Problem 5 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following compounds are formed whe \(\mathrm{BCl}_{3}\) is treated with water? (a) \(\mathrm{B}_{2} \mathrm{O}_{3}+\mathrm{HCl}\) (b) \(\mathrm{H}_{3} \mathrm{BO}_{3}+\mathrm{HCl}\) (c) \(\mathrm{B}_{2} \mathrm{H}_{6}+\mathrm{HCl}\) (d) none of these

Step-by-Step Solution

Answer

(b) \( \mathrm{H}_{3} \mathrm{BO}_{3} + \mathrm{HCl} \)

Step 1: Understand the Reaction

When boron trichloride \( \mathrm{BCl}_{3} \) is treated with water, a chemical reaction occurs. Remember, \( \mathrm{BCl}_{3} \) is a Lewis acid and reacts with water to form boric acid \( \mathrm{H}_3\mathrm{BO}_3 \) and hydrochloric acid \( \mathrm{HCl} \). The key here is to recognize the products of the hydrolysis reaction.

Step 2: Identify the Hydrolysis Products

The reaction between \( \mathrm{BCl}_{3} \) and water can be written as follows: \[ \mathrm{BCl}_{3} + 3\mathrm{H}_2\mathrm{O} \rightarrow \mathrm{H}_3\mathrm{BO}_3 + 3\mathrm{HCl} \] This equation shows that boron trichloride undergoes hydrolysis to yield boric acid and hydrochloric acid.

Step 3: Match Reaction Products with Options

Now, compare the products from Step 2 with the provided options: (a) is \( \mathrm{B}_{2} \mathrm{O}_{3} + \mathrm{HCl} \)(b) is \( \mathrm{H}_{3} \mathrm{BO}_{3} + \mathrm{HCl} \)(c) is \( \mathrm{B}_{2} \mathrm{H}_{6} + \mathrm{HCl} \)(d) is none of these.The correct answer is (b) because it directly matches the hydrolysis products.

Key Concepts

Hydrolysis

Hydrolysis is a chemical reaction where water interacts with another compound, leading to the breakdown of that compound. In the case of boron trichloride (\( \mathrm{BCl}_3 \)), it undergoes hydrolysis when mixed with water. This process involves the breaking of bonds in the \( \mathrm{BCl}_3 \) molecule by the action of water.

• During hydrolysis, \( \mathrm{BCl}_3 \) reacts with three molecules of water.
• The chlorine atoms in \( \mathrm{BCl}_3 \) are replaced by hydroxyl groups.
• This reaction forms boric acid (\( \mathrm{H}_3\mathrm{BO}_3 \)) and hydrochloric acid (\( \mathrm{HCl} \)).

The hydrolysis reaction can be represented by the equation:\[ \mathrm{BCl}_3 + 3\mathrm{H}_2\mathrm{O} \rightarrow \mathrm{H}_3\mathrm{BO}_3 + 3\mathrm{HCl} \]This simple, direct equation captures the essence of the transformation from boron trichloride to boric acid via hydrolysis.

Lewis Acid

A Lewis acid is a chemical species that can accept an electron pair. This concept was introduced by Gilbert N. Lewis and is fundamental to understanding many chemical reactions. Boron trichloride (\( \mathrm{BCl}_3 \)) is an excellent example of a Lewis acid.

• \( \mathrm{BCl}_3 \) has an incomplete octet, making it eager to accept an electron pair.
• This tendency makes \( \mathrm{BCl}_3 \) very reactive, especially with water, in hydrolysis.
• The ability to accept lone pairs classifies \( \mathrm{BCl}_3 \) as a Lewis acid, facilitating its conversion to boric acid.

Thus, the interaction with water in hydrolysis showcases \( \mathrm{BCl}_3 \)'s role as a classic Lewis acid. Understanding this concept helps in predicting and explaining the substances formed during such reactions.

Boric Acid

Boric acid (\( \mathrm{H}_3\mathrm{BO}_3 \)) is a weak acid derived from boron. It is one of the products formed when boron trichloride undergoes hydrolysis.

• Boric acid is often used in various applications, such as antiseptics and preservatives.
• It features a trigonal planar structure and is mildly acidic in solution.
• In the hydrolysis reaction, boric acid forms alongside hydrochloric acid.

Boric acid's formation during the hydrolysis of \( \mathrm{BCl}_3 \) showcases its chemical significance and provides practical insight into its characteristics and uses.