Question

Amorphous boron on burning in air forms (a) Mixture of \(\mathrm{B}_{2} \mathrm{O}_{3}\) and \((\mathrm{BN})_{\mathrm{x}}\) (b) \(\mathrm{B}(\mathrm{OH})_{3}\) (c) Only (BN) \(_{x}\) (d) Only \(\mathrm{B}_{2} \mathrm{O}_{3}\)

Short answer

The correct answer is (d) Only \( \mathrm{B}_{2}\mathrm{O}_{3} \).

Step-by-step solution

Understanding the Reactants and Reaction Conditions

When amorphous boron burns in the presence of air, it reacts primarily with oxygen, which is the most abundant component.

Considering the Products

The reaction of boron with oxygen typically forms boron trioxide, represented chemically as \( \mathrm{B}_{2}\mathrm{O}_{3} \).

Reviewing Possible Compounds

In addition to forming \( \mathrm{B}_{2}\mathrm{O}_{3} \), other compounds like \((\mathrm{BN})_x\) are not generally formed under standard burning conditions.

Determining the Correct Answer

The primary product when amorphous boron burns in air is boron trioxide, \( \mathrm{B}_{2}\mathrm{O}_{3} \); options involving \((\mathrm{BN})_x\) and other compounds are not accurately formed in this reaction.

Key concepts

Amorphous Boron Combustion

Amorphous boron is a unique form of the element boron characterized by its lack of a long-range ordered crystal structure. This makes it more reactive as compared to its crystalline counterpart. When amorphous boron is exposed to air, particularly oxygen, it undergoes a combustion reaction. Combustion, in simple terms, is a chemical process where a substance reacts with oxygen, often producing heat and light.
This type of reaction can be observed in many day-to-day occurrences such as burning wood or gasoline. However, for amorphous boron, the chemical process is slightly different from common combustibles due to its unique properties.

• Unlike standard combustibles, amorphous boron does not produce a visible flame when burned.
• The reaction primarily occurs because of boron's affinity to bond with oxygen to form boron trioxide.

This process helps us understand why certain compounds are formed when elements like boron interact with typical atmospheric gases.

Boron Trioxide Formation

The primary product formed when amorphous boron burns in the presence of oxygen is boron trioxide. Boron trioxide is denoted by its chemical formula \( \mathrm{B}_{2}\mathrm{O}_{3} \). This compound is known for its vitreous or glass-like appearance when solidified.
The formation of boron trioxide involves a direct oxidation process. Here's what happens at the molecular level:

• Boron atoms in their amorphous form are highly reactive toward oxygen molecules.
• When oxygen molecules come in contact with amorphous boron at higher temperatures, a reaction takes place, forming \( \mathrm{B}_{2}\mathrm{O}_{3} \).

The formation of \( \mathrm{B}_{2}\mathrm{O}_{3} \) is a classic example of a synthesis reaction where smaller reactants (boron and oxygen) combine to form a complex product. Understanding this specific outcome is crucial as it explains why other potential compounds such as \((\mathrm{BN})_x\) are not the result under standard conditions.

Boron Reaction with Oxygen

Boron's reaction with oxygen is a highly significant chemical process. The affinity of boron towards oxygen is why \( \mathrm{B}_{2}\mathrm{O}_{3} \) is predominantly formed during combustion. But why does boron react so readily with oxygen?

• Oxygen, being a highly electronegative element, tends to form strong bonds with metals or metalloids like boron.
• This bond formation results in the stabilization of both elements in the compound \( \mathrm{B}_{2}\mathrm{O}_{3} \).

The reaction can be represented by the oxidation equation:
\[ 4\mathrm{B} + 3\mathrm{O}_{2} \rightarrow 2\mathrm{B}_{2}\mathrm{O}_{3} \]This balanced equation highlights the stoichiometric ratio in which boron and oxygen react to form boron trioxide. The resulting compound \( \mathrm{B}_{2}\mathrm{O}_{3} \), due to its stable structure, is generally the only product formed in significant quantities under normal atmospheric conditions.