what causes oxygen to decompose from hydrogen peroxide and what causes hydrogen peroxide fizzing?

Answer 1

Second part first, because you already answered that -- when hydrogen peroxide (H2O2) decomposes, one of the products is dioxygen gas (O2). So you see bubbles.

What causes hydrogen peroxide to decompose? It's thermodynamically unstable. That is, the reaction

2 H2O2 ===> 2 H2O + O2

is downhill, releasing a lot of energy. In effect, this is a disproportionation reaction, where the O atoms act as both oxidant and reductant. O in H2O2 is a –1 oxidation state, while that in water and O2 is –2 and zero, respectively. In the reaction above, two O atoms are oxidized from –1 to 0, and two are reduced from –1 to –2. Because the bonds are stronger in the products, the products are more stable than the reactants, and the decomposition reaction is favoured.

But the reaction is usually slow, kinetically. You can put H2O2 in a bottle, and it won't fall apart by itself, unless you provide a catalyst or some energy to get the reaction going. But metal ions, metal surfaces, excessive heat, even light, these will all trigger the reaction, so if you want the H2O2 to hang around, you need to store it in a plastic bottle in the dark, ideally in a fridge.

The most common use is as a disinfectant. The general idea is that the Fe ions in your blood can act as catalytic sites to trigger the decomposition reaction, so you start generating O2 (and water). The reason for the disinfectant action has nothing to do with "bleaching", or with free O atoms, but the nature of the O2 you generate. Usually O2 has two unpaired electrons (called "triplet" oxygen). Oxidation of any organic material by O2 is a favourable process (i.e. "burning"), but again it's kinetically slow, precisely because O2 is a triplet -- it's the only reason you aren't bursting into flame right now. What's cool about H2O2 decomposition is that it generates "singlet" O2, with no unpaired electrons, and that material has no barrier to the oxidation of organic matter, so in a biological system it rips apart and destroys cells. You have a lot of cells, so using H2O2 to make singlet O2 to kill a bunch of them won't hurt you, but bacteria are single-celled -- rip one cell apart and it's dead. You're using H2O2 as a convenient source of bacteria-destroying singlet O2 (which is actually a technique many of your enzymes employ as well -- lots of metalloenzymes make singlet O2 as a pathogen to kill intruding organisms).

Answer 2

H2O2 is H2O with an extra O atom, an unstable compound that is ready to give up its extra O to any substance that can form a higher-energy bond with it. However, there is competition for the monatomic O atoms; they also tend to join together and form O2. When you apply it, say to blood or a fresh bloodstain, it fizzes because some of the liberated O atoms don't actually complete the oxidation process and instead join another O atoms to form O2 which bubbles off. Meanwhile the O atoms that do engage in oxidation bleach out the blood.
Edit: The next answer is a much better explanation. But I have a question: Is the singlet O2 a gas? I'd guess not, because if it were a gas it wouldn't react much before it escaped.