Study of the First Element - Hydrogen

• (i) Resemblance to Alkali Metals:
1. Electronic Configuration: Hydrogen has $1$ valence electron in its outermost shell ($1s^1$), resembling alkali metals ($\text{Li}: 2, 1; \text{Na}: 2, 8, 1$).
2. Electropositive Ion Formation: Like alkali metals, it readily loses its single valence electron to form a unipositive cation: $\text{H} \to \text{H}^+ + e^-$.
• (ii) Resemblance to Halogens:
1. Diatomic Molecular State: Hydrogen forms a stable diatomic gas ($\text{H}_2$), exactly like halogens ($\text{F}_2, \text{Cl}_2, \text{Br}_2$).
2. Electronegative Hydride Formation: Like halogens needing $1$ electron to achieve an octet, hydrogen needs $1$ electron to complete its duplet, forming hydride anions ($\text{H}^-$) in metallic hydrides like $\text{NaH}$.

• (i) Why Granulated Zinc: Pure zinc reacts extremely slowly with dilute acids. Granulated zinc contains trace impurities of Copper, which establishes an internal electrochemical couple that acts as a catalyst, speeding up the rate of hydrogen evolution.
• (ii) Why NOT Dilute Nitric Acid: Nitric acid ($\text{HNO}_3$) is a powerful oxidizing agent. As soon as hydrogen gas is formed, $\text{HNO}_3$ instantly oxidizes it to water ($\text{H}_2\text{O}$) while being reduced itself to toxic brown fumes of nitrogen dioxide ($\text{NO}_2$):

$$\text{Zn} + 4\text{HNO}_3 \to \text{Zn}(\text{NO}_3)_2 + 2\text{H}_2\text{O} + 2\text{NO}_2(g)$$

• Method of Collection: Hydrogen is collected over water by the Downward Displacement of Water because it is virtually insoluble in water.
• Why NOT collected in Air: Although hydrogen is much lighter than air, it CANNOT be collected by downward displacement of air because hydrogen mixes intimately with atmospheric oxygen to form an extremely dangerous, violently explosive mixture that detonates upon contact with any stray flame or spark.

• Step 1 (Production of Water Gas): Superheated steam is passed over white-hot coke at $1000^\\circ\text{C}$:

$$\mathbf{\text{C}(s) + \text{H}_2\text{O}(g) \xrightarrow{1000^\\circ\text{C}} [\text{CO} + \text{H}_2] - \Delta \quad (\text{Water Gas})}$$

• Step 2 (Catalytic Oxidation): Water gas mixed with excess steam is passed over heated Iron(III) oxide catalyst ($\text{Fe}_2\text{O}_3$) with $\text{Cr}_2\text{O}_3$ promoter at $450^\\circ\text{C}$:

$$\mathbf{[\text{CO} + \text{H}_2] + \text{H}_2\text{O}(g) \xrightarrow[450^\\circ\text{C}]{\text{Fe}_2\text{O}_3 + \text{Cr}_2\text{O}_3} \text{CO}_2 + 2\text{H}_2 + \Delta}$$

• Step 3 (Purification): $\text{CO}_2$ is dissolved in water under $30\text{ atm}$ pressure; residual $\text{CO}$ is absorbed in ammoniacal $\text{CuCl}$ solution, yielding pure $\text{H}_2$.

• Observation: The black copper(II) oxide powder turns into a shining reddish-brown deposit of metallic copper, and droplets of water condense on the cooler parts of the tube.
• Balanced Chemical Equation:

$$\mathbf{\text{CuO}(s) \text{ [Black]} + \text{H}_2(g) \xrightarrow{\Delta} \text{Cu}(s) \text{ [Reddish-brown]} + \text{H}_2\text{O}(l)}$$

• Redox Analysis:
- $\text{CuO}$ loses oxygen $\implies$ $\text{CuO}$ is REDUCED to $\text{Cu}$ (acting as oxidizing agent).
- $\text{H}_2$ gains oxygen $\implies$ $\text{H}_2$ is OXIDIZED to $\text{H}_2\text{O}$ (acting as reducing agent).

• The Pop Test: When a burning wooden matchstick or splinter is brought to the mouth of a test tube containing hydrogen, the gas catches fire and burns with a pale blue flame accompanied by a sharp, characteristic "POP" sound.
• Demonstration: This confirms the identity of hydrogen gas and proves that a small volume of hydrogen mixed with air burns with explosive velocity.

• Naturally occurring vegetable oils (like groundnut oil, soybean oil) are unsaturated liquid organic hydrocarbons containing double bonds.
• In the presence of finely divided Nickel ($\text{Ni}$) catalyst at a temperature of approximately $200^\\circ\text{C}$, hydrogen gas is bubbled through the liquid oil under pressure.
• The double bonds saturate by adding hydrogen atoms, converting the liquid oil into solid saturated fat: Vanaspati Ghee.

$$\mathbf{\text{Vegetable Oil (Liquid)} + \text{H}_2 \xrightarrow[200^\\circ\text{C}]{\text{Ni}} \text{Vanaspati Ghee (Solid)}}$$

• Zero Harmful Emissions: The only product of hydrogen combustion is pure water vapor ($2\text{H}_2 + \text{O}_2 \to 2\text{H}_2\text{O}$); it produces zero carbon dioxide, zero carbon monoxide, zero soot, and zero particulates.
• Highest Energy Density: Hydrogen has the highest calorific value of any chemical fuel ($142\text{ kJ/g}$), releasing over three times more energy per gram than gasoline or diesel.