Hydrogen

• Laboratory Preparation: Hydrogen gas is prepared by adding dilute hydrochloric acid (or dilute sulphuric acid) through a thistle funnel onto granulated zinc placed in a flat-bottomed Woulfe's flask:

$$\mathbf{\text{Zn (s)} + 2\text{HCl (aq)} \to \text{ZnCl}_2\text{ (aq)} + \text{H}_2\text{ (g)} \uparrow}$$

• Why Granulated Zinc? Pure zinc metal is chemically unreactive and reacts with dilute acids extremely slowly. Granulated zinc contains microscopic copper impurities that form local electrochemical couples, acting as a natural catalyst to accelerate the rapid evolution of hydrogen gas.

• Dilute nitric acid ($\text{HNO}_3$) is a powerful oxidizing agent.
• As soon as hydrogen gas is produced at the zinc surface, nitric acid immediately oxidizes hydrogen into water ($\text{H}_2\text{O}$).
• In the process, the nitric acid is itself reduced to poisonous oxides of nitrogen (such as nitric oxide $\text{NO}$ or nitrogen dioxide $\text{NO}_2$).
• Hence, no free hydrogen gas can be collected.

• Method of Collection: Hydrogen gas is collected over water by the Downward Displacement of Water in a pneumatic trough.
• Two Reasons:
1. Insolubility in Water: Hydrogen is almost completely insoluble in water, so none of the collected gas dissolves or is lost in the trough.
2. Explosion Hazard in Air: Although hydrogen is much lighter than air, it cannot be collected by downward displacement of air because it mixes instantly with atmospheric oxygen to form a violently explosive mixture.

• Observation: The hydrogen gas burns with a pale blue flame accompanied by a characteristic sharp "POP" sound.
• Tiny droplets of water condense on the cool inner walls of the test tube.
• Chemical Equation:

$$\mathbf{2\text{H}_2\text{ (g)} + \text{O}_2\text{ (g)} \to 2\text{H}_2\text{O (l)} + \text{Heat}}$$

• Chemical Equation:

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

• Color Change: The jet black powder of copper(II) oxide turns into a shining reddish-brown metallic powder of pure copper.
• Hydrogen acts as a reducing agent by removing oxygen from the copper oxide.

• Hydrogenation: The chemical addition of hydrogen gas across the unsaturated double bonds of liquid organic compounds in the presence of a metal catalyst.
• Manufacture of Vanaspati Ghee:
Liquid polyunsaturated vegetable oils (such as groundnut oil or cottonseed oil) are heated to approximately $200^{\circ}\text{C}$ in an autoclave.
Dry hydrogen gas is bubbled through the liquid in the presence of finely divided Nickel (Ni) catalyst.
The unsaturated liquid oil molecules absorb hydrogen and become fully saturated, solidifying upon cooling into solid Vanaspati Ghee.

1. Stage 1 (Production of Water Gas): Superheated steam is blown through a tall furnace over white-hot coke at $1000^{\circ}\text{C}$:
   

$$\text{C} + \text{H}_2\text{O} \xrightarrow{1000^{\circ}\text{C}} [\text{CO} + \text{H}_2]\text{ (Water Gas)} - \text{Heat}$$

2. Stage 2 (Catalytic Oxidation of Carbon Monoxide): Water gas is mixed with excess steam and passed over heated iron(III) oxide ($\text{Fe}_2\text{O}_3$) with chromium oxide ($\text{Cr}_2\text{O}_3$) promoter at $450^{\circ}\text{C}$:

$$[\text{CO} + \text{H}_2] + \text{H}_2\text{O} \xrightarrow{\text{Fe}_2\text{O}_3, 450^{\circ}\text{C}} \text{CO}_2 + 2\text{H}_2 + \text{Heat}$$

The $\text{CO}_2$ is dissolved in pressurized water, yielding pure hydrogen gas.

• If the bottom of the thistle funnel were positioned above the acid surface in the air space of the flask, the newly generated hydrogen gas would escape directly backward out into the room through the thistle funnel.
• Submerging the stem under the acid creates a liquid airlock, forcing the expanding hydrogen gas to travel exclusively forward through the delivery tube to the water trough.