why H2O2 decomposes to H2O and O2 when we put it in a cup with dish soap?

Answer 1

Well, it isn't really the soap that causes the decomposition of H2O2, but the soap is put in there so you can see a rather dramatic visual. It looks like a toothpaste snake, or a volcano of foam. But the decomposition of H2O2 usually requires a catalyst such as iodide ion (I^-), otherwise the reaction is too slow and takes a long time.

The decomposition reaction is: 2 H₂O₂ (aq)   ==> 2 H₂O (l)  +  O₂ (g)   +  heat

The decomposition of hydrogen peroxide in the presence of iodide ion occurs in two steps:

H₂O₂ (aq)  +   I- (aq)   ==>   H₂O (l)  +  OI^- (aq)

H₂O₂ (aq)  +  OI- (aq)   ==>   H₂O (l)  +  O₂ (g)  +  I^- (aq)

Other catalysts are yeast, catalase, etc.