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)

Step 1: Identify the diagonal relationship

The diagonal relationship is a concept in the periodic table where elements show similar properties diagonally across the periods and groups, like Be (Beryllium) and Al (Aluminum). We need to compare which given statements align with this relationship.

Step 2: Evaluate each statement for truth or falsity

1. Both Be and Al react with NaOH to liberate hydrogen. Be behaves similarly to Al in this reaction, making it true. 2. Their oxides are basic. BeO is amphoteric, whereas Al2O3 is also amphoteric, making this reasoning incorrect if you interpret 'basic' as not amphoteric. 3. They are made passive by nitric acid because both form an oxide layer on their surface, hindering further reaction. This holds true for both elements. 4. Their carbides give acetylene on treatment with water. Be forms Be2C, which reacts with water to give a different product, while Al4C3 with water gives methane, not acetylene. This is false.

Step 3: Match incorrect statements with given options

Considering the incorrect statements, statements (2) and (4) are not true. Because their oxides are not purely basic due to their amphoteric nature, and their carbides do not produce acetylene on treatment with water.

Key Concepts

Beryllium and Aluminum Chemistry

Beryllium (Be) and Aluminum (Al) are two elements that exhibit a notable diagonal relationship in the periodic table. This relationship leads to some surprising similarities between these elements, despite being in different groups. One of the most interesting chemical behaviors of these elements is their interaction with sodium hydroxide (\(\mathrm{NaOH}\)). Both Be and Al react with \(\mathrm{NaOH}\) to produce hydrogen gas, a property typical for amphoteric compounds. This characteristic aligns with the behavior of elements across diagonal relationships, showing how elements such as Be (in period 2) and Al (in period 3) can react similarly under certain conditions. However, in practical terms, the reactions they undergo and their conditions may slightly differ due to their unique atomic structures.

Amphoteric Oxides

When discussing Be and Al, it is crucial to understand the nature of their oxides. Amphoteric oxides are compounds that can behave as either acidic or basic, depending on the environment. Beryllium oxide (\(\mathrm{BeO}\)) and aluminum oxide (\(\mathrm{Al_2O_3}\)) are classic examples. These oxides do not conform to simply being 'basic' or 'acidic'. They react with both acids and bases to form salts and water. In the context of the diagonal relationship, both \(\mathrm{BeO}\) and \(\mathrm{Al_2O_3}\) demonstrate this amphoteric nature, emphasizing the versatile chemical behavior shared among period and group diagonal neighbors. Such properties challenge straightforward classification, demanding a nuanced understanding of chemical contexts.

Carbide Reactions with Water

Carbide compounds of Be and Al present fascinating chemistry when they interact with water. Typically, carbides are compounds containing carbon in a negative oxidation state, often bound to metals. Beryllium forms beryllium carbide (\(\mathrm{Be_2C}\)), while aluminum forms aluminum carbide (\(\mathrm{Al_4C_3}\)). These compounds, however, do not yield acetylene (\(\mathrm{C_2H_2}\)) when reacting with water. Instead, beryllium carbide reacts with water to produce methane (\(\mathrm{CH_4}\)), and aluminum carbide similarly produces methane rather than acetylene. These reactions contravene some initial expectations of carbide reactivity with water and underline the importance of precise chemical understanding in real-world applications.

Passivation by Nitric Acid

The process of passivation is central to the chemistry and stability of metals. Both beryllium and aluminum become passive upon exposure to nitric acid. Passivation occurs because these metals form a protective oxide layer on their surface, which acts as a barrier against further chemical reaction. This oxide layer is stable and adherent, effectively isolating the underlying metal from contact with the harsh surroundings. The resulting stability is critical for applications where resistance to corrosion and chemical attack is needed. This property highlights another similarity between Be and Al, despite their positions on different portions of the periodic table, reaffirming the significance of diagonal relationships in understanding shared chemical properties.