Tag: the p-block elements - group 13

Questions Related to the p-block elements - group 13

Which of the following statements regarding orthoboric acid $(H _3BO _3)$ is false?

chemistry the p-block elements - group 13 study of orthoboric acid some important compounds of boron study of boron

  1. It acts as a weak monobasic acid.

  2. It is soluble in hot water.

  3. It has a planar structure.

  4. It acts as a tribasic acid.

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Correct Option: D
Explanation:

Boric acid, also called hydrogen borate, , is a weak, monobasic lewis acid of boron

Option D only is the wrong statement because it acts as monobasic acid not tri basic .

Hence option D is correct.

$B(OH) _3+NaOH\rightarrow Na[B(OH) _4] _{(aq)}$

Then addition of which of the following proceeds the reaction in the forward direction :

chemistry the p-block elements - group 13 study of orthoboric acid some important compounds of boron study of boron

  1. cis-1, 2 diol

  2. trans 1, 2 diol

  3. borax

  4. $Na _2HPO _4$

Show answer
Correct Option: A
Explanation:

$B(OH) _3+NaOH\rightarrow Na[B(OH) _4] _{(aq)}$

This reaction is reversible reaction because sodium metaborate, $ Na [B(OH) _4]$ formed by the reaction between $B(OH) _3$ and NaOH gets hydrolysed to regenerate $B(OH) _3$ and NaOH.

$ Na [B(OH) _4]$ $\overset {hydrolysis}\rightarrow$ $NaOH$ + $B(OH) _3$

If, some quantity of polyhydroxy compounds like cis- 1, 2-diol, catechol, glycerol etc is added to the reaction mixture then the $B(OH) _3$ combines with such polyhydroxy compounds to give chelated complex compound. Due to complex compound formation, stability increases and due to higher stability of complex, reaction moves in forward direction.

Which one of the following is/are incorrect statement(s)?

chemistry the p-block elements - group 13 study of orthoboric acid some important compounds of boron study of boron

  1. The hydroxide of aluminium is more acidic than that of boron.

  2. The hydroxide of boron is basic while that of aluminium is amphoteric.

  3. The hydroxide of boron is acidic while that of aluminium is amphoteric.

  4. The hydroxides of both boron and aluminium are amphoteric.

Show answer
Correct Option: A,B,D
Explanation:

All the elements of this group 13 form oxides and hydroxide.


Boron is the only element in group III of the periodic table which is non-metallic. Metal hydroxides must form a stable cation and anion i.e $OH^-$. Boron does not form a stable cation. This is the reason why metals form basic hydroxides while boron forms a weak acid called boric acid.

 Boron forms acidic oxides and hydroxides, while Al and Ga form amphoteric oxides and hydroxides. In and Tl forms basic oxides and hydroxides.The acidic nature of hydroxides decreases down the group.

Hence options A,B & D are correct.

Boric acid is a very weak acid but in the presence of certain organic compounds, it acts as a strong acid. Which one of the following organic compound(s) cannot affect such change?

chemistry the p-block elements - group 13 study of orthoboric acid some important compounds of boron study of boron

  1. Glycerol

  2. Acetic acid

  3. Ethyl alcohol

  4. Ethylene

Show answer
Correct Option: B,C,D
Explanation:


Option (B),(C),(D) are correct.
Boric acid forms a stable cyclic complex with polyhydroxy compounds like glycerol. This helps in the release of $H^+$ as $H _3O^+$ and therefore boric acid acts as a strong acid. This is not possible with compounds such as acetic acid, ethyl alcohol and ethylene.

$2B+3H _{2}SO _{4}\rightarrow 2H _{3}BO _{3} +A$ 


The hybridization of central atom in compound A is :

chemistry the p-block elements - group 13 study of orthoboric acid some important compounds of boron study of boron

  1. sp

  2. $sp^{2}$

  3. $sp^{3}$

  4. $sp^{3}d$

Show answer
Correct Option: B
Explanation:

The balanced chemical reaction is as given below.
$2B+3H _{2}SO _{4}\rightarrow 2H _{3}BO _{3}\rightarrow + \underset {A} {SO _2}$
S atom in sulphur dioxide undergoes $sp^2$ hybridization.

$H _{3}BO _{3}\xrightarrow{375K} A \xrightarrow{Red \, Heat}B _{2}O _{3}$
$H _{3}BO _{3}\xrightarrow{435K} B \xrightarrow{Red \, Heat}B _{2}O _{3}$


The compounds A & B are:

chemistry the p-block elements - group 13 study of orthoboric acid some important compounds of boron study of boron

  1. Orthoboric acid, metaboric acid

  2. Metaboric acid, Tetra boric acid

  3. Tetra boric acid, Metaboric acid

  4. Tetra boric acid, orthoboric acid

Show answer
Correct Option: B
Explanation:

$H _{3} BO _{3} \overset{375K}{\rightarrow} HBO _{2} \overset{red heat}{\rightarrow} B _{2} O _{3} $
$H _{3} BO _{3} \overset{435K}{\rightarrow} H _{2} B _{4} O _{7} \overset{red heat}{\rightarrow} B _{2} O _{3} $

Orthoboric acid when heated to red hot gives:

chemistry the p-block elements - group 13 study of orthoboric acid some important compounds of boron study of boron

  1. metaboric acid

  2. pyroboric acid

  3. boron and water

  4. boric anhydride

Show answer
Correct Option: D
Explanation:

$H _{3} BO _{3} \overset{375K}{\rightarrow} HBO _{2} \overset{red heat}{\rightarrow} B _{2} O _{3} $ (Boric anhydride)

Which is the correct statement for $H _{3}BO _{3}$ ?

chemistry the p-block elements - group 13 study of orthoboric acid some important compounds of boron study of boron

  1. It is used as antiseptic

  2. As preservative of foods

  3. It gives green colour to flame

  4. All are correct

Show answer
Correct Option: D
Explanation:

It is antiseptic and its water solution is used as an eyewash. It is used as a preservative.
Boron burns with green flame when heated strongly with $O _{2} $.

$BCl _{3}$ on hydrolysis gives:

chemistry the p-block elements - group 13 study of orthoboric acid some important compounds of boron study of boron

  1. $B _{2}O _{3}$ and $HCl$

  2. $B _{2}H _{6},\ HCl$ and $O _{2}$

  3. $H _{3}BO _{3}$ and $HCl$

  4. $B _{2}O _{3},\ H _{2}$ and $Cl _{2}$

Show answer
Correct Option: C
Explanation:

$BCl _3$ on hydrolysis gives $H _3BO _3$ and $HCl$.

The reaction is as follows:
$BCl _{3} + 3H _{2} O \rightarrow B(OH) _{3}  + 3HCl$

Boric acid on heating yields:

chemistry the p-block elements - group 13 study of orthoboric acid some important compounds of boron study of boron

  1. B

  2. $B _{2}H _{6}$

  3. $BO _{2}$

  4. $B _{2}O _{3}$

Show answer
Correct Option: D
Explanation:

$H _{3} BO _{3} \overset{375K}{\rightarrow} HBO _{2} \overset{red heat}{\rightarrow} B _{2} O _{3} $

Thus boric acid on heating ginves boric oxide.