The boiling point of hydrogen peroxide is 151°C. It decomposes on heating, decomposes rapidly at boiling point. Therefore, it is distilled under low pressure. Its boiling point is 68°C at 26 mm pressure. At a pressure lower than this, its boiling point is even lower.
Its dilute solution is distilled at about 26 mm of pressure to concentrate hydrogen peroxide. This pressure can be easily achieved with the help of a water pump. Hydrogen peroxide of about 50% concentration can be obtained by this method.
Hydrogen peroxide is distilled at about 14 mm of pressure for higher concentrations. Hydrogen peroxide of about 90% concentration is obtained by this method.
Hydrogen peroxide of 99% concentration is obtained by drying it in a vacuum desiccator at 90% concentration of concentrated H2SO4. To remove the final amount of water, H2O2 is cooled by placing it in a freezing mixture of solid CO2 and ether, from which it separates into a solid, which on heating gives Hydrogen peroxide of 100% concentration.
Decomposition: It is a temporary substance. In the presence of sunlight, heat, rough surface etc., it decomposes into water and oxygen.
2H2O2 → 2H2O + O2
Oxidizing agent: It accepts and discards electron. Hence it acts as both an oxidising agent and a reducing agent. The oxidation number of O in H2O2 is -1. The permanent oxidation numbers of O are 0 and -2.
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Therefore, the oxidation number of O in H2O2 may increase or decrease in chemical reactions. Therefore, it can act as both an oxidising agent and a reducing agent. When the oxidation number of O in H2O2 becomes -1 and -2, it acts as an oxidising agent.
In its oxidising reactions, it provides nascent oxygen according to the following half reaction –
H2O2 → H2O + O2
Its major oxidising reactions are as follows –
1) Iodine is released from potassium iodide
H2O2 → H2O + O2
2KI + H2O + O2 → 2KOH + I2
2KI + H2O2 → 2KOH + I2
2) Oxidizes black colored lead sulfide to white colored lead sulfate.
[H2O2 → H2O + O2] x 4
PbS + 4O → PbSO4
PbS + 4H2O2 → PbSO4 + 4H2O
3) oxidizes ferrous sulfate [FeSO4] to ferric sulfate [Fe2(SO4)3] in the presence of dilute H2SO4.
2FeSO4 + H2SO4 + H2O2 → Fe2(SO4)3 + 2H2O
4) Oxidizes sulfurus and arsenius acid to sulfuric and orcenic acid.
H2SO3 + H2O2 → H2SO4 + H2O
H3AsO3 + H2O2 → H3AsO4 + H2O
5) It oxidises nitrites to nitrate, sulfites to sulfate and orsanits to orsanate.
KNO2 + H2O2 → KNO3 + H2O
K2SO3 + H2O2 → K2SO4 + H2O
Na3AsO3 + H2O2 → Na3AsO4 + H2O
6) oxidizes acidic potassium ferocinide to potassium fericinide.
2K4Fe(CN)6 + H2O2 + H2SO4 → 2K3Fe(CN)6 + K2SO4 + 2H2O
7) In the presence of ferrous sulphate, it converts benzene to phenol.
C6H6 + H2O2 → C6H5OH + H2O
8) Converts formaldyhide to pharmic acid in basic solution.
2HCHO + H2O2 → 2HCOOH + H2
9) H2S gives out S.
H2O2 + H2S → 2H2O + S
Reducing agent: It reacts with any oxidising substance to form a sub-oxygen. In this process the oxidising number of O in H2O2 is reduced from -1 to 0. Hence it is also a waste. Its main reducing reactions are as follows –
It reacts with ozone to form oxygen.
O3 + H2O2 → 2O2 + H2O
In this reaction H2O2 is oxidised to O2 and H2O2 is reduced to H2O. Thus, in this reaction, H2O2 itself acts as both an oxidising agent and a reducing agent.
It reduces chlorine to HCl.
H2O2 + Cl2 → 2HCl + O2
It reduces KMnO4 to magnus salt in acidic medium.
5H2O2 + 2KMnO4 + 3H2SO4 → K2SO4 + 2MnSO4 + 8H2O + 5O2
In neutral medium it converts KMnO4 to MnO2 and the solution becomes alkaline.
H2O2 + 2KMnO4 → KOH + 2MnO2 + 2O2
It reduces the oxides of silver and mercury to silver and mercury.
Ag2O + H2O2 → 2Ag + H2O + O2
HgO + H2O2 → Hg + H2O + O2
It reduces lead dioxide to lead mono oxide.
PbO2 + H2O2 → PbO + H2O + O2
Reduces MnO2 to magnus salt in acidic solution.
MnO2 + H2SO4 + H2O2 → MnSO4 + O2 + 2H2O
Reduces basic potassium fericinide to potassium ferocinide.
2K3Fe(CN)6 + 2KOH + H2O2 → 2K4Fe(CN)6 + 2H2O + O2
Sodium removes oxygen from hipochlorite.
H2O2 + NaOCl → NaCl + H2O + O2
Reaction with potassium dicromate: The reaction of potassium dicromate and hydrogen peroxide in etheric solution in the presence of concentrated sulfuric acid results in the formation of blue percromate (CrO5).
K2Cr2O7 + H2SO4 + 4H2O2 → 2CrO5 + 5H2O + K2SO4
Blue percromate (CrO5) has two peroxide bonds (- O – O -) and its structure formula is as follows –
In both K2Cr2O7 and CrO5, the oxidation number of Cr is +6. Hence the above reaction is not a radox reaction. In the above reaction, K2Cr2O7 acts as an oxidising agent and H2O2 does not act as an oxidising agent or a reducing agent.
CrO5 is soluble in ether and is stable in ether solution. It is stable in aqueous solution and decomposes according to the following equation:
4CrO5 → 2Cr2O3 + 7O2
Acidic properties: Hydrogen peroxide reacts with bases to form salts. Hence it has acidic property.
2NaOH + H2O2 → Na2O2 + 2H2O
Na2CO3 + H2O2 → Na2O2 + H2O +CO2
Ca(OH)2 + H2O2 → CaO2 + 2H2O
Ba(OH)2 + H2O2 → BaO2 + 2H2O
Both its hydrogen are displaced. Hence it is a bi-phasic acid.
Bleaching property: It is a good oxidizer. Therefore, it spoils the color of many materials, such as silk, hair, etc. Its bleaching property is due to this reaction –
H2O2 → H2O + O