Is Cl2 + 2e- -> 2Cl- an oxidation half-reaction?
See your post above.
To determine if the given half-reaction is an oxidation or reduction, we need to remember the definition of oxidation and reduction in terms of electron transfer.
Oxidation refers to the loss of electrons, while reduction refers to the gain of electrons. In a half-reaction, the species that is being oxidized loses electrons, and the species that is being reduced gains electrons.
In the given half-reaction: Cl2 + 2e- -> 2Cl-, we can see that chlorine (Cl2) is being converted into chloride ions (Cl-). Chlorine gains two electrons, which indicates that it is being reduced. Since reduction involves the gain of electrons, this half-reaction is a reduction half-reaction.
Therefore, Cl2 + 2e- -> 2Cl- is a reduction half-reaction.