Hey, how do I recognize a peroxide based on the reaction equation?

This always results in connection with the other elements which form the connection. Only a few peroxides are really common, and luckily you can recognize them immediately (e.g. Na2O2, BaO2). Generally, you’re about to do this:

1. Do I have a special reason to assume that it is a peroxide? If so, then −I, otherwise
2. Suppose it is a normal oxide, i.e. −II. Calculate the remaining oxidation numbers under this assumption. Does something implausible or perhaps even a clear contradiction arise to what is surely known? If no, we’ll stay with −II.
3. If a contradiction occurs, we need fingertip feeling and possibly additional information to balance it. In the simplest case, the acceptance of O ̄I immediately results in sensible and plausible oxidation numbers for the other atoms, then we leave it here

In practice, this may look like:

• TiO2. The naïve assumption is Ti+IVO2, which is plausible, because titanium has four outer electrodes and can therefore be four-valued (it is also in the fourth subgroup).
• BaO2. The naïve assumption is Ba+IVO2, which is not plausible at all because barium has only two external electrons and can therefore be impossible to be four-valued (it is in the second main group). But with Ba+IIO ̄I2, a shoe gets out, so it’s probably a peroxide.
• CrO5: The chromium may not be tense, so it must be a peroxide. Since the material can be produced from Cr+VIO42 ̄ solutions with H2O2, we assume that Cr+VI occurs in this compound, then four O atoms must be peroxidic and the last oxidic, Cr+VIO ̄I4O ̄II. This desperate assumption is also true (and can also be demonstrated experimentally, e.g. by an X-ray structure)
• K2S2O8: The naïve assumption K+I2S+VII2O ̄II8 does not make any sense because sulfur in the 6th. The main group is and can therefore be a maximum of six. With the adoption of S+VI it will be correct: K+I2S+VI2O ̄II6. When one comes to a structural formula of the peroxidisulfate ion, one can see it very clearly: ̄O3S– OO– SO3 ̄
• Ag2O2: That this is a silver(II) oxide is hardly to be believed because silver has the oxidation number +II as the plague. Thus, a peroxide Ag+I2O ̄I2 could be expected, but this is unfortunately incorrect, because in reality this compound contains silver in the unusual oxidation number +III: Ag+IAg+IIIO ̄II2. Although this is not to be seen in the formula, it is to be noted in the reactivity (no H2 O2 is formed in acidification, as is otherwise made from peroxides), and the crystal structure proves that two different types of silver atoms are present side by side.
• You have a similar problem with disulfides. Pyrite is, for example, FeS2, and no one with awake mind will assume that it could be a Fe+IVS II2 because tetravalent iron never occurs (Sr2Fe+IVO4 as an example). So it is probably Fe+IIS ̄I2, and the crystal structure confirms it.