Hydrogen peroxide foams in presence of blood, this reaction is a redox. But what is "redox"?
The reduction-oxidation is a chemical reaction that can be sum up as a transfer of electron beetween two chemical species. Those can either be atoms, ions or molecules. The reason why electrons are moving from species to other is because they are mobile, lightweight and ubiquitous in all forms of matter. There are two half-equations: an oxidation and a reduction, both part of the overall equation, the reduction-oxidation. Each of them are representative of a "couple" composed by:
- A reductant, the species which loses electrons.
- An oxidant, the species which gains electrons.
But in this case, two things differ from this explanation:
- The fact that hydrogen peroxide reacts with itself.
- The presence of blood and its impact on the reaction.
Indeed, if you let hydrogen peroxide a certain time, the solution will dismutate and form oxygen and water. This is because hydrogen peroxide acts as the reductant and the oxidant at the same time. If we take a closer look to the hydrogen peroxide redox, we can, once again, observe two half-equations, one for each couple.
- The first one is represented by:
H2O2(aq) -> O2(g)
This half-equation explains hydrogen peroxide is (slowly) turning into oxygen. To understand why, we need to balance the equation:
- The oxygens are already balanced.
- To balance the hydrogens, 2H+ are added to the products (O2).
- 2 electrons are added to balance the equation, which yields:
H2O2 -> O2 + 2H+ + 2è
This actually means H2O2 lost 2H+, thus 2 electrons, to form the O2 molecule. It's a loss of a electrons: the oxidation (where O2 is the oxidant)
The second half-equation represent the hydrogen peroxide transforming into water:
H2O2(aq) -> H2O(aq)
The equation need to be balanced:
- To balance the oxygens, 2 goes in front of H2O.
- 2H+ are added to the reactants (H2O2).
- 2 electrons are added to balance the equation:
H2O2 + 2H+ + 2è -> 2H2O
In this equation, H2O2 gains 2H+, thus 2 electrons, to become the H2O molecule. This is a gain of electrons: the reduction (where H2O is the reductant).
In conclusion we can see H2O2 acts both as a reductant (the species that loses electrons), in the first half-equation, and as an oxidant (the species that gains electrons) in the second half-equation. That's why the hydrogen peroxide is said and indeed reacts with itself, the (balanced and simplified) overall reaction being:
H2O2(aq) + H2O2(aq)
The sign (aq) meaning aqueous, (g) for gas and (l) for liquid ((s) for solid).
The molecule H2O2 loses and gains 2 electrons and 2H+, canceling each other out.
The hydrogen peroxide is said to disproportionate and produces oxygen and water.
But in fact, this disproportionation takes a very long time. And this is where the blood intervenes:
In this chemical reaction, blood is a catalysis. A catalysis is a compound able to speed a chemical reaction. It's involved in the reaction but is not listed in the reactants or the products. It is generaly written up the reaction's arrow symbol.
Here, the catalysis is actually iron. More precisaly iron atoms, contained in heme: a chemical compound present in hemoglobin.
When it is in contact with blood, hydrogen peroxide's reaction is immediate. And because the oxygen is separating from the water this quickely, the solution foams.