Question

What happens when (a) Borax is heated strongly, (b) Boric acid is added to water, (c) Aluminium is treated with dilute NaOH, (d) \(\mathrm{BF}_{3}\) is reacted with ammonia?

Short answer

(a) Forms boric oxide; (b) Forms borate ions; (c) Forms sodium aluminate and hydrogen; (d) Forms a boron-nitrogen adduct.

Step-by-step solution

Borax Heating Reaction

Upon strong heating, borax ( ext{Na}_2 ext{B}_4 ext{O}_7 ext{·}10 ext{H}_2 ext{O}) loses its water of crystallization and subsequently converts into a glass-like material known as boric oxide ( ext{B}_2 ext{O}_3). The reaction is as follows:\[ ext{Na}_2 ext{B}_4 ext{O}_7 ext{·}10 ext{H}_2 ext{O} ightarrow ext{Na}_2 ext{B}_4 ext{O}_7 + 10 ext{H}_2 ext{O} ightarrow 2 ext{NaBO}_2 + ext{B}_2 ext{O}_3\]

Boric Acid Dissolution

When boric acid (H₃BO₃) is added to water, it does not dissociate as a typical acid but rather interacts with water to form a weak monobasic acid. It reacts with water to produce borate ions and hydronium ions:\[ ext{H}_3 ext{BO}_3 + 2 ext{H}_2 ext{O} ightleftharpoons ext{B}( ext{OH})_4^- + ext{H}_3 ext{O}^+\]

Aluminium Reacting with NaOH

Aluminium reacts with dilute NaOH solution to form sodium aluminate and hydrogen gas is evolved in the process. The balanced chemical equation is:\[2 ext{Al} + 2 ext{NaOH} + 6 ext{H}_2 ext{O} ightarrow 2 ext{NaAl(OH)}_4 + 3 ext{H}_2\]

Reaction of BF3 with Ammonia

Boron trifluoride ( ext{BF}_3), a Lewis acid, reacts with ammonia ( ext{NH}_3), a Lewis base, to form an adduct. The product is a boron-nitrogen complex, ext{F}_3 ext{B}· ext{NH}_3, where ammonia donates its lone pair to BF₃:\[ ext{BF}_3 + ext{NH}_3 ightarrow ext{F}_3 ext{B}· ext{NH}_3\]

Key concepts

borax heating

When borax, which has the chemical formula \( \text{Na}_2 \text{B}_4 \text{O}_7 \cdot 10 \text{H}_2 \text{O} \), is heated strongly, it undergoes a two-stage decomposition process. First, it loses its water of crystallization. This means the 10 molecules of water attached to each borax molecule are driven off as vapor. This can be thought of as similar to drying or dehydrating a substance.

Following this, the anhydrous borax that remains is further heated and breaks down into sodium metaborate (\( \text{NaBO}_2 \)) and boric oxide (\( \text{B}_2 \text{O}_3 \)), a glassy material. This glassy form is used in applications like soldering and creating ceramics because it melts at a higher temperature without crystallizing.

Understanding the reaction:

• The first step: Borax loses its water molecules to become anhydrous borax.
• The second step: Anhydrous borax breaks into smaller compounds - sodium metaborate and boric oxide (\( \text{B}_2 \text{O}_3 \)).
• Boric oxide forms an amorphous, non-crystalline glassy solid.

boric acid dissolution

Boric acid (\( \text{H}_3 \text{BO}_3 \)) exhibits interesting behavior when it is introduced to water. Unlike strong acids, which immediately release hydrogen ions \( (\text{H}^+) \), boric acid acts differently. It interacts with water molecules to form borate ions, specifically the tetrahydroxyborate ion \( (\text{B(OH)}_4^-) \), and hydronium ions \( (\text{H}_3 \text{O}^+) \).

This reaction does not involve a straightforward dissociation of hydrogen ions as typical in other acids but involves a chemical equilibrium process.

To understand this process:

• Boric acid comes into contact with water and doesn't immediately dissociate like a strong acid.
• Instead, it accepts hydroxide ions from water, forming \( \text{B(OH)}_4^- \) and releases \( \text{H}_3 \text{O}^+ \).
• This makes boric acid a weak, monobasic acid because it forms only one type of borate ion.

aluminium reaction with NaOH

When aluminium comes into contact with a dilute solution of sodium hydroxide \( (\text{NaOH}) \), a fascinating chemical transformation takes place. The aluminium reacts with the sodium hydroxide and water, producing sodium aluminate \( (\text{NaAl(OH)}_4) \) and releasing hydrogen gas \( (\text{H}_2) \).

This reaction is notable because it highlights the amphoteric nature of aluminium, meaning it can react with both acids and bases. In this particular reaction, a base (NaOH) is involved.Key points about the reaction:

• Two aluminium atoms react with two NaOH molecules.
• Water is essential in this reaction as it supplies the additional hydroxide ions needed.
• The reaction results in sodium aluminate formation and bubbles of hydrogen gas, which are visible during the reaction.
• This bubbling indicates hydrogen gas being produced, which can be tested with a lit splint producing a 'pop' sound due to hydrogen's flammability.

BF3 and ammonia reaction

The reaction between boron trifluoride \( (\text{BF}_3) \) and ammonia \( (\text{NH}_3) \) is a classic example of a Lewis acid-base reaction. Boron trifluoride is considered a Lewis acid because it can accept an electron pair, while ammonia is a Lewis base due to its ability to donate an electron pair.

In this reaction, ammonia donates its lone pair of electrons to the boron atom in \( \text{BF}_3 \), resulting in the formation of a coordination complex known as \( \text{F}_3 \text{B} \cdot \text{NH}_3 \).Understanding the process:

• Boron in \( \text{BF}_3 \) is electron-deficient and seeks electrons to complete its octet.
• Ammonia has a lone pair of electrons it is willing to share.
• The reaction forms a stable adduct by sharing electrons, creating \( \text{F}_3 \text{B} \cdot \text{NH}_3 \).
• This adduct involves a coordinate covalent bond, which is a type of bonding where both electrons in the bond come from the same atom (here, from ammonia).