Question

Which of the following statements is/are not true about the diagonal relationship of \(\mathrm{Be}\) and \(\mathrm{Al} ?\) (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)

Short answer

The answer is (d) (2) and (4).

Step-by-step solution

Analyze Statement (1)

Beryllium (Be) and Aluminum (Al) both react with \(\text{NaOH} \) to release hydrogen gas. Thus, statement (1) is true.

Analyze Statement (2)

Check the nature of the oxides of Be and Al. BeO is amphoteric, meaning it can react with both acids and bases. Similarly, Al\(_2\)O\(_3\) is also amphoteric. Since neither BeO nor Al\(_2\)O\(_3\) are basic, statement (2) is not true.

Analyze Statement (3)

Investigate their passivation by nitric acid. Both Be and Al form protective oxide layers when they react with \(\text{HNO}_3\), preventing further reaction, so they undergo passivation. Therefore, statement (3) is true.

Analyze Statement (4)

Consider the reaction of their carbides with water. Be\(_2\)C and Al\(_4\)C\(_3\) do not produce acetylene, \(\text{C}_2\text{H}_2\). Instead, Be\(_2\)C forms methane (\(\text{CH}_4\)) and Al\(_4\)C\(_3\) forms methane, not acetylene. Thus, statement (4) is not correct.

Select Incorrect Statements

From the analysis, statements (2) and (4) are not true.

Key concepts

Beryllium (Be) and Aluminum (Al) reactions

Beryllium (Be) and Aluminum (Al) have a fascinating diagonal relationship on the periodic table. This relationship is due to the similar properties they exhibit, despite being in different groups. One of the key similarities is their ability to react with sodium hydroxide (NaOH) to release hydrogen gas. Beryllium behaves differently than most metals, as it does not react with water but will react with NaOH to produce hydrogen. This is similar to aluminum, which also reacts with NaOH to release hydrogen gas and form soluble aluminates. This reaction is a hallmark of their unusual chemistry, making both Be and Al interesting subjects of study. The overall process occurs as metals are introduced to NaOH, forming their respective hydroxides and releasing hydrogen:

Amphoteric oxides

Beryllium oxide (BeO) and aluminum oxide (AlO) are not merely basic or acidic. They are classified as amphoteric, meaning they can react with both acids and bases. This property is rather exceptional and deviates from the typical behavior of many metal oxides. For BeO, it can dissolve in acidic solutions to form beryllium salts, while in basic solutions, it forms beryllates. Similarly, AlO behaves in much the same way, dissolving in acid to form aluminum salts and in bases to produce aluminate ions. In summary, the amphoteric nature of these oxides reflects their versatility in chemical reactions. They do not fit neatly into the category of a purely basic oxide, which differentiates them distinctly from many other metal oxides.

Passivation by Nitric Acid

Both beryllium and aluminum have the ability to undergo passivation when exposed to concentrated nitric acid (HNO). Passivation is a process where a material becomes less affected by environmental factors through the formation of an outer layer. In this case, the exposure to nitric acid results in the formation of an oxide layer that adheres closely to the surface of the metal. This layer acts as a protective barrier, preventing further reaction of the metal with the acid. This protective behavior is important as it helps preserve the metal's integrity in environments that could otherwise cause corrosion or wear, making it a crucial property for practical applications in various industries.

Carbides reactions with water

Carbides are compounds composed of carbon and a metal. Beryllium carbide (BeC) and aluminum carbide (AlC) showcase interesting reactions when combined with water; however, they do not produce acetylene (CH) as might be expected. When BeC reacts with water, it forms methane (CH) instead. Similarly, AlC's reaction with water also results in the production of methane, not acetylene. This outcome is contrary to the behavior of some other metal carbides, which do react with water to produce acetylene. These reactions highlight the importance of understanding specific carbide properties, as they can vary quite significantly among different metals, leading to a range of products upon hydrolysis.