Q10.73CP - Chemistry: Molecular Nature Of Matter And Change

Question

Like several other bonds, carbon-oxygen bonds have lengths and strengths that depend on the bond order. Draw Lewis structures for the following species, and arrange them in order of increasing carbon-oxygen bond length and then by increasing carbon-oxygen bond strength:

1. $\mathbf{CO}$
   
2. ${\mathbf{CO}}_3^{2-}$
   
3. ${\mathbf{H}}_2\mathbf{CO}$
   
4. ${\mathbf{H}}_4{\mathbf{CO}}_{}^{}$
   
5.  ${\mathbf{HCO}}_3^{-}$(H attached to O).

Step-by-Step Solution

Answer

a) The order of increasing carbon-oxygen bond length. 

     ${\text{CO < H}}_{\text{2}}{\text{CO < CO}}_{\text{3}}^{\text{2-}}{\text{ < HCO}}_{\text{3}}^{\text{-}}{\text{ < H}}_{\text{4}}\text{CO}$

 

b) The order of increasing carbon-oxygen bond strength.

      ${\text{H}}_{\text{4}}{\text{CO < HCO}}_{\text{3}}^{\text{-}}{\text{ < CO}}_{\text{3}}^{\text{2-}}{\text{ < H}}_{\text{2}}\text{CO < CO}$

Step 1: Definition

Bond order shows the number of bonds that is present between any two atoms in a molecule. 

Bond length is the average distance present between the nuclei of two atoms in a molecule.

Bond strength is the strength that helps in holding two atoms together with the chemical bond.

Step 2: Subpart (a) The Lewis structure of CO.

Ten valence electrons make up the Lewis structure of CO. In order to satisfy the octet-rule of each atom, a triple bond is formed between the carbon and oxygen atom.

Since the triple bond is present in the CO molecule, the bond order will be 3 and a triple bond is a very strong bond, it increases the bond strength of the molecule.

Bond length decreases with an increase in bond strength.

Step 3: Subpart (b) The Lewis structure of CO 3 2 &#8722; .

The Lewis structure of  ${{\text{CO}}_{\text{3}}}^{\text{2-}}$ has 24 valence electrons. For ${{\text{CO}}_{\text{3}}}^{\text{2-}}$, three different resonance configurations are feasible. In  ${{\text{CO}}_{\text{3}}}^{\text{2-}}$there are two oxygen atoms which has -1 charge on each of them. In ${{\text{CO}}_{\text{3}}}^{\text{2-}}$carbon is attached with two oxygens by single bonds and with one oxygen by a double bond. In the resonance structures of carbonate, it is observed that bonds are not fixed in carbonate ion and show resonance therefore all C-O bonds are equal in length.

 

In carbonate ion four bonding pairs are present between C-O thus bond order $\mathbf{=}\frac{4}{3}\mathbf{=}\mathbf{1}\mathbf{.}\mathbf{33}$

Step 4: Subpart (c) The Lewis structure of H 2 CO .

This molecule is formaldehyde. In this molecule central carbon atom is attached with two hydrogen atoms through single bonds and one oxygen atom through double bond. It has two lone pair of electrons on oxygen atom. It contains 12 valence electrons.

Bond order = 2

In formaldehyde molecule there is two equivalents $\mathbf{C}\mathbf{-}\mathbf{H}$ bond length and one $\mathbf{C}\mathbf{=}\mathbf{O}$ bond length.

Step 5: Subpart (d) The Lewis structure of H 4 CO .

This molecule is methanol. Three C-H bonds, one C-O bond, and one O-H bond are present. The oxygen atom has two isolated pairs. There are total of 14 valence electrons in valence shell in molecule.

 ${\mathrm{H}}_3\mathrm{C}-\mathrm{OH}$

 

There is only single bond present between C-O so the bond order will be 1.

Step 6: Subpart (e) The Lewis structure of HCO 3 -

This is a bicarbonate ion. It contains one carbon atom, three oxygen atom, and one hydrogen atom. It has one C=O bond, two C-o bonds and one O-H bond.  There are 24 valence electrons present in bicarbonate ion.

 

Here, four bonding pairs between carbon and three oxygen atoms are present.

So, the bond order  $\mathbf{=}\frac{4}{3}\mathbf{=}\mathbf{1}\mathbf{.}\mathbf{33}$.

Step 7: The order of increasing carbon-oxygen bond length.

${\text{CO < H}}_{\text{2}}{\text{CO < CO}}_{\text{3}}^{\text{2-}}{\text{ < HCO}}_{\text{3}}^{\text{-}}{\text{ < CH}}_{\text{4}}\text{O}$

As the bond order has an inverse relationship with bond length. Therefore, the bond length in the given molecules increases as shown above.

$\text{Bond Length = }\frac{1}{\text{Bond Order}}$

Here, CO has the smallest bond length and maximum strength. The bond in methanol is the longest and therefore weakest.

 

Step 8: The order of increasing carbon-oxygen bond strength.

${\text{H}}_{\text{4}}{\text{CO < HCO}}_{\text{3}}^{\text{-}}{\text{ < CO}}_{\text{3}}^{\text{2-}}{\text{ < H}}_{\text{2}}\text{CO < CO}$

Bond strength is directly proportional to bond order so the order of increasing bond strength is similar to what shown above.

 

$\text{Bond Strength }\propto \text{ Bond Order}$

 

Here, CO has the maximum strength because of the three bonds present in between .