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In my classwork on the decomposition reaction of hydrogen peroxide by catalase, I wrote the following equation: $$\ce{2H2O2 -> 2H2O + O2}$$ However, my teacher said that I should go further in depth and write down the (quote unquote) "mechanism" of this reaction. I'm not sure what he is talking about, can anyone help me?

EDIT: I am an IB student studying chemistry in Standard Level so the answer to the above problem should remain within the scope of the IB syllabus. Here's more information from my teacher:

“Although we show reactions in one neat equation, the majority of reactions occur in a series of steps. (…) There’s three step to that (the one above) reaction and catalase will act on one of those steps to reduce the activation energy and therefore make the reaction faster”.

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    $\begingroup$ According to the Wikipedia entry for Catalase the mechanism is not currently known however some details are, there is an interaction with Fe(III) bound to the enzyme producing a Fe(IV) species with a bound oxygen. More details here: en.wikipedia.org/wiki/Catalase $\endgroup$
    – Waylander
    Commented Feb 3, 2020 at 10:56
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    $\begingroup$ Probably the teacher would like to know whether the two O atoms of the future of O2 are coming from the same H2O2 molecule or from two different molecules. H2O2 may loose one O atom produce H2O, and the free O join another one coming from the next same situation in the next H2O2 molecule. Another mechanism would be to suppose that one H2O2 looses 2 H to produce O2, and then the two H are reacting with the the next H2O2 to produce two H2O. You may imagine a third mechanism. $\endgroup$
    – Maurice
    Commented Feb 3, 2020 at 10:59
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    $\begingroup$ See:chemistry.stackexchange.com/questions/116006/… $\endgroup$
    – Nilay Ghosh
    Commented Feb 3, 2020 at 13:45
  • $\begingroup$ @Maurice that may seem like a possible solution. My teacher also mentioned that the reaction occurs in three steps. See edit above. $\endgroup$
    – Chris28
    Commented Feb 3, 2020 at 14:22
  • $\begingroup$ Related: Mechanism for Hydrogen Peroxide decomposition? $\endgroup$
    – user7951
    Commented Feb 3, 2020 at 20:40

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Below is one way to write it that shows what the enzyme does and avoids the uncertainty of the details:

$$\ce{E + H2O2 -> E-O + H2O}\tag{1}$$ $$\ce{E-O + H2O2 -> E-O2 + H2O}\tag{2}$$ $$\ce{E-O2 -> E + O2}\tag{3}$$

In words, the enzyme binds to the first molecule of hydrogen peroxide, release one molecule of water and holds on to the other oxygen atom (via the iron bound to heme bound to the enzyme). Then, the second molecule of hydrogen peroxide binds to the enzyme, which again release water and holds on to the second oxygen atom. Finally, the enzyme releases the two oxygen atoms in the form of the dioxygen molecule.

Alternatively, you could write the following three steps, giving even less information about what is happening in the enzyme:

$$\ce{E + 2H2O2 -> [E\cdot H2O2\cdot H2O2]}\tag{A}$$ $$\ce{[E\cdot H2O2\cdot H2O2]-> [E\cdot H2O\cdot H2O\cdot O2]}\tag{B}$$ $$\ce{[E\cdot H2O\cdot H2O\cdot O2] -> E + 2H2O + 2O2}\tag{C}$$

In words, the enzyme first binds to the substrates, then catalyzes the reaction, and finally releases the products. While this in general might be a good way of describing the action of an enzyme, it is not quite correct for catalase, which releases one product before the second substrate binds. However, this might be the answer you are looking for at the IB level. I would ask the teacher for further clarification.

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