Theory: Industrial Preparation of Important Gases
1. Steam Reforming of Methane
Steam reforming: CH₄ + H₂O →(Ni catalyst, 700-1100°C) CO + 3H₂. This is endothermic (ΔH > 0) — needs high temperature. The mixture of CO + H₂ is called syngas (synthesis gas). It's the primary feedstock for the Haber process (NH₃) and Fischer-Tropsch process (liquid fuels). About 95% of the world's hydrogen is produced by steam reforming of natural gas (methane). The nickel catalyst is crucial — it lowers activation energy and speeds up the reaction.
2. Water-Gas Shift Reaction
After steam reforming, CO can be converted to CO₂ + more H₂ using the water-gas shift reaction: CO + H₂O ⇌ CO₂ + H₂ (exothermic, low T favoured). This is done to increase H₂ yield and remove CO (which poisons Haber process catalyst). The CO₂ is then removed by scrubbing. Combined reactions: CH₄ + 2H₂O → CO₂ + 4H₂ (overall, using both reforming + shift). The shift reaction is done at 150-350°C using Fe₃O₄/Cr₂O₃ catalyst.
3. Water Gas and Producer Gas
📌 Water gas: C + H₂O →(1000°C) CO + H₂ (equal volumes, used as fuel)
📌 Producer gas: 2C + O₂ + 4N₂(air) → 2CO + 4N₂ (cheap industrial fuel, ~25% CO + 70% N₂)
📌 Syngas: CO + H₂ from steam reforming of CH₄ — ratio varies by process
📌 Water gas reaction: same as steam reforming but uses carbon (coke) instead of CH₄
📌 CO is toxic but excellent reducing agent and fuel: 2CO + O₂ → 2CO₂ (combustion)
📌 Mixture of water gas + producer gas = "town gas" (used before natural gas)
4. Haber Process — Industrial NH₃ Synthesis
N₂ + 3H₂ ⇌ 2NH₃ (Fe catalyst, 400-500°C, 200 atm). The H₂ used in Haber process comes primarily from steam reforming of methane. So the sequence is: (1) CH₄ + H₂O → CO + 3H₂ (steam reforming). (2) CO + H₂O → CO₂ + H₂ (shift reaction). (3) CO₂ removed. (4) N₂ + 3H₂ → NH₃ (Haber process). This is one of the most important industrial chemical sequences — NH₃ → fertilisers → food for ~half of world's population.
5. Fischer-Tropsch Process
Syngas (CO + H₂) can be converted to liquid fuels (hydrocarbons) using Fischer-Tropsch synthesis: nCO + (2n+1)H₂ →(Co or Fe catalyst, 150-300°C) CₙH₂ₙ₊₂ + nH₂O. Produces synthetic diesel, petrol, wax. Used commercially in South Africa (SASOL) using coal as feedstock. The process is important for energy security (converts coal or natural gas to liquid fuel). Modern interest: using green H₂ + captured CO₂ to make synthetic "e-fuels" for aviation.
6. Preparation of H₂ in Laboratory
📌 Zn + H₂SO₄ (dil): Zn + H₂SO₄ → ZnSO₄ + H₂↑ (most common lab method)
📌 Zn + NaOH: Zn + 2NaOH → Na₂ZnO₂ + H₂ (amphoteric Zn reacts with alkali)
📌 Electrolysis of water: 2H₂O →(electrolysis) 2H₂ + O₂ (purest method)
📌 Na + H₂O: 2Na + 2H₂O → 2NaOH + H₂ (vigorous, dangerous)
📌 NaH + H₂O: NaH + H₂O → NaOH + H₂ (metal hydride hydrolysis)
7. Green Hydrogen vs Grey Hydrogen
Grey hydrogen: from steam reforming of fossil fuels (CH₄) — produces CO₂ as byproduct (~95% of current production). Blue hydrogen: steam reforming + CCS (carbon capture and storage) — lower CO₂ emissions. Green hydrogen: from electrolysis of water using renewable electricity (solar, wind) — zero CO₂. Currently expensive but rapidly getting cheaper. Hydrogen economy: using H₂ as clean fuel — in fuel cells (H₂ + O₂ → H₂O + electricity), for transportation (fuel cell vehicles), industrial reduction. H₂ has highest energy density per kg of any fuel (141 MJ/kg vs 44 MJ/kg for petrol).
8. Methane Sources and Natural Gas
Natural gas = primarily methane (CH₄, ~80-95%) + ethane, propane, butane + CO₂ + H₂S (impurities). Sources: (1) Fossil natural gas from geological formations (petroleum wells). (2) Biogas: CH₄ from anaerobic decomposition of organic matter (sewage, manure, landfill) by methanogens (archaebacteria). (3) Methane clathrates (hydrates): CH₄ trapped in ice-like structures on ocean floors — estimated to contain more carbon than all other fossil fuels combined, but difficult to extract. Methane is also a potent greenhouse gas (28× more warming than CO₂ over 100 years by mass).
Frequently Asked Questions
1. Why CO and not CO₂ at 1273K? ⌄
The reaction CH₄ + H₂O → CO + 3H₂ is a partial oxidation of carbon (C goes from −4 in CH₄ to +2 in CO). At very high temperature (1273 K), CO is thermodynamically stable (Boudouard equilibrium favours CO at high T). If CO₂ formed initially, it would be reduced back by excess H₂: CO₂ + H₂ → CO + H₂O (reverse shift reaction — favoured at high T). The direct reaction CH₄ + 2H₂O → CO₂ + 4H₂ (complete reforming) occurs at lower temperatures with different conditions.
2. What is the role of the nickel catalyst? ⌄
Nickel (Ni) catalyst: (1) Lowers activation energy — without catalyst, C-H bonds in CH₄ are very strong (414 kJ/mol each) and the reaction rate would be extremely slow. (2) Provides active surface where CH₄ and H₂O adsorb and react. (3) Does not get consumed — regenerated after each catalytic cycle. Mechanism: CH₄ adsorbs on Ni surface → C-H bonds weaken → H₂O adsorbs → C reacts with O from water → CO formed → desorbs. Iron (Fe) and Ruthenium (Ru) can also catalyse this reaction. Nickel is preferred industrially due to low cost.
3. Why is H₂ important industrially? ⌄
H₂ is a critical industrial chemical: (1) Haber process: N₂ + 3H₂ → 2NH₃ (fertilisers — ~55% of H₂ use). (2) Hydrogenation of oils to fats (margarine production). (3) Hydrocracking in petroleum refining (breaking heavy hydrocarbons). (4) Methanol synthesis: CO + 2H₂ → CH₃OH (plastics, fuels). (5) Steel production (replacing coke with H₂ for DRI). (6) Fuel cells (clean electricity). (7) Desulphurisation of petroleum (H₂ + S compounds → H₂S → removed). World H₂ production: ~75 million tonnes/year — almost entirely for industrial use, not energy.
4. What is biogas and how is it produced? ⌄
Biogas = ~60% CH₄ + 40% CO₂ (+ traces H₂S, H₂O vapour). Produced by anaerobic digestion of organic waste by methanogenic archaea (e.g., Methanobacterium). Feedstocks: animal dung (gobar gas plants in India), sewage, food waste, agricultural waste, landfill. Process: (1) Hydrolysis (complex → simpler molecules). (2) Acidogenesis (organic acids). (3) Acetogenesis (acetic acid + H₂). (4) Methanogenesis (CH₄ + CO₂). Uses: cooking fuel, electricity generation, vehicle fuel (after CO₂ removal). India is second largest biogas producer — over 5 million family-scale biogas plants.
5. What is the isotopic composition of hydrogen? ⌄
Three isotopes: ¹H (protium, 99·985%) — most common, ²H (deuterium, D, 0·015%) — heavy hydrogen, ³H (tritium, T, trace) — radioactive (t½ = 12·3 years). Heavy water D₂O: used as moderator in CANDU nuclear reactors, used to prepare deuterated compounds for NMR spectroscopy. Deuterium oxide (D₂O) is 10% heavier than H₂O — slightly different physical properties (bp 101·4°C, mp 3·8°C). Tritium: used in nuclear fusion research (D + T → ⁴He + n), hydrogen bombs, luminous watch dials.
6. Why is methane (CH₄) a greenhouse gas? ⌄
Methane absorbs IR radiation in the wavelength region that CO₂ doesn't fully cover — making it an important greenhouse gas. GWP (Global Warming Potential): CH₄ = 84× CO₂ (over 20 years) or 28× over 100 years by mass. Sources: natural gas leaks, cattle (ruminant digestion — belching), rice paddies (anaerobic decomposition), landfills, wetlands, permafrost thawing. CH₄ is broken down in the atmosphere: OH• + CH₄ → •CH₃ + H₂O (atmospheric lifetime ~12 years). NEET context: methane is a significant driver of climate change alongside CO₂.
7. What happens when methane burns completely? ⌄
Complete combustion: CH₄ + 2O₂ → CO₂ + 2H₂O, ΔH = −890 kJ/mol. Incomplete combustion (limited O₂): 2CH₄ + 3O₂ → 2CO + 4H₂O (CO = toxic carbon monoxide). Very limited O₂: CH₄ + O₂ → C + 2H₂O (soot/carbon black formation). The blue flame in gas stoves = complete combustion (CO₂ + H₂O). Yellow/orange flame = incomplete combustion (hot soot particles glowing). Natural gas stoves are efficient and cleaner than coal/wood because CH₄ has highest H:C ratio (4:1) → produces most H₂O relative to CO₂ per unit energy.
8. What is the difference between syngas and natural gas? ⌄
Natural gas: primarily CH₄ — a fuel directly burned for heating/electricity. Syngas (synthesis gas): CO + H₂ mixture — not directly used as fuel but as feedstock for chemical synthesis. Syngas is produced FROM natural gas (via steam reforming) or from coal (via coal gasification). Syngas is then converted to: NH₃ (Haber), methanol (CO + 2H₂ → CH₃OH), synthetic fuels (Fischer-Tropsch), or purified to pure H₂. "Town gas" (used before natural gas infrastructure) was also CO + H₂ — made from coal. It was toxic (CO) and eventually replaced by safer natural gas.