Why is hydrated barium peroxide used in the preparation of hydrogen peroxide instead of anhydrous barium peroxide ?

Explanation

If anhydrous barium peroxide is used in the preparation of `H_2O_2`, the barium sulphate formed during the reaction forms an insoluble protective coating on the surface of solid barium peroxide.
`BaO_2 (s) + H_2 SO_4(aq) to underset("Insoluble")(BaSO_4)(s) + H_2 O_2 (aq)`
This prevents further action of acid and therefore, stops the reaction. If, however, hydrated barium peroxide (in the form of thin paste) is used, the water of crystallisation does not allow `BaSO_4` to deposit on the surface of `BaO_2` and the reaction goes to completion.