Question

(a) Why is hydrated barium peroxide used in the preparation of hydrogen peroxide instead of anhydrous barium peroxide ?
(b) Phosphoric acid is preferred to sulphuric acid in the preparation of `H_(2)O_(2)` from barium peroxide. Explain.
(c) Hydrogen peroxide acts both as an oxidising agent as well as reducing agent in alkaline solution towards certain first row transition metal ions. Illustrate both these properties of `H_(2)O_(2)` using chemical equatons.

Answer 1

(a) If anhydrous barium perioxide is used in the preparation of `H_(2)O_(2)` , the `BaSO_(4)` formed during the reaction forms an insoluble protective coating on the surface of solid barium peroxide . This prevents
`BaO_(2) (s) + H_(2)SO_(4)(aq) to BaSO_(4) (s) + H_(2)O_(2) (aq)`
further action of the acid and utlimately the reaction stops. If, however, hydrated barium peroxide (in the form of thin paste) is used, the water of crystallization does not allow `BaSO_(4)` to deposit on the surface of `BaO_(2)` and the reaction goes to completion.
(b) The aqueous solution of `H_(2)O_(2)` prepared by the action of dil. `H_(2)SO_(4)` on hydrate `BaO_(2)` has impurities of heavy metal ions like `Ba^(2+),Pb^(2+), ` etc. These catalyse the decomposition of `H_(2)O_(2)` . Therefore, `H_(2)O_(2)` prepared by the action of dil. `H_(2)SO_(4)` on hydrated `BaO_(2)` does not have good keeping qualities . If, however, phosphoric acid is used, the impurities of heavy metal ioins are precipitated as insoluble phosphates. As a result, the resulting solution of `H_(2)O_(2)` has good keeping qualities.
(c) Oxidising agent : `2 Cr(OH)_(3) + 4NaOH + 3H_(2)O_(2) to 2Na_(2)CrO_(4) + 8H_(2)O`
Here, `Cr^(3+)` gets oxidised to `Cr^(6+)`
Reducing agent : `2K_(3)[Fe(CN)_(6)]+2KOH + H_(2)O_(2) to 2K_(4)[Fe(CN)_(6)]+2H_(2)O+O_(2)`
Here, `Fe^(3+)` gets reduced to `Fe^(2+)`.