Why is the following a redox reaction?

Question

C2H4 +3O2 →2CO2 + 2H2O

Why is this considered a redox reaction?

Answer 1

Anytime an element changes oxidation number in a reaction, it would be a redox reaction.

The oxidation number of oxygen on the left (in O2) is zero. The oxidation of oxygen on the right (in CO2) is 2-, so it has been reduced meaning something else must be oxidized, and thus you have a redox reaction. The other thing being oxidized happens to be the carbon atom (C),which is 1- in C₂H₄ and 4+ in CO₂.