Which of the following is a correctly balanced half-reaction? A) Cr₂O₇²⁻ reacts to form Cr³⁺ plus 5H₂O. B) Fe³⁺ reacts to form Fe plus 3e⁻. C) H₂ plus OH⁻ react to form 2H₂O. D) 2Cl⁻ react to form Cl₂ plus 2e⁻. Or E) Pb²⁺ plus Zn react to form Pb plus Zn²⁺.
A half-reaction is a reaction where we see electrons on one side or another. We use half-reactions so we can look at reduction and oxidation processes separately. And we combine them together to make redox reactions. Options A, C, and E can be automatically eliminated because they don’t show electrons. We don’t need to check if they’re correctly balanced because they don’t conform to the basic definition of a half-reaction.
So let’s have a look at B. Iron three plus reacts to form iron in its neutral state plus three electrons. For this half-reaction, the iron is balanced. But what about the charge? The reactants have a total charge of three plus. But the products have a total charge of three minus. So half-reaction B cannot be a correct answer because the charge is not balanced.
So let’s have a look at D. In half-reaction D, we have two chlorines on both sides. So the chlorines are balanced. And we have two minus as the total charge of the reactants and two minus as the total charge of the products. So this is our only example of a correctly balanced half-reaction.
In half-reaction A, what was missing were 14 protons in the reactants, six electrons to balance charge, a second chromium three plus ion, and two water molecules. In half-reaction B, the electrons had been added on the wrong side. Move them to the reactant side, and we have the reduction equation for iron three ions to iron solid.
Reaction C isn’t straightforward to balance. So I’ll just move on to reaction E. Reaction E is a redox reaction that might occur between lead two plus ions and zinc metal. And as we mentioned before, it can be eliminated because it’s a full reaction equation, even though it’s balanced. So we can be fully confident saying that, of the five equations given, the only correctly balanced half-reaction is 2Cl⁻ react to form Cl₂ plus 2e⁻.