Which of the following half-reactions is an oxidation?
a
AuCl4- -----> AuCl2-
b
Co3+ -----> Co2+
c
Mn7+ -----> Mn2+
d
Cl2 -----> ClO3-
c. Mn7+ -----> Mn2+
In order to determine which half-reaction is an oxidation, we need to identify the change in oxidation state for each element involved.
a) AuCl4- -----> AuCl2-
The oxidation state of Au decreases from +4 to +2. Therefore, this is a reduction half-reaction.
b) Co3+ -----> Co2+
The oxidation state of Co decreases from +3 to +2. Therefore, this is a reduction half-reaction.
c) Mn7+ -----> Mn2+
The oxidation state of Mn decreases from +7 to +2. Therefore, this is a reduction half-reaction.
d) Cl2 -----> ClO3-
The oxidation state of Cl increases from 0 to +5. Therefore, this is an oxidation half-reaction.
Therefore, the correct answer is (d) Cl2 -----> ClO3-, which represents oxidation.