Final answer:
In the reaction between carbon dioxide and hydrogen to produce carbon monoxide and water, hydrogen is being oxidized as it goes from an oxidation state of 0 to +1, indicating a loss of electrons.
Step-by-step explanation:
To determine what is being oxidized in the given reaction, we need to look at the changes in oxidation states of the elements involved. By definition, oxidation is the loss of electrons or an increase in oxidation state by a molecule, atom, or ion.
In the reaction CO2(g) + H2(g) → CO(g) + H2O(l), the carbon in carbon dioxide (CO2) is going from an oxidation state of +4 to an oxidation state of +2 in carbon monoxide (CO), thus it is being reduced. On the other hand, the hydrogen in H2 is initially at an oxidation state of 0 and ends up in H2O with an oxidation state of +1. Since the hydrogen is going from an oxidation state of 0 to +1, it is losing electrons and is therefore being oxidized.