NDA (II) 2018 General Ability Test · Question 66 of 146

Correct — C, CO(g) + H₂(g). Synthesis gas, shortened to syngas and long known as water gas, is a mixture of carbon monoxide and hydrogen. It is made by passing steam over red-hot coke at about 1270 K: C(s) + H₂O(g) → CO(g) + H₂(g). The name records what the mixture is for — it is the industrial feedstock from which methanol, ammonia and, through the Fischer-Tropsch route, synthetic liquid hydrocarbons are synthesised. Notice that the name attaches to the products of that reaction, not to the reactants, which is exactly the distinction the option set is built to test.

• (a)C(s) + H₂O(g) — Coke and steam are the reactants of the water-gas reaction, the pair you start with. The gas mixture is what comes out of the reaction, so this option names the input rather than the product.
• (b)CO(g) + H₂O(g) — Carbon monoxide together with steam. This pair does have a name in industry — it is the feed to the water-gas shift reaction, CO + H₂O → CO₂ + H₂, used to convert carbon monoxide into extra hydrogen — but it is not syngas.
• (d)NO₂(g) + H₂(g) — Nitrogen dioxide has no part in the gasification of coal or of hydrocarbons. NDA planted this same false pairing in 2021 as a statement to be rejected, which is a fair indication of how often it is tried.

Gasification turns solid or heavy carbon feedstock into a burnable and chemically useful gas. Blowing steam through red-hot coke gives water gas, carbon monoxide plus hydrogen, and because the reaction absorbs heat the coke bed cools and has to be reheated by blowing air through it in alternating cycles. Blowing air alone through hot coke gives producer gas, mainly carbon monoxide diluted with the nitrogen of the air. Coal gas, made by heating coal out of contact with air, is a different mixture again — hydrogen, methane and carbon monoxide.

Sort the fuel gases by their constituents once and the whole family becomes answerable. Water gas or syngas is CO + H₂. Producer gas is CO + N₂. Coal gas is H₂ + CH₄ + CO. Liquefied petroleum gas is butane with propane, and compressed natural gas is mostly methane. The particular trap in this item is subtler than the names, though: three of the four options are chemically sensible pairings drawn from the same industrial chemistry, and the question is whether you know which side of the arrow the name sits on. Syngas is the product side.

• Syngas, also called water gas, is a mixture of carbon monoxide and hydrogen.
• It is produced by the action of steam on red-hot coke at about 1270 K.
• It is the industrial starting point for methanol and, through Fischer-Tropsch synthesis, for liquid hydrocarbon fuels.
• The water-gas shift reaction, CO + H₂O → CO₂ + H₂, is used to raise the hydrogen content, for example in ammonia manufacture.
• Producer gas, a different mixture, is mainly carbon monoxide diluted by nitrogen, made by blowing air through hot coke.

• Choosing the reactants of the water-gas reaction because the name 'water gas' sounds like coke and steam.
• Confusing syngas with producer gas, which is carbon monoxide with nitrogen.
• Accepting a nitrogen-dioxide pairing simply because hydrogen appears in it.

Fuel-gas items either give a mixture and ask for its name, or give the name and ask for the constituents — and NDA has used both directions for this one gas.

• practice — not a real PYQ

  Producer gas is a mixture of

  • (a)carbon monoxide and hydrogen
  • (b)carbon monoxide and nitrogen
  • (c)carbon dioxide and hydrogen
  • (d)methane and hydrogen

  Answer(b) carbon monoxide and nitrogen — it is made by blowing air through hot coke, so the nitrogen of the air comes through with the carbon monoxide.

• practice — not a real PYQ

  Water gas is produced by passing

  • (a)air over red-hot coke
  • (b)steam over red-hot coke
  • (c)carbon dioxide over red-hot coke
  • (d)steam over heated iron

  Answer(b) steam over red-hot coke — the reaction C + H₂O → CO + H₂ yields the carbon monoxide and hydrogen mixture called water gas or syngas.