Question

Which one of the following statements is incorrect?

• A. Bond dissociation enthalpy of \( \text{H}_2 \) is highest among diatomic gaseous molecules which contain a single bond.
• B. At around 2000 K, the dissociation of dihydrogen into its atoms is nearly 8.1%.
• C. Dihydrogen is produced on reacting zinc with \( \text{HCl} \) as well as \( \text{NaOH}_{(aq)} \).
• D. Atomic hydrogen is produced when \( \text{H}_2 \) molecules at a high temperature are irradiated with UV radiation.

Hint:

Remember that \( \text{H}_2 \) dissociation is extremely low at moderate temperatures. It only becomes significant (\(>90% \)) in an electric arc or at temperatures exceeding 5000 K.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
This question covers the physical properties, chemical preparations, and dissociation behavior of dihydrogen (\( \text{H}_2 \)).
Step 2: Detailed Explanation:
(A) The H-H bond is very strong due to its small size. Its bond dissociation enthalpy is \( 435.8 \text{ kJ/mol} \), which is the highest for any single bond between two identical atoms. This is correct.
(B) Dihydrogen is extremely stable. At 2000 K, the degree of dissociation is only about 0.081%. It reaches approximately 95.5% only at very high temperatures like 5000 K. Thus, the value 8.1% is incorrect.
(C) Zinc is an amphoteric metal.
Reaction with acid: \( \text{Zn} + 2\text{HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2 \)
Reaction with base: \( \text{Zn} + 2\text{NaOH} \rightarrow \text{Na}_2\text{ZnO}_2 \text{ (Sodium zincate)} + \text{H}_2 \). This is correct.
(D) High temperatures and UV radiation provide the energy needed to overcome the high bond dissociation enthalpy, resulting in the formation of atomic hydrogen. This is correct.
Step 3: Final Answer:
Statement (B) is incorrect.