Answer:
Cu⁺² + 2e⁻ → Cu
Explanation:
A redox reaction is the half-equation where the element is reduced (gains electrons).
Compounds always have to have oxidation numbers that add to 0.
Oxygen's oxidation number is -2.
Stand alone elements always have an oxidation number of 0.
Reactants
Because Copper's oxidation number + Oxygen's oxidation number = 0, and Oxygen's oxidation number = -2, we know Copper's oxidation number is +2.
H₂ has to have a net oxidation of 0, so Hydrogen's oxidation number is 0.
Products
Copper's oxidation number is 0 because it's a standalone element.
2 * Hydrogen's oxidation number + Oxygen's oxidation number = 0, so that means Oxygen's oxidation number is -2, and Hydrogen's is +1.
Because Copper's oxidation number went down from +2 to 0, that means it gained electrons and therefore was reduced. So your answer is:
Cu⁺² + 2e⁻ → Cu
