What is the oxidation number of phosphorus in H₃PO₄? A) +2, B) +4, C) +5, D) +6, or E) +8.
The oxidation number of phosphorus in this case can be thought off as the charge phosphorus would have if all the bonds it has at the moment were purely ionic. It’s a way of keeping track and detecting whether some components are reduced or oxidized if they undergo a chemical reaction. To answer this question, we’re going to need to know a few things.
Firstly, what is the structure of H₃PO₄? It might be helpful if we give it its name — phosphoric acid. We know each unit of phosphoric acid can dissociate in water, releasing three hydrogen ions. To be acidic in the first place, these hydrogens are better off being attached to the oxygen than the phosphorus because oxygen is more electronegative. And the oxygens are attached to the phosphorus atom in the middle and the last oxygen is double-bonded to the phosphorus. The structure is important, but we will come to why later.
The next thing we need to work out is the total oxidation state for our molecule. If we’re dealing with a molecule or complex ion, all the oxidation states must add up to the charge. Phosphoric acid is neutral. So all the oxidation states of our hydrogens, oxygens, and phosphorus must add up to zero.
The next thing we have to do is use our fundamental rules for oxidation states to figure out the oxidation states of our hydrogens and our oxygens. Unless bonded to itself, the oxidation state for hydrogen is pretty much always positive one, although it’s occasionally negative one when hydrogen is bonded to metals. The oxidation state for oxygen in compounds is typically negative two. However, this oxidation number goes up when oxygen is bonded to itself. For instance, in hydrogen peroxide, each oxygen has an oxidation number of negative one.
This is why it was important to figure out the structure of phosphoric acid first. All the oxygens in phosphoric acid are bonded to other elements. So they all have an oxidation number of negative two. This leaves us with just phosphorus’s oxidation number to figure out. We know that our oxidation states sum to zero. So we know that all the oxidation states for hydrogen — that’s three times one — and all the oxidation states for oxygen — that’s four times negative two — plus the oxidation state for phosphorus is equal to zero. Therefore, the oxidation of phosphorus must be equal to eight minus three, which is positive five.
We could do a quick check just to be safe. Three times one plus four times minus two plus five should be equal to zero. Three minus eight plus five equals zero looks correct. So we can be confident that the oxidation number of phosphorus in H₃PO₄ is positive five.