Chapter 9.6 - Structure of Hydrogen Peroxide

In the previous section, we saw the basic details about hydrogen peroxide. In this section, we will see it's structure and chemical properties.

Structure of hydrogen peroxide

This can be written in 8 steps:
1. We know that, hydrogen peroxide has two H atoms and two O atoms.
• In the fig.9.7 below,
   ♦ the H atoms are named as ₁H and ₂H .
   ♦ the O atoms are named as ₁O and ₂O.

Fig.9.7

2. We know that H-O-O-H has three covalent bonds.
   ♦ Two O-H bonds and one O-O bond.
• In the fig.9.7 above,
   ♦ the two O-H bonds are shown in yellow color.
   ♦ the O-O bond is shown in magenta color.
3. If there are three points in space, we will be able to draw a unique plane through all those three points. The plane will be ‘unique’ because, no other plane will contain those three points.
4. Let us take ₁H, ₁O and ₂O as the three points.
• The unique plane through those three points is shown in fig.9.8 below. The plane is shown in gray color.

Fig.9.8

5. Let us take ₂H, ₁O and ₂O as another set of three points.
• The unique plane through those three points is also shown in fig.9.8 above. The plane is shown in blue color.
6. Any two non-parallel planes will intersect along a line.
• In our present case, ₁O ans ₂O are present in both the planes.
   ♦ So ₁O and ₂O will lie on the line of intersection of the two planes.
7. From fig.9.8, it is clear that H₂O₂ has a non-planar structure.
• Let us write the bond lengths and bond angles. Fig.9.8 above shows the measurements when H₂O₂ is in gas phase.   
   ♦ ₁O-₁H bond length is 95.0 pm.
   ♦ ₂O-₂H bond length will also be 95.0 pm.
   ♦ Bond angle between ₁O-₂O and ₂H-₂O is 94.8^o.
   ♦ Bond angle between ₁O-₂O and ₁H-₁O will also be 94.8^o.
   ♦ Dihedral angle (angle between two intersecting planes) is 111.5^o.
8. Fig.9.9 below shows the measurements when H₂O₂ is in solid phase.

Fig.9.8

   ♦ ₁O-₁H bond length is 98.8 pm.
   ♦ ₂O-₂H bond length will also be 98.8 pm.
   ♦ Bond angle between ₁O-₂O and ₂H-₂O is 101.9^o.
   ♦ Bond angle between ₁O-₂O and ₁H-₁O will also be 101.9^o.
   ♦ Dihedral angle is 90.2^o.

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Chemical properties of hydrogen peroxide

This can be written in 6 steps:
1. Hydrogen peroxide as an oxidizing agent:
   ♦ Hydrogen peroxide can act as an oxidizing agent in acidic medium.
   ♦ Hydrogen peroxide can act as an oxidizing agent in basic medium also.
2. Hydrogen peroxide as a reducing agent
   ♦ Hydrogen peroxide can act as a reducing agent in acidic medium.
   ♦ Hydrogen peroxide can act as a reducing agent in basic medium also.
3. Let us see an example where hydrogen peroxide acts as an oxidizing agent in acidic medium.

$\rm{2 Fe^{2+} (aq)~+~2 H^{+} (aq) ~+~H_2 O_2 (aq) ~\rightarrow~2 Fe^{3+} (aq) ~+~ 2H_2O (l)}$

• Let us write the oxidation numbers:

$\rm{2 \overset{+2}{Fe^{2+}} (aq)~+~2 \overset{+1}{H^{+}} (aq) ~+~\overset{+1}{H}_2 \overset{-1}{O}_2 (aq) ~\rightarrow~2 \overset{+3}{Fe^{3+}} (aq) ~+~ 2 \overset{+1}{H}_2 \overset{-2}{O} (l)}$

• There are three atoms involved: Fe, H and O
   ♦ Oxidation number (O.N) of Fe increases.
   ♦ O.N of H remains the same.
   ♦ O.N of O decreases.
• It is clear that, Fe²⁺ is oxidized by O.
   ♦ The O is supplied by H₂O₂.
• So we can write:
H₂O₂ acts as an oxidizing agent in this reaction.

◼ Let us see another example where hydrogen peroxide acts as an oxidizing agent in acidic medium.

$\rm{PbS (s) ~+~4 H_2 O_2 (aq) ~\rightarrow~PbSO_4 (s) ~+~ 4H_2O (l)}$

• Let us write the oxidation numbers:

$\rm{\overset{+2}{Pb} \overset{-2}{S} (s) ~+~4 \overset{+1}{H}_2 \overset{-1}{O}_2 (aq) ~\rightarrow~\overset{+2}{Pb} \overset{+6}{S} \overset{-2}{O_4} (s) ~+~ 4 \overset{+1}{H}_2 \overset{-2}{O} (l)}$

• There are three atoms involved: Pb, S, H and O
   ♦ O.N of Pb remains the same.
   ♦ O.N of S increases.
   ♦ O.N of H remains the same.
   ♦ O.N of O decreases.
• It is clear that, S is oxidized by O.
   ♦ The O is supplied by H₂O₂.
• So we can write:
H₂O₂ acts as an oxidizing agent in this reaction.

4. Let us see an example where hydrogen peroxide acts as an oxidizing agent in basic medium.

$\rm{2 Fe^{2+} ~+~H_2 O_2 ~\rightarrow~2Fe^{3+}~+~ 2OH^{-}}$

• Let us write the oxidation numbers:

$\rm{2 \overset{+2}{Fe^{2+}} ~+~\overset{+1}{H}_2 \overset{-1}{O}_2 ~\rightarrow~2 \overset{+3}{Fe^{3+}}~+~ 2 \overset{-2}{O} \overset{+1}{H^{-}}}$

• There are three atoms involved: Fe, H and O
   ♦ O.N of Fe increases.
   ♦ O.N of H remains the same.
   ♦ O.N of O decreases.
• It is clear that, Fe²⁺ is oxidized by O.
   ♦ The O is supplied by H₂O₂.
• So we can write:
H₂O₂ acts as an oxidizing agent in this reaction.

◼ Let us see another example where hydrogen peroxide acts as an oxidizing agent in basic medium.

$\rm{Mn^{2+} ~+~H_2 O_2 ~\rightarrow~Mn^{4+}~+~ 2OH^{-}}$

• Let us write the oxidation numbers:

$\rm{\overset{+2}{Mn^{2+}} ~+~\overset{+1}{H}_2 \overset{-1}{O}_2 ~\rightarrow~\overset{+4}{Mn^{4+}}~+~ 2 \overset{-2}{O} \overset{+1}{H^{-}}}$

• There are three atoms involved: Mn, H and O
   ♦ O.N of Mn increases.
   ♦ O.N of H remains the same.
   ♦ O.N of O decreases.
• It is clear that, Mn²⁺ is oxidized by O.
   ♦ The O is supplied by H₂O₂.
• So we can write:
H₂O₂ acts as an oxidizing agent in this reaction.

5. Let us see an example where hydrogen peroxide acts as a reducing agent in acidic medium.

$\rm{2 MnO^{-}_4 ~+~6 H^{+} ~+~5 H_2 O_2 ~\rightarrow~2 Mn^{2+}~+~ 8 H_2O~+~5 O_2}$

• Let us write the oxidation numbers:

$\rm{2 \overset{+7}{Mn} \overset{-2}{O}^{-}_4 ~+~6 \overset{+1}{H^{+}} ~+~5 \overset{+1}{H}_2 \overset{-1}{O}_2 ~\rightarrow~2 \overset{+2}{Mn^{2+}}~+~ 8 \overset{+1}{H}_2 \overset{-2}{O}~+~5 \overset{0}{O_2}}$

• There are three atoms involved: Mn, H and O
   ♦ O.N of Mn decreases.
   ♦ O.N of H remains the same.
   ♦ O.N of O increases.
• It is clear that, Mn⁷⁺ is reduced by O.
   ♦ The O is supplied by H₂O₂.
• So we can write:
H₂O₂ acts as a reducing agent in this reaction.

◼ Let us see another example where hydrogen peroxide acts as a reducing agent in acidic medium.

$\rm{HOCl ~+~H_2 O_2 ~\rightarrow~H_3 O^{+}~+~ Cl^{-}~+~O_2}$ 

• Let us write the oxidation numbers:

$\rm{\overset{+1}{H} \overset{-2}{O} \overset{+1}{Cl} ~+~\overset{+1}{H}_2 \overset{-1}{O}_2 ~\rightarrow~\overset{+1}{H_3} \overset{-2}{O}^{+}~+~ \overset{-1}{Cl^{-}}~+~\overset{0}{O_2}}$

• There are three atoms involved: Cl, H and O
   ♦ O.N of Cl decreases.
   ♦ O.N of H remains the same.
   ♦ O.N of O increases.
• It is clear that, Cl is reduced by O.
   ♦ The O is supplied by H₂O₂.
• So we can write:
H₂O₂ acts as a reducing agent in this reaction.

6. Let us see an example where hydrogen peroxide acts as a reducing agent in basic medium.

$\rm{I_2 ~+~H_2 O_2 ~+~2 OH^{-}~\rightarrow~2 I^{-}~+~ 2H_2O~+~O_2}$

• Let us write the oxidation numbers:

$\rm{\overset{0}{I_2} ~+~\overset{+1}{H}_2 \overset{-1}{O}_2 ~+~2  \overset{-2}{O} \overset{+1}{H^{-}}~\rightarrow~2 \overset{-1}{I^{-}}~+~ 2 \overset{+1}{H}_2 \overset{-2}{O}~+~\overset{0}{O_2}}$

• There are three atoms involved: I, H and O
   ♦ O.N of I decreases.
   ♦ O.N of H remains the same.
   ♦ O.N of O increases.
• It is clear that, I is reduced by O.
   ♦ The O is supplied by H₂O₂.
• So we can write:
H₂O₂ acts as a reducing agent in this reaction.

◼ Let us see another example where hydrogen peroxide acts as a reducing agent in basic medium.

$\rm{2 MnO^{-}_4 ~+~3 H_2 O_2 ~\rightarrow~2 MnO_2~+~3O_2 ~+~ 2 H_2O~+~2 OH^{-}}$ 

• Let us write the oxidation numbers:

$\rm{2 \overset{+7}{Mn} \overset{-2}{O}^{-}_4 ~+~3\overset{+1}{H}_2 \overset{-1}{O}_2 ~\rightarrow~2 \overset{+4}Mn \overset{-2}{O_2}~+~3\overset{0}{O_2} ~+~ 2 \overset{+1}{H}_2 \overset{-2}{O}~+~2 \overset{-2}{O} \overset{+1}{H^{-}}}$

• There are three atoms involved: Mn, H and O
   ♦ O.N of Mn decreases.
   ♦ O.N of H remains the same.
   ♦ O.N of O increases.
• It is clear that, Mn is reduced by O.
   ♦ The O is supplied by H₂O₂.
• So we can write:
H₂O₂ acts as a reducing agent in this reaction.

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In the next section, we will see storage and uses of Hydrogen peroxide.

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