Note 19 — Chapter IV · OZONE AND HYDROGEN PEROXIDE—DALTON'S LAW

The formation of hydrogen peroxide from barium peroxide by a method of double decomposition is an instance of a number of indirect methods of preparation. A substance A does not combine with B, but A B is obtained from A C in its action on B D (see Introduction) when C D is formed. Water does not combine with oxygen, but as a hydrate of acids it acts on the compound of oxygen with barium oxide, because this oxide gives a salt with an acid anhydride; or, what is the same thing, hydrogen with oxygen does not directly form hydrogen peroxide, but when combined with a haloid (for example, chlorine), under the action of barium peroxide, BaO2, it leads to the formation of a salt of barium and H2O2. It is to be remarked that the passage of barium oxide, BaO, into the peroxide, BaO2, is accompanied by the evolution of 12,100 heat units per 16 parts of oxygen by weight combined, and the passage of H2O into the peroxide H2O2 does not proceed directly, because it would be accompanied by the absorption of 22,000 units of heat by 16 parts by weight of oxygen combined. Barium peroxide, in acting on an acid, evidently evolves less heat than the oxide, and it is this difference of heat that is absorbed in the hydrogen peroxide. Its energy is obtained from that evolved in the formation of the salt of barium.