Which one of the following statements is incorrect ?
(a) Atomic hydrogen is produced when H2 molecul

Q: Why is the bond dissociation enthalpy of $H_2$ so high?

Hydrogen atoms are very small, so the bond length is very short. This leads to a very strong electrostatic attraction between the nuclei and the shared electrons, making the bond very difficult to break.

Q: What does it mean for a metal to be amphoteric?

An amphoteric metal, like Zinc or Aluminum, has the unique ability to react chemically with both acids (like $HCl$) and bases (like $NaOH$) to form salts and typically release hydrogen gas.

Q: Why do we need UV radiation to produce atomic hydrogen?

The energy of UV photons is high enough to overcome the strong bond dissociation enthalpy of the $H-H$ bond, effectively splitting the molecule into individual hydrogen atoms.

Q: Is $8.1\%$ dissociation at $2000 \text{ K}$ really that far off?

Yes, it is off by a factor of 100! The actual dissociation at $2000 \text{ K}$ is only about $0.081\%$. It takes much higher temperatures, around $5000 \text{ K}$, to achieve near-complete dissociation.

Q: What is the formula for sodium zincate?

The chemical formula for sodium zincate, formed when zinc reacts with sodium hydroxide, is $Na_2ZnO_2$.

Question

Which one of the following statements is incorrect ?

Options:

(A) Atomic hydrogen is produced when $	ext{H}_2$ molecules at a high temperature are irradiated with UV radiation.

(B) At around $2000	ext{ K}$, the dissociation of dihydrogen into its atoms is nearly $8.1\%$.

(C) Bond dissociation enthalpy of $	ext{H}_2$ is highest among diatomic gaseous molecules which contain a single bond .

(D) Dihydrogen is produced on reacting zinc with $	ext{HCl}$ as well as $	ext{NaOH(aq)}$.

Sigma Percentile · Accepted Answer

### Analyzing the Setup
The question tests our factual knowledge about the properties and preparation of dihydrogen ($H_2$). We need to evaluate four distinct statements covering its dissociation, bond enthalpy, and chemical reactions.

### The Strength of the H-H Bond
Let's start by understanding the nature of the hydrogen molecule. Hydrogen atoms are the smallest atoms in the periodic table. When two hydrogen atoms bond to form $H_2$, their nuclei are very close to each other. This proximity allows for a very strong electrostatic attraction between the positively charged nuclei and the shared pair of electrons.

Because of this intense attraction, the **bond dissociation enthalpy** of the $H-H$ bond is exceptionally high—in fact, it is the highest for a single bond between any two atoms of the same element, standing at $435.88 	ext{ kJ mol}^{-1}$. This makes statement (c) absolutely correct.

### Breaking the Unbreakable
Since the $H-H$ bond is so strong, dihydrogen is relatively inert at room temperature. To break this bond and produce atomic hydrogen, we must supply a massive amount of energy. This is typically achieved by subjecting the gas to very high temperatures (like in an electric arc) or by irradiating it with high-energy ultraviolet (UV) light. Thus, statement (a) is also a correct fact.

But how much of it actually breaks apart when heated? If we heat $H_2$ to $2000 	ext{ K}$, the thermal energy is still not enough to cause widespread dissociation. At this temperature, only about **$0.081\%$** of the molecules dissociate into atoms. You have to push the temperature up to around $5000 	ext{ K}$ to see a significant dissociation of about $95.5\%$. Statement (b) claims the dissociation is $8.1\%$ at $2000 	ext{ K}$, which is an overestimation by a factor of 100! Therefore, statement (b) is the incorrect one.

### The Amphoteric Nature of Zinc
Finally, let's look at the chemical preparation of hydrogen mentioned in statement (d). Zinc is a classic example of an **amphoteric** metal, meaning it can react with both acids and bases.

When zinc reacts with a strong acid like hydrochloric acid ($HCl$), it undergoes a single displacement reaction to liberate hydrogen gas:
$$Zn + 2HCl ightarrow ZnCl_2 + H_2 \uparrow$$

Similarly, when zinc is treated with a strong aqueous base like sodium hydroxide ($NaOH$), it forms a complex salt called sodium zincate and again liberates hydrogen gas:
$$Zn + 2NaOH ightarrow Na_2ZnO_2 + H_2 \uparrow$$

This confirms that statement (d) is perfectly correct.

### Final Conclusion
By systematically evaluating each option against the known chemical and physical properties of hydrogen, we can confidently conclude that statement (b) is the only incorrect statement among the choices provided.

Animated Solution for Chemistry - s and p-Block Elements: Which one of the following statements is incorrect ?

Select Answer:

Visualized Solution

\text{Production of Atomic Hydrogen}

\text{Thermal Dissociation of } \text{H}_2

\text{Bond Dissociation Enthalpy}

\text{Reaction of Zinc}

\text{Final Conclusion}

The Sigma Insight: Hydrogen, Hydrides and Water

Analyzing the Setup The question tests our factual knowledge about the properties and preparation of dihydrogen ( $H_{2}$ )

The Strength of the H-H Bond Let's start by understanding the nature of the hydrogen molecule

Breaking the Unbreakable Since the $H - H$ bond is so strong, dihydrogen is relatively inert at room temperature

The Amphoteric Nature of Zinc Finally, let's look at the chemical preparation of hydrogen mentioned in statement (d)

Final Conclusion

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