Question

Sodium sulfate reacts with carbon to produce the products sodium sulfide and carbon monoxide. Identify the reducing agent in the following reaction.

Na2SO4 + 4C --> Na2S + 4CO

Answer 1

A reducing agent is that chemical which is oxidized during the reaction, which means it losses electrons, and gets more positive.
You need to know the oxidation state changes in each atom during the reaction

2Na(+1) + SO4(-2) +4C(0) --> 2Na(+1) + S(-2) + 4C(+2) + 4O(-2)

so to be more specific, the sodium atoms are not changing; they are spectators.
Sulfur in SO4(-2) has an oxidation state of +6 and it goes to -2 so it gains 8 electrons.
Pure Carbon has an oxidation state of 0 by definition and it goes to +2 in CO so it loses 8 electrons.
Note that there are 4 carbon atoms each losing 2 electrons = total of 8 electrons.
Note too that the number of electrons lost must equal the number of electrons gained, here it is 8.
So the bottom line is carbon is the reducing agent since it is losing the electrons.
Another way to say the same thing is that carbon is being oxidized since it is combining with oxygen.