For statements (I) in (II), state for each if they are true or false. (I) Ce4+ plus e− react to form Ce3+ is a correctly balanced reduction half reaction. (II) In the half reaction Ce4+ plus e− react to form Ce3+, both mass and charge are conserved. If both are true, state if (II) is a correct explanation for (I).
A half reaction is one part or half of a reduction–oxidation reaction which involves the transfer of electrons between two different chemical species. We can remember how electrons move in a redox or reduction–oxidation reaction by using the acronym OILRIG, or “Oxidation Is Loss of electrons and Reduction Is Gain of electrons.”
This question says that the reaction we’re looking at is a reduction half reaction, so it should be gaining electrons. This certainly seems to be the case. Ce4+ gains an electron over the course of the reaction to form Ce3+. So the reaction we’re given is a reduction reaction. But is it correctly balanced?
In order to see if this equation is correctly balanced, we need to check two things. The first thing we need to check is if the mass is conserved. This simply means that we need to ensure that there is the same number of each type of element on both sides of the equation. Looking at our equation, we can see that we have one cerium on the reactant side and one cerium on the product side. So mass is definitely conserved for this reaction.
The second thing that we need to check to make sure that our reaction is balanced is we need to check if the charge is conserved. That is, if it has the same charge on both sides of the equation. On the reactant side of our equation, the cerium has a charge of four plus and the electron has a charge of one minus. This means that we have a total charge of three on our reactant side. And on our product side, our cerium ion also has a charge of three plus. So charge is conserved for this reaction.
That means statement (I) is true. Our reaction is correctly balanced. And it’s a reduction half reaction. Statement (II) says that, in our reaction, both mass and charge are conserved. As we’ve just discussed, this statement is also true.
Since both parts of this question were true, we need to state if (II) is a correct explanation for (I). So is the fact that in our half reaction both mass and charge are conserved the reason that our half reaction is correctly balanced? We had to check if mass and charge were conserved in order to ensure our equation was correctly balanced. So this is true as well.