Problem 12 - Chemistry in 30 days

Question

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

Step-by-Step Solution

Answer

The correct product formed is (a) \( \text{H}_3\text{BO}_3 + \text{HCl} \).

Step 1: Identify the Reactants

The problem involves boron trichloride (\( \text{BCl}_3 \)) being treated with water (\( \text{H}_2\text{O} \)). We need to determine the reaction that occurs between these two substances.

Step 2: Determine Possible Reaction Products

Boron trichloride (\( \text{BCl}_3 \)) is a Lewis acid and will react with water to form boric acid (\( \text{H}_3\text{BO}_3 \)) and hydrochloric acid (\( \text{HCl} \)). The reaction can be represented as: \[ \text{BCl}_3 + 3\text{H}_2\text{O} \rightarrow \text{H}_3\text{BO}_3 + 3\text{HCl} \] This indicates the formation of boric acid (\( \text{H}_3\text{BO}_3 \)) and hydrochloric acid (\( \text{HCl} \)).

Step 3: Match with Provided Options

Looking at the given options, option (a) \( \text{H}_3\text{BO}_3 + \text{HCl} \) matches the products we determined in the previous step. The other options involve different compounds, such as diborane \( (\text{B}_2\text{H}_6) \) and boron trioxide \( (\text{B}_2\text{O}_3) \), which are not formed in this reaction.

Step 4: Confirm with Chemical Principles

By confirming the reaction with known chemical principles, \( \text{BCl}_3 \) is a typical Lewis acid that can hydrolyze in the presence of water to form boric acid and hydrochloric acid, which matches our prediction of the reaction products.

Key Concepts

Lewis Acid

Boron trichloride (\( \text{BCl}_3 \)) is characterized as a Lewis acid. But what exactly does that mean? A Lewis acid is defined as a compound that can accept a pair of electrons. In other words, it has the capacity to form bonds by accepting electron pairs from Lewis bases. This electron pair acceptance is critical in chemical reactions where Lewis acids participate.

In the case of boron trichloride, the boron atom has an incomplete octet, meaning it doesn't have a full set of electrons in its outer shell. Because of this electron deficiency, \( \text{BCl}_3 \) readily seeks out other molecules, such as water, to provide these missing electrons.

This reaction tendency makes \( \text{BCl}_3 \) a classic example of a Lewis acid. When it encounters water, it effectively accepts electron pairs, facilitating the hydrolysis reaction to occur.

Hydrolysis Reaction

A hydrolysis reaction involves the breaking of bonds of a compound and the incorporation of water elements into the resulting parts. With boron trichloride, this process is quite vivid. When \( \text{BCl}_3 \) comes into contact with water, it undergoes hydrolysis. This means that the existing bonds in \( \text{BCl}_3 \) break, and new bonds form, incorporating hydrogen and oxygen atoms from water.

During the reaction, water molecules donate electron pairs to \( \text{BCl}_3 \), enabling the breakdown of the trichloride compound. As water splits into its components (\( \text{H}^+ \) and \( \text{OH}^- \)), these fragments attach to the boron atom, leading to the formation of hydroxyl groups and hydrochloric acid (\( \text{HCl} \)).

• Thus, the overall reaction can be represented by:
  \( \text{BCl}_3 + 3\text{H}_2\text{O} \rightarrow \text{H}_3\text{BO}_3 + 3\text{HCl} \)

This hydrolysis results in final products that are a blend of boric acid and hydrochloric acid.

Boric Acid Formation

The culmination of the hydrolysis of boron trichloride is the formation of boric acid (\( \text{H}_3\text{BO}_3 \)). Boric acid is a weak acid but serves as an essential product from the treatment of \( \text{BCl}_3 \) with water. The step-by-step hydrolysis process stabilizes the boron atom by pairing it with three \( \text{OH}^- \) groups obtained from the water.

Boric acid can be visually depicted as a compound where the boron atom is surrounded by three hydroxyl groups. It is commonly found in many household and industrial applications, often used in mild antiseptics and various chemical solutions.

It's important to note that the formation of boric acid is a direct consequence of the electron acceptance through the Lewis acid-base reaction and subsequent hydrolysis of boron trichloride. This underscores the interconnectedness of these chemical processes and the fruitful result they yield in forming stable, common chemical compounds like boric acid.