Question

Which of the following statement(s) is/are not correct?
(A) All $C - O$ bonds in $CO_3^{2 - }$ are equal but not in ${H_2}C{O_3}$ .
(B) All $C - O$ bonds in $HC{O^{2 - }}$ are equal but not in $HC{O_2}H$ .
(C) $C - O$ bond length in $HCO_2^ -$ is longer than $C - O$ bond length in $CO_3^{2 - }$
(D) $C - O$ bond length in $HCO_2^ -$ and $C - O$ bond length in $CO_3^{2 - }$ are equal.

Answer 1

To determine the correct option, one must know about the bond length, bond order and the relation between the bond order and bond length. Higher will be the bond order shorter will be the bond length.

Complete step by step answer:
Bond order is defined as the number of bonding electrons present between the two atoms. In the single bond of the covalent bond between the two atoms the bond order is one, a double bond contains the bond order two, the triple bond contains a bond order three.
Bond length is defined as the distance between the centers of the two atoms bonded by a covalent bond.
Relation between bond order and bond length: The length of the bond present between the two atoms is determined by the bond order. Higher is the bond order, the stronger the pull will be between the two atoms and shorter will be the bond length.
(A) All $C - O$ bonds in $CO_3^{2 - }$ are equal but not in ${H_2}C{O_3}$ .
In ${H_2}C{O_3}$ two single bond and one double bond is present.
All $C - O$ bonds in $CO_3^{2 - }$ are equal due to similar resonating structure.
(B) All $C - O$ bonds in $HC{O^{2 - }}$ are equal but not in $HC{O_2}H$ .
All $C - O$ bonds in $HC{O^{2 - }}$ are equal due to similar resonating structure.
$HC{O_2}H$ does not have resonating structure, carbon and oxygen is bonded by one double bond and one single bond.
(C) $C - O$ bond length in $HCO_2^ -$ is longer than $C - O$ bond length in $CO_3^{2 - }$
All $C - O$ bonds in $CO_3^{2 - }$ are equal due to similar resonating structure but not in $HC{O_2}H$ due to resonance.
Bond order of carbonate is 1.33
Bond order of formate is 1.5
Higher is the bond order, shorter is the bond length
So formate has a shorter bond length as compared to carbonate.
(D) $C - O$ bond length in $HCO_2^ -$ and $C - O$ bond length in $CO_3^{2 - }$ are equal.
All $C - O$ bonds in $CO_3^{2 - }$ are equal due to similar resonating structure but not in $HC{O_2}H$ due to resonance.
Bond order of carbonate is 1.33
Bond order of formate is 1.5
Higher is the bond order, shorter is the bond length
So formate has a shorter bond length as compared to carbonate.

So, the correct answer is OptionC, D.

Note: For polyatomic molecules the bond order is determined by first drawing the Lewis structure, then count the total number of bonds then count the total number of bond groups between the individual atoms. Divide the number of bonds between the atoms by the total number of bond groups between the molecules.