bleach - a catalyst

[Chenga]

Bleach is a catalyst for the decomposition of hydrogen peroxide. I have only ever worked with solid catalysts. Can anyone explain how aquious catalysts work?

[charco]

There are two kinds of catalyst homogeneous and heterogenous - if it's in the same phase (as is bleach) it is a homogeneous catalyst. These work by providing an alternative mechanism with a lower activation energy when compared to the catalyst free reaction.

The catalyst usually chemically combines with one of the intermediate species and then is regenerated in a later step of the reaction mechanism.

Classic example is the catalytic oxidation of the tartrate ion using hydrogen peroxide with a cobalt II catalyst. The pink cobalt II turns green during the course of the reaction showing the formation of Co III ions - as the reaction finishes the CoII is regenerated and the pink colour reappears.

[Nick]

isnt the definition of a catalyst that it does NOT change during the reaction.
what you describe here is a redox - reaction.
Hydrogen Peroxide is nearly decomposed with every substance. you dont really need a catalyst for that.

aquious catalysts work pretty much the same way as any other catalyst. the only difference is the homogeneous phase instead of the heterogenous one.

[RobJim]

Not really. The definition of a catalyst includes the characteristic that at the end of the reaction, the catalyst is either still there or has been replaced by an identical molecule. The catalyst can change during the reaction as long as you end up with an identical copy of it afterwards.

[Nick]

hmmm funny. we always learned, a catalyst doesn´t change in any way, just takes part in a reaction on that way that it lowers the energylevel for bondings to break up ... or something like this.

[charco]

All catalysts take part in the reaction otherwise any effect they were to have on the reaction would be ..... 'magic'!! (a la David Copperfield)

As explained above, catalysts either act as templates facilitating reaction (which effectively means that they are part of the reaction) or provide low energy intermediates for the reaction mechanisms...

... and don't believe anything that your teachers telll you otherwise!

[RobJim]

Chemistry teachers sometimes lie to keep things simple, and sometimes they are plain wrong about the details. They are people after all.

For example, there is a reaction in which ozone changes to oxygen gas. It's catalyzed by hydroxyl radical and the mechanism is believed to be

OH + O3 --> HO2 + O2
HO2 + O3 --> OH + 2O2

Notice there is no OH in the intermediate step. It doesn't exist any more because it's reacted and become something else. However, by the end of the reaction, it's back again - it's been regenerated.

Note that it's not a good idea to point out to the teacher things like this. Sometimes they get annoyed. Sometimes they'll argue with you, and in an argument like that, you lose even if you're right and the teacher is wrong. Just accept what they tell you and spit it back out on the test.

[Pop Eye]

Hi everyone, I have one question.
Should I say: Alpha amylase decomposes amylose. (I know that water decomposes amylose, but...)
Or maybe: Alpha amylase helps (i.e. speeds up) the amylose decomposition.
I know that the latter statement is correct, but I wonder if the former is wrong.
Thank you

[RobJim]

The former statement should be correct. It's similar to saying that you cooked an egg, even though what actually cooked it was the heat from the stove.