Problem 34 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is a false statement? (a) boron hydrides are formed when dil. HCl reacts with \(\mathrm{Mg}_{3} \mathrm{~B}_{2}\) (b) all the B-H bond distances in \(\mathrm{B}_{2} \mathrm{H}_{6}\) are equal (c) \(\mathrm{BH}_{3}\) is not a stable compound (d) the boron hydrides are readily hydrolyzed

Step-by-Step Solution

Answer

Statement (b) is false; not all B-H bond distances in B_2H_6 are equal.

Step 1: Understanding the Chemical Reactions

Review statement (a): Boron hydrides are generally formed from a reaction involving boron with hydrogen sources. When magnesium boride (Mg_3B_2) reacts with dilute hydrochloric acid (HCl), boron hydrides are indeed formed. Thus, this statement is true.

Step 2: Analyzing Boron Hydride Structures

Review statement (b): B_2H_6 (diborane) has a structure where there are two types of B-H bonds: terminal B-H bonds and bridged B-H-B bonds. The bond distances for terminal B-H bonds and bridged B-H-B bonds are not equal, making this statement false.

Step 3: Studying the Stability of Compounds

Review statement (c): BH_3 (borane) is a known compound, but it is not stable in isolation under normal conditions because it polymerizes to form B_2H_6. Hence, this statement is true.

Step 4: Reactivity of Boron Hydrides with Water

Review statement (d): Boron hydrides, like B_2H_6, react with water and are readily hydrolyzed to form hydrogen gas among other products. Therefore, this statement is true.

Key Concepts

Chemical Reactions

Chemical reactions involve the transformation of reactants into products, often with observable changes such as emission of gas, change of temperature, or the formation of a new substance. Boron hydrides, a class of intriguing compounds formed from boron and hydrogen, undergo particular reactions that are fascinating to study.

When magnesium boride (\(\mathrm{Mg}_3 \mathrm{B}_2\)) reacts with dilute hydrochloric acid (\(\mathrm{HCl}\)), boron hydrides are produced. This is a chemical reaction where the acid reacts with magnesium boride, replacing the magnesium with hydrogen to form a new compound with boron under acidic conditions. Boron hydrides formed this way, demonstrate how boron's versatile bonding capability allows for a rich chemistry including the formation of multiple hydrides.

Molecular Structure

Understanding the molecular structure of boron hydrides such as diborane (\(\mathrm{B}_2 \mathrm{H}_6\)) is crucial for grasping their behavior and reactivity. Unlike simpler molecules, diborane contains what is known as a 'bridge' bond that is quite unique.

• Terminal B-H bonds: These are typical single bonds.
• Bridged B-H-B bonds: Important for understanding the three-center two-electron bonding, a remarkable feature in diborane's structure.

This structure means that not all B-H bond distances are the same, contrary to what might be intuitively expected in symmetrical molecules. This results in interesting properties and reactivity patterns that are characteristic of boron compounds.

Compound Stability

The stability of a compound is a measure of how resistant it is to change or decomposition under certain conditions. In the case of \(\mathrm{BH}_3\) (borane), stability is compromised in isolation due to its tendency to polymerize.

Borane molecules exhibit an inherent instability when not part of a larger structure, rapidly forming diborane (\(\mathrm{B}_2 \mathrm{H}_6\)) through polymerization. This transformation occurs spontaneously under normal conditions because lone borane is electron-deficient and seeks stability by sharing electrons through polymerization. Understanding why certain compounds like \(\mathrm{BH}_3\) are unstable sheds light on chemical bonding and molecular interactions.

Hydrolysis

Hydrolysis is a chemical process in which a compound reacts with water, often breaking down into simpler molecules. Boron hydrides such as diborane (\(\mathrm{B}_2 \mathrm{H}_6\)) are readily hydrolyzed.

During hydrolysis, boron hydrides react with water to produce hydrogen gas along with boric acid or other boron-containing compounds. This readiness to undergo hydrolysis highlights not only their reactivity but also offers practical applications such as the controlled release of hydrogen gas. The study of hydrolysis reactions is essential as it demonstrates how boron hydrides interact with their environment and transform into more stable forms when water is present.