When molecular hydrogen (H2) and oxygen (O2) are combined and allowed to react together, energy is released and the molecules of hydrogen and oxygen can combine to form either water or hydrogen peroxide. These two processes are represented by the two chemical equations shown at right. Chemists use redox half-reactions to describe thermodynamic processes like the ones embodied by such equations. For both of the reactions shown, the hydrogen molecules are oxidized and the oxygen atoms are reduced. Accordingly, each of the reactions below is described by a combination of two half-reactions--one corresponding to a chemical oxidation and another corresponding to a reduction.