Chemistry Q68 – NEET-UG 2023

Home Chemistry Basic ChemistryQuestion #60

Which of the following statements are NOT correct? A. Hydrogen is used to reduce heavy metal oxides to metals. B. Heavy water is used to study reaction mechanism. C. Hydrogen is used to make saturated fats from oils. D. The H-H bond dissociation enthalpy is lowest as compared to a single bond between two atoms of any element. E. Hydrogen reduces oxides of metals that are more active than iron. Choose the most appropriate answer from the options given below :

Clue 1 of 3: Properties of Hydrogen and Heavy Water

Recall the common uses and chemical properties of hydrogen and heavy water, including their roles as reducing agents and in reaction studies.

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Ninja Strategy•Factual Recall and Elimination

Quickly identify the factually correct statements (A, B, C) and eliminate options that include these as 'NOT correct'. This narrows down the choices significantly.

🧠 Full Solution Path

Step 1: Evaluate Statements A, B, and C✦ Active

Statement A: Hydrogen is a good reducing agent and is used to reduce oxides of less reactive metals (often referred to as heavy metals) to their elemental form. For example, $C u O + H_{2} \to C u + H_{2} O$ . This statement is **correct**.

Statement B: Heavy water ( $D_{2} O$ ) is used as a tracer in studying reaction mechanisms due to the isotopic difference between deuterium (D) and hydrogen (H), which allows its path to be tracked. This statement is **correct**.

Statement C: Hydrogen is used in the hydrogenation of unsaturated vegetable oils to produce saturated fats (e.g., vanaspati ghee) in the presence of a catalyst like nickel. This statement is **correct**.

Step 2: Evaluate Statements D and E○ Expand

Statement D: The H-H bond dissociation enthalpy is approximately $435.8 kJ/mol$ . This is a relatively high value. Many other single bonds, such as F-F ( $158 kJ/mol$ ), Cl-Cl ( $242 kJ/mol$ ), and O-O in $H_{2} O_{2}$ ( $146 kJ/mol$ ), have lower bond dissociation enthalpies. Therefore, the statement that H-H bond dissociation enthalpy is the lowest is **incorrect**.

Statement E: Hydrogen can reduce oxides of metals that are less reactive than hydrogen or moderately reactive metals (like iron, lead, copper). However, it cannot reduce oxides of highly reactive metals (e.g., Na, K, Ca, Mg, Al) which are more active than iron. For instance, hydrogen cannot reduce $A l_{2} O_{3}$ . Therefore, this statement is **incorrect**.

💡 Teacher's Secret Hint

Remember the reactivity series of metals to determine the reducing power of hydrogen.

Step 3: Identify Incorrect Statements and Select Option○ Expand

From the evaluation, statements D and E are NOT correct. We need to choose the option that lists these incorrect statements.

Option (3) 'D, E only' correctly identifies the incorrect statements.

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Chemistry Question 68 – NEET-UG 2023

Clue 1 of 3: Properties of Hydrogen and Heavy Water

🧠 Full Solution Path