Which of the following are true for the reaction shown? Fe plus 4HNO3 react to form Fe(NO3)2 plus 2H2O plus 2NO2 gas. (I) Some of the nitrogen is reduced. (II) It is an oxidation–reduction reaction. (III) The oxidation number of iron changes from zero to minus two. (A) II only, (B) III only, (C) I and II, (D) II and III, or (E) I, II and III.
Let’s look at some of the key words in these statements. We see that we have the words “reduction” or “reduced” and “oxidation” a couple of times in the statements. This means that oxidation and reduction are clearly an important part of answering this question. Let’s begin by reminding ourselves what we know already about oxidation and reduction.
From the acronym OIL RIG, you may remember that oxidation is the loss of electrons, while reduction is the gain of electrons. OIL RIG is the common way to remember which is which, but it’s okay if you have your own way.
Statement (I) asks whether some of the nitrogen is reduced. If this is true, some of our nitrogen atoms will gain electrons over the course of our reaction, but not all of them. This is because we have this keyword “some” in the statement. If statement (II) is true, this means that we have both oxidation and reduction processes occurring in the same reaction. You may hear this referred to as a redox reaction, which is short for both reduction and oxidation.
The last statement says that the oxidation number of iron changes from zero to negative two. But is this change oxidation or reduction? To work this out, we can think of this as a change in charge. Imagine that you had an Fe atom with a charge of zero and it becomes an Fe atom with a charge of minus two. Would it have gained electrons or lost electrons? Since our iron has gained a negative charge, we can say that it’s gained electrons, which are negatively charged. Since we know that a gain of electrons is reduction, this statement is asking whether the iron is reduced.
So, how do we work out which of these statements are true? Let’s do this by looking at the oxidation numbers of various elements in our reaction. Let’s start by looking at the iron. In our reactants, iron exists on its own in a neutral state. So, this has the oxidation number of zero. On the
reactants [products] side, the oxidation number of iron is slightly more complicated. If you imagine NO3 as a polyatomic ion, you should remember that it exists as NO3−. In our iron-containing product, we have two of these NO3 ions. Two lots of negative one gives us negative two.
Because this product is overall neutral, our iron needs to balance out our negative two from the NO3. This means that our iron must exist in a plus-two oxidation state. If we logic-check our answer, Fe2+ is a commonly seen iron. So, we’ve probably worked this out correctly. From working this out, we can already tell that statement (III) is incorrect. The oxidation number of iron changes from zero to plus two not minus two. So, any answer involving statement (III) can be ruled out.
Let’s turn our attention to nitrogen. Looking carefully, we can see that nitrogen exists in one of our reactants but in two of our products. So, we need to be careful. Let’s start with the oxidation state of the nitrogen in our reactant, which we’ve already kind of worked out. We’ve already remembered that NO3 has an oxidation state of negative one. This means that in HNO3, our hydrogen must have an oxidation state of plus one. When we think logically about hydrogen, this makes sense. So, now, we just need to work out what the state of nitrogen is in our NO3.
We know that the nitrogen needs to balance the oxidation state of O3 to leave an overall negative one state. But what oxidation number might oxygen have? You may be able to remember this off the top of your head or we can look to the periodic table to give us a hand. We know that oxygen is in group 16. And this can tell us which oxidation state oxygen tends to have. We know that elements in group 16 tend to form two minus anions like O2 minus. So usually, oxygen has the oxidation number of negative two. Of course, we have three oxygen atoms here, so we need to multiply this negative two by three, which gives us minus six.
Since the nitrogen needs to balance the negative six to leave us with an overall oxidation number of negative one, we can see that the oxidation number of nitrogen must be positive five. When we compare this to the first of our products, we can see that our nitrogen is in a very similar environment. So, the oxidation number of the nitrogen in our first product is the same, positive five.
But what about the last product, NO2? Here, we have two oxygens each with an oxidation number of negative two. So, both of our oxygens together contribute negative four. Overall, our NO2 gas is neutral, so the nitrogen needs to balance out this negative four completely. So, the oxidation number of our nitrogen here must be plus four.
Now, let’s tidy this up. We now have enough information to answer statement (I), whether some of our nitrogen is reduced. When we compare the nitrogen in our two products to the nitrogen in our reactant, we see that in one case, there is no change in oxidation state, but in the other case, we have a decrease in the oxidation number. A decrease in our oxidation number means that the number has become more negative. If something has become more negative, it must have gained negative electrons. So, this is a gain in electrons. So, statement (I) is indeed correct; some of the nitrogen is unchanged, but some of the nitrogen has been reduced.
We’ve seen so far that statement (I) is true and statement (III) is false. So, let’s look, finally, at statement (II). Is this an oxidation–reduction reaction? We’ve actually already worked out the answer to this. We know that some of our nitrogen is reduced, so there is reduction in our reaction. We also worked out that the oxidation number of iron increases from zero to plus two. This increase means that our iron has been oxidized. So, we do have both oxidation and reduction processes going on at the same time in our reaction. So, statement (II) is also true. Looking at our list of answers, the only answer which says that both statement (I) and (II) are true but not (III) is answer (C).