What NEET Asks
- Direct questions on reactants, products, and conditions for hydrogen preparation.
- Identification of specific industrial processes and their catalysts/temperatures.
- Relative purity of hydrogen obtained from different methods.
Key Points
- Lab Preparation: Usually by reacting granular zinc with dilute acids (HCl or H₂SO₄) or with aqueous alkali (NaOH).
- Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)
- Zn(s) + 2NaOH(aq) → Na₂ZnO₂(aq) + H₂(g) (Sodium zincate)
- Industrial Preparation: Primarily from water, hydrocarbons, or steam.
- Electrolysis of acidified water: 2H₂O(l) --(Electrolysis)--> 2H₂(g) + O₂(g). Produces very high purity hydrogen.
- From Steam (Bosch Process/Steam Reforming): Passing steam over red-hot coke or hydrocarbons.
- C(s) + H₂O(g) --(1270 K)--> CO(g) + H₂(g) (Water gas)
- CH₄(g) + H₂O(g) --(Ni catalyst, 1173 K)--> CO(g) + 3H₂(g) (Syngas)
- Water-Gas Shift Reaction: To increase hydrogen yield from syngas.
- CO(g) + H₂O(g) --(Fe₂O₃/Cr₂O₃ catalyst, 773 K)--> CO₂(g) + H₂(g)
Must-Know Formula / Reaction
Water-Gas Shift Reaction: CO(g) + H₂O(g) --(Fe₂O₃/Cr₂O₃ catalyst, 773 K)--> CO₂(g) + H₂(g)
- CO: Carbon monoxide from steam reforming.
- H₂O: Steam used as a reactant.
- Fe₂O₃/Cr₂O₃: Iron chromate acts as a catalyst.
- 773 K: Optimal temperature for the reaction.
- CO₂: Carbon dioxide byproduct.
- H₂: Additional hydrogen produced.
Common Mistakes
- Students often confuse the conditions (catalysts, temperature) for steam reforming and water-gas shift reaction.
- Don't confuse the purity of hydrogen obtained; electrolysis generally gives the highest purity.
- Forgetting that amphoteric metals like Zn, Al react with both acids and strong bases to produce hydrogen.
Rapid Revision
Hydrogen is prepared in the lab using active metals (Zn) with dilute acids or alkalis. Industrially, electrolysis of water, steam reforming of hydrocarbons (Bosch process), and the water-gas shift reaction are key. Remember specific catalysts and temperatures for industrial methods.