Concept understanding — Dihydrogen — Occurrence and Preparation
Dihydrogen is prepared in the laboratory by the action of dilute acids on active metals such as zinc, or by electrolysis of acidified or alkaline water; pure water conducts electricity poorly, so a small quantity of an electrolyte such as dilute H2SO4 or NaOH is added to furnish free ions and carry the current efficiently, releasing H2 at the cathode and O2 at the anode. Zinc also reacts directly with hot concentrated NaOH solution, liberating H2 and forming sodium zincate: Zn+2NaOH→Na2ZnO2+H2. On the industrial/bulk scale, dihydrogen is obtained mainly from hydrocarbons and coal. Coal gasification passes steam over red-hot coke to give water gas, a mixture of CO and H2: C(s)+H2O(g)1270KCO(g)+H2(g); its hydrogen yield can be increased by mixing the water gas with more steam over a catalyst in the water-gas shift reaction, CO(g)+H2O(g)catalystCO2(g)+H2(g), followed by scrubbing out the CO2 with sodium arsenite solution. Similarly, hydrocarbons or coke react with steam over a nickel catalyst (steam reforming) to generate syngas, from which H2 is further enriched the same way.
Water gas is the CO + H₂ mixture obtained by passing steam over red-hot coke.
✓Final answer
(c) CO + H₂
Step 1. Water gas is produced by passing steam over red-hot coke: C+H2O1000∘CCO+H2.
Step 2. The product is a mixture of carbon monoxide and hydrogen gas -- not steam alone (a), not CO plus unreacted steam (b), and not CO plus nitrogen (d, which has nothing to do with this reaction).
✓Final answer
(c) CO + H₂
Recall the water-gas reaction (steam + red-hot coke) and read off its two gaseous products.
Confusing water gas (CO + H₂) with producer gas (CO + N₂), which is option (d) and a different industrial gas mixture made with air instead of pure steam.