Hydrogen Peroxide - Practice Questions & MCQ

Hydrogen peroxide is an important chemical used in pollution control treatment of domestic and industrial effluents.
It can be prepared by the following methods.

• Acidifying barium peroxide and removing excess water by evaporation under reduced pressure gives hydrogen peroxide.
  
• Peroxodisulphate, obtained by electrolytic oxidation of acidified sulphate solutions at high current density, on hydrolysis yields hydrogen peroxide.
   
• Industrially it is prepared by the autooxidation of 2-alklylanthraquinols.
  
  In this case 1% H₂O₂ is formed. It is extracted with water and concentrated to ~30% (by mass) by distillation under reduced pressure. It can be further concentrated to ~85% by careful distillation under low pressure. The remaining water can be frozen out to obtain pure H₂O₂.

Various physical properties of hydrogen peroxide are as follows:

• Pure anhydrous hydrogen peroxide is a syrupy liquid. It is colourless but gives a bluish tinge in thick layers. It is odourless.
• Its specific gravity is 1.45 at 0^oC.
• It is soluble in water, alcohol and ether.
• It has a bitter taste. It is injurious to the skin.
• It boils at 152^oC and freezes at -0.89^oC. It begins to decompose at boiling point, however, it can be distilled under reduced pressure.
• It is an associated liquid due to hydrogen bonding.
• The dipole moment of H₂O₂ is little more than that of H₂O.

It acts as an oxidising as well as reducing agent in both acidic and alkaline media. Simple reactions are described below.

• Stability: It is unstable in nature. It decomposes on standing and heating. It is an example of auto oxidation-reduction reaction.
  
• Acidic nature: The pure liquid has weak acidic nature but its aqueous solution is neutral towards litmus. It reacts with alkalies and carbonates to give their corresponding peroxides.
  
• Oxidising nature: It is a powerful oxidising agent. It is an electron acceptor in acidic and alkaline solutions.
  
  
• Reducing nature: H₂O₂ can also act as a reducing agent towards powerful oxidising agents.
  
  In alkaline medium, its reducing agent is more effective.
  
• H₂O₂ decomposes slowly on exposure to light. ,
   
• In the presence of metal surfaces or traces of alkali (present in glass containers), the above reaction is catalysed. It is, therefore, stored in wax-lined glass or plastic vessels in dark. Urea can be added as a stabiliser.
• It is kept away from dust because dust can induce explosive decomposition of the compound.

The vapour density as determined by Victor Meyer method at 90^oC is 17. Hence, the molecular mass of H₂O₂ is 34. This value has been confirmed by depression of the freezing point method also. Thenard found that 17 parts by mass of hydrogen peroxide give 9 parts by mass of water and 8 parts by mass of oxygen. Thus the empirical formula is OH and the molecular is H₂O₂.
The calculated value of the single bond O-O distance is 1.48 and X-ray measurements do show that in hydrogen peroxide, O-O bond distance is 1.460.03. The value of the dipole moment of H₂O₂ is 2.1D. This suggests that all four atoms do not lie in the same plane. The molecule can be pictured as lying on the spine of a book open to an angle of 94^o. The hydrogen atoms are present one on each cover and H-O bonds making angles of 97^o with the O-O bond as shown in the figure given below

Structure of H₂O₂ is slightly different in gas phase and crystalline phase.