Problem 107 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following statements is/are not true about the diagonal relationship of Be and Al? (1) Both react with \(\mathrm{NaOH}\) to liberate hydrogen (2) Their oxides are basic (3) They are made passive by nitric acid (4) Their carbides give acetylene on treatment with water (a) only (1) (b) (2) and (3) (c) only (4) (d) (2) and (4)

Step-by-Step Solution

Answer

(d) (2) and (4) are not true; they're both false statements.

Step 1: Analyze Statement 1

The statement (1) asserts that both Beryllium (Be) and Aluminum (Al) react with sodium hydroxide (\(\mathrm{NaOH}\)) to liberate hydrogen. Beryllium reacts with \(\mathrm{NaOH}\) to form beryllate and evolves hydrogen gas, while Aluminum also reacts with \(\mathrm{NaOH}\) similarly to give aluminate and hydrogen. Thus, statement (1) is true.

Step 2: Evaluate Statement 2

Statement (2) claims that the oxides of Be and Al are basic. Beryllium oxide (\(\mathrm{BeO}\)) is amphoteric, not basic—it reacts with both acids and bases. Similarly, aluminum oxide (\(\mathrm{Al_2O_3}\)) is also amphoteric. Hence, statement (2) is not true.

Step 3: Review Statement 3

For statement (3), it is stated that Be and Al are made passive by nitric acid. This is correct because both metals form a protective oxide layer when treated with concentrated \(\mathrm{HNO_3}\), preventing further reaction. So, statement (3) is true.

Step 4: Examine Statement 4

The fourth statement suggests that the carbides of Be and Al give acetylene on reaction with water. Beryllium carbide (\(\mathrm{Be_2C}\)) reacts with water to produce methane, not acetylene. Similarly, aluminum carbide (\(\mathrm{Al_4C_3}\)) reacts with water to yield methane. Therefore, statement (4) is incorrect.

Step 5: Conclusion

Based on the evaluations, statements (2) and (4) are incorrect. The correct answer, which corresponds to statements (2) and (4) being false, is option (d).

Key Concepts

Amphoteric Oxides

Amphoteric oxides are intriguing because they can behave like both acid and base. This dual nature is what defines their amphoteric character. Beryllium oxide (\( \text{BeO} \)) is a classic example of an amphoteric oxide. It reacts with acids to form beryllium salts and with strong bases like \( \text{NaOH} \) to form compounds such as beryllates.
Aluminum oxide (\( \text{Al}_2\text{O}_3 \)) also shares this characteristic. It reacts with acids to yield aluminum salts and can react with bases to form aluminates.

• Such dual behavior makes amphoteric oxides unique, as they can help neutralize both acidic and basic substances.
• Their ability to participate in diverse chemical reactions makes them useful in various industrial applications.

Understanding amphoteric oxides is crucial in the study of chemistry and helps in making informed predictions about chemical reactivity.

Hydrogen Liberation

The liberation of hydrogen gas is a fascinating reaction studied in chemistry. Alcali, like sodium hydroxide (\( \text{NaOH} \)), can react with some metals such as Beryllium (\( \text{Be} \)) and Aluminum (\( \text{Al} \)) to produce hydrogen.
When \( \text{Be} \) reacts with \( \text{NaOH} \), beryllate is formed, and hydrogen gas is released. Similarly, \( \text{Al} \) forms aluminate and hydrogen when reacting with \( \text{NaOH} \).

• These reactions are useful in hydrogen gas production and have applications in various chemical processes.
• Knowing which metals can liberate hydrogen is key in designing reactions for practical and industrial purposes.

This consistent behavior across Be and Al highlights the diagonal relationship they share in the periodic table, exhibiting similar chemical properties.

Passivation by Nitric Acid

Passivation is the process of making a material "passive" or less reactive to environmental factors like air or moisture. Nitric acid (\( \text{HNO}_3 \)) can passivate certain metals by forming a thin oxide layer on their surfaces.
The interaction with concentrated \( \text{HNO}_3 \) generates a protective oxide layer, effectively insulating the metal underneath. Both Beryllium and Aluminum become "passive" due to passivation, preventing them from further reacting with acids or bases.

• This property is particularly important for preventing corrosion and sustaining the longevity of metal utility in practical applications.
• Understanding passivation helps in choosing appropriate materials for equipment that will be exposed to environments where corrosion is a risk.

Passivation confirms the unique behavior of metals like Be and Al, further accentuating their diagonal relationship in the periodic table.