NDA (I) 2021 General Ability Test · Question 61 of 149

Correct — D, 'a mixture of NO₂ and H₂ is known as Syngas' — this is the incorrect statement. Syngas (synthesis gas, also called water gas) is a mixture of carbon monoxide (CO) and hydrogen (H₂), not nitrogen dioxide and hydrogen. The other three statements are all true: H₂ is lighter than air and only slightly soluble in water; it is fairly unreactive at room temperature because of the strong H-H bond (high bond-dissociation enthalpy); and it reacts with hot alkali metals to form ionic metal hydrides, for example 2Na + H₂ → 2NaH.

• (a)H₂ is lighter than air and insoluble in water. — This is true — hydrogen is the lightest gas and only very slightly soluble in water — so it is not the incorrect statement.
• (b)H₂ is inert at room temperature due to high H–H bond dissociation enthalpy. — True — the strong H-H bond (about 436 kJ/mol) makes hydrogen sluggish at ordinary temperatures.
• (c)H₂ reacts with alkali metals at high temperature to yield metal hydrides. — True — hot sodium or potassium react with hydrogen to give ionic hydrides such as NaH.

Dihydrogen (H₂) is the lightest, colourless, odourless gas; it is fairly unreactive at room temperature because of its strong covalent bond, but reacts with reactive metals to form hydrides. Syngas, or water gas, is an industrial fuel-gas mixture of carbon monoxide and hydrogen (CO + H₂), used to make ammonia, methanol and other chemicals.

Three statements are standard true facts about hydrogen; the odd one out plants a false definition of syngas, swapping CO for NO₂. Knowing that syngas is CO + H₂ flags option (d) at once.

• Syngas (water gas) is CO + H₂; it is a feedstock for ammonia and methanol.
• H₂ is the lightest gas, lighter than air, and only slightly soluble in water.
• The H-H bond enthalpy (about 436 kJ/mol) makes H₂ fairly inert at room temperature.
• H₂ reacts with hot alkali metals to form ionic metal hydrides such as NaH.

• Being distracted by three correct statements and missing the planted false definition.
• Confusing syngas (CO + H₂) with a nitrogen-based mixture.

Framed as 'which statement is NOT correct' — three true properties of hydrogen hide one false definition of syngas.

• practice — not a real PYQ

  Water gas (syngas) is a mixture of

  • (a)CO and H₂
  • (b)CO₂ and H₂
  • (c)NO₂ and H₂
  • (d)CH₄ and O₂

  Answer(a) CO and H₂.

• practice — not a real PYQ

  Hydrogen is relatively unreactive at room temperature mainly because of its

  • (a)low molecular mass
  • (b)high H-H bond dissociation enthalpy
  • (c)insolubility in water
  • (d)low density

  Answer(b) high H-H bond dissociation enthalpy.