Hydrogen

Correct Answer: Option 2 – D, E only

This question asks you to identify which statements about hydrogen and its properties are NOT correct. Let’s analyze each statement individually:

Statement A: "Hydrogen is used to reduce heavy metal oxides to metals." – This is correct. Hydrogen acts as a reducing agent and is commonly used in metallurgical processes to reduce oxides of metals like tungsten (WO₃ to W) at high temperatures.

Statement B: "Heavy water is used to study reaction mechanism." – This is correct. Heavy water (D₂O) contains deuterium, which is used in kinetic isotope effect studies to trace reaction pathways and mechanisms in organic and inorganic chemistry.

Statement C: "Hydrogen is used to make saturated fats from oils." – This is correct. This process is called hydrogenation, where unsaturated vegetable oils are converted into saturated fats (like margarine) using hydrogen gas in the presence of a catalyst like nickel.

Statement D: "The H–H bond dissociation enthalpy is lowest as compared to a single bond between two atoms of any element." – This is incorrect. The H–H bond has a very high bond dissociation enthalpy (~436 kJ/mol), which means a large amount of energy is required to break the bond. It is not the lowest among elemental bonds.

Statement E: "Hydrogen reduces oxides of metals that are more active than iron." – This is incorrect. Hydrogen typically reduces oxides of less active metals than iron. More reactive metals (like sodium or calcium) have stronger oxides that hydrogen cannot reduce under normal conditions.

Therefore, the incorrect statements are D and E.

This makes Option 2 the correct choice.

Conclusion: Understanding the industrial and chemical behavior of hydrogen—including its reducing properties, bond energy, and role in synthesis and mechanisms—is crucial for acing NEET and other competitive exams. This question tests both factual recall and conceptual clarity in inorganic and general chemistry.