Hydrogen Peroxide - Practice Questions & MCQ

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Chemical Properties of H2O2 is considered one the most difficult concept.
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Hydrogen peroxide acts both as an oxidising and as a reducing agent depending upon the nature of the reacting species. In which of the following cases H₂O₂ acts as a reducing agent in acid medium ?

$MnO{_{4}}^{-}$

$Cr_{2}O_{7}^{2-}$

$SO_{3}^{2-}$

$KI$

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Hydrogen Peroxide(H2O2) - Methods of Preparation

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.
$\mathrm{BaO}_{2} \cdot 8 \mathrm{H}_{2} \mathrm{O}(\mathrm{s})+\mathrm{H}_{2} \mathrm{SO}_{4}(\mathrm{aq}) \rightarrow \mathrm{BaSO}_{4}(\mathrm{s})+\mathrm{H}_{2} \mathrm{O}_{2}(\mathrm{aq})+8 \mathrm{H}_{2} \mathrm{O}(1)$
Peroxodisulphate, obtained by electrolytic oxidation of acidified sulphate solutions at high current density, on hydrolysis yields hydrogen peroxide.
$\\2\mathrm{HSO}_{4}^{-}(\mathrm{aq})\overset{\mathrm{Electrolysis}}{\longrightarrow}\mathrm{HO}_{3} \mathrm{SOOSO}_{3} \mathrm{H}(\mathrm{aq})\overset{\mathrm{Hydrolysis}}{\longrightarrow}2 \mathrm{HSO}_{4}^{-}(\mathrm{aq})+2 \mathrm{H}^{+}(\mathrm{aq})\: +$ $\mathrm{H}_{2} \mathrm{O}_{2}(\mathrm{aq})$
Industrially it is prepared by the autooxidation of 2-alklylanthraquinols.
$\mathrm{2-ethylanthraquinol\: \rightleftharpoons \: H_{2}O_{2}\: +\: (Oxidised\: product)}$
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.

Physical Properties of 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.

Chemical Properties of H2O2

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.
$\mathrm{2H_{2}O_{2}\: \rightleftharpoons \: 2H_{2}O\: +\: O_{2}}$
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.
$\mathrm{H_{2}O_{2}\: +\: 2NaOH\: \rightarrow \: Na_{2}O_{2}\: +\: 2H_{2}O}$
Oxidising nature: It is a powerful oxidising agent. It is an electron acceptor in acidic and alkaline solutions.
$\mathrm{H_{2}O_{2}\: +\: 2H^{+}\: +\: 2e^{-}\: \rightarrow \: 2H_{2}O \quad\quad\quad\quad\quad(In \: acidic \: medium)}$
$\mathrm{H_{2}O_{2}\: +\: 2e^{-}\: \rightarrow \: 2OH^{-} \quad\quad\quad\quad\quad(In \: alkaline \: medium)}$
Reducing nature: H₂O₂ can also act as a reducing agent towards powerful oxidising agents.
$\mathrm{H_{2}O_{2}\: \rightarrow \: 2H^{+}\: +\: O_{2}\: +\: 2e^{-}}$
In alkaline medium, its reducing agent is more effective.
$\mathrm{H_{2}O_{2}\: +\: 2OH^{-}\rightarrow \: 2H_{2}O\: +\: O_{2}\: +\: 2e^{-}}$

Structure of H2O2

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 $\mathrm{\AA}$ and X-ray measurements do show that in hydrogen peroxide, O-O bond distance is 1.46 $\mathrm{\pm}$ 0.03 $\mathrm{\AA}$ . 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