Question

Using the definitions for 'oxidation' and for 'reduction', in each of the following redox reactions identify the exact species that is getting oxidized and species that is getting reduced. And, indicate the total number of electrons transferred in the process:
Total e— transferred

a. 4Cr (s) + 3O2 (l)  2Cr2O 3 (s)



b. 6H+(aq) + 2Al (s) --> 2Al3+ (aq) + 3H2(g)

Answer 1

If something is oxidized, it means it has lost electrons.
If something is reduced, it means it has gained electrons.

In these reactions, it's easiest to compare the oxidation states of the products and reactants to determine what is oxidized and what is reduced.

For the first reaction, the oxidation state of Cr is 0, and the oxidation state of O in O2 is also 0 because both compounds are neutral. In Cr2O3, however, the oxidation state of Cr is +3, while the oxidation state of O is -2 (2*3 + 3*(-2) = 0). This means Cr has been oxidized (it has become more positive, so it has lost electrons), and O has been reduced (it has become more negative, so it has gained electrons).

Electrons have been transferred from Cr to O, so we just need to look at one of these elements to figure out the total number transferred. We have 2 Cr atoms in Cr2O3, and each has lost 3 electrons, meaning 6 electrons have been transferred. However, notice that in the equation, you actually have 2 molecules of Cr2O3, meaning that twice as many electrons have been transferred. For this specific equation, 12 electrons have been transferred.

Now just do the same for the second equation! Hope that helps!