}$) due to very weak London dispersion forces.
Thermal Conductivity: — Very high (second only to He among gases).
Diffusion/Effusion: — Very rapid (Graham's Law: $r propto 1/sqrt{M}$).
Ortho & Para Hydrogen: — Nuclear spin isomers. Ortho (parallel spins, higher energy, 75% at RT). Para (anti-parallel spins, lower energy, 25% at RT, 100% at low T).

II. Chemical Properties:

Kinetic Inertness: — High H-H bond dissociation enthalpy ($435.88,\text{kJ/mol}$) makes it kinetically inert at room temperature. Requires high activation energy (heat, light, catalyst) to react.
Reactions with Halogens ($X_2$): — Forms hydrogen halides ($HX$).

* $H_2 + F_2 \rightarrow 2HF$ (Explosive, even in dark, low T) * $H_2 + Cl_2 xrightarrow{\text{light/heat}} 2HCl$ * $H_2 + Br_2 xrightarrow{\text{heat}} 2HBr$ * $H_2 + I_2 \rightleftharpoons 2HI$ (Reversible, requires heat, catalyst) * Reactivity Trend: $F_2 > Cl_2 > Br_2 > I_2$

Reaction with Oxygen ($O_2$): — Forms water.

* $2H_2(g) + O_2(g) xrightarrow{\text{spark/catalyst}} 2H_2O(l)$ (Highly exothermic, explosive)

Reaction with Nitrogen ($N_2$): — Forms ammonia.

* $N_2(g) + 3H_2(g) xrightarrow{\text{Fe catalyst, high T, P}} 2NH_3(g)$ (Haber-Bosch Process)

Reaction with Metals (Hydride Formation):

* Ionic (Saline) Hydrides: With Group 1 & 2 metals (highly electropositive). $H$ acts as $H^-$. E.g., $2Na + H_2 \rightarrow 2NaH$; $Ca + H_2 \rightarrow CaH_2$. * Interstitial (Metallic) Hydrides: With transition metals. Non-stoichiometric. E.g., $TiH_{1.7}$.

Reducing Properties (Important!): — Dihydrogen is a strong reducing agent, especially at elevated temperatures.

* Reduction of Metal Oxides: Reduces oxides of less reactive metals. * $CuO(s) + H_2(g) xrightarrow{\text{heat}} Cu(s) + H_2O(g)$ * $Fe_3O_4(s) + 4H_2(g) xrightarrow{\text{heat}} 3Fe(s) + 4H_2O(g)$ * *Cannot reduce oxides of highly electropositive metals (e.

g., $Na_2O$, $CaO$, $Al_2O_3$).* * Hydrogenation of Unsaturated Hydrocarbons: Converts oils to fats. * $R-CH=CH-R' + H_2 xrightarrow{\text{Ni/Pd/Pt catalyst}} R-CH_2-CH_2-R'$ (e.g.

III. Common Misconceptions:

Not always explosive; only with air/oxygen in specific ratios.
Not reactive at all temperatures; kinetically inert at room temp.
Not a universal reducing agent; cannot reduce oxides of very reactive metals.

Vyyuha Quick Recall

To remember Dihydrogen's key properties and reactions, think: Hydrogen Is Light, Catalyst Required, Always Reducing.

Hydrogen Is Light: Lightest gas, low MP/BP.
Catalyst Required: High bond energy means kinetic inertness; needs catalyst/heat/light for reactions (e.g., Haber-Bosch, hydrogenation).
Always Reducing: Strong reducing agent (reduces metal oxides, hydrogenates oils). Remember it *doesn't* reduce highly electropositive metal oxides.