Hello, my name is Robert Smith and I'm with New River Community College's Academic Assistance. And today we are discussing oxidation and reduction reactions. So we are given the task of assigning oxidation states, or otherwise called oxidation numbers, to each element in the following compound. There are several rules for assigning oxidation states to various elements. But there are a few that show up regularly throughout your core.
One of them is identifying that group 1 elements will be having an oxidation state of plus 1. And group 2 elements will have an oxidation state of plus 2. Hydrogen will have a plus 1 charge when it is attached to a non-metal. and it will have a minus one charge when attached to a metal. And then oxygen is usually our telltale whenever we're trying to assign oxidation states. It normally has a minus two as its oxidation state.
There are two exceptions. If it is in a peroxide ion, then... it'll have a minus one oxidation state. And if it is a superoxide as part of an ion, then it'll have a minus one half oxidation state. But those two do not show up very regularly.
So let's begin. We're going to start by rewriting our compound, Na2O. And above each element, we're going to put the oxidation states, recognizing that because there is no charge on this compound, that must mean that the oxidation states have to equal zero when summed. So let's look at...
How we're going to assign this. Well, oxygen has a minus 2 whenever it's not a peroxide or superoxide. So, we can make the equation down beneath that whatever this ends up being has to equal 0, and we have a minus 2 as our oxygen.
Therefore, the total of sodium must be plus 2, because only 2 minus 2 equals 0. That's the only way to solve this particular equation in a reasonable fashion. But because there are two sodiums, we have to take this two and divide it by two to get the actual oxidation for sodium. So the oxidation state of sodium is plus one. Because we can then rewrite this if we need to as one sodium plus one sodium, because there are two sodiums, minus two for the oxygen. And that equals zero.
So those are our oxidation states for that compound. Let's look at the next one. When we rewrite this and assign the oxidation states, again, our key indicator is going to be oxygen. So HClO3.
Oxygen has a minus 2. And hydrogen is also a key indicator, because hydrogen is attached to a non-metal. So whenever hydrogen is attached to a non-metal, it has a plus one oxidation state. And we know that the total has to equal zero. So we have one plus some number, let's call it x, plus one.
Minus, or 2 times 3, because there are 3 oxygens. So that's the same as saying 1 plus x minus 6 is equal to 0. Well, if you isolate x, you'll find that x is equal to 5, because you would add 6 to both sides. and then subtract one from both sides or you can combine like turn and make it so that x minus 5 is equal to 0, and then add 5 to both sides to get x by itself, so x equals 5. So our determination is that chlorine has a plus 5 oxidation state. That's the odd man out. That's the one that is going to be a little trickier to find out because chlorine can have several different oxidation states.
Let's look at this one more example, HPO4 Let's rewrite this, and we'll assign the oxidation states for it. PO4 So we know that oxygen, again, is going to be a minus 2, and we know that hydrogen is going to be a plus 1. That's relatively simple. Now we need to find what phosphorus is. But because the charge on this particular ion This particular compound is a minus 2. It's 2 minus. We're going to say that our oxidation states have to equal negative 2 instead of 0. So we have 1 for the hydrogen plus some unknown value, let's call it x, plus 4 times negative 2, because there are four oxygens, so plus negative 8. You have four oxygens here.
Each one has a minus two oxidation state. Four minus four times negative two is negative eight. If you combine like terms, you end up with x minus seven is equal to negative two. And then you would add seven to both sides to isolate x. So you find that x is equal to five.
because negative 2 plus 7 is equal to 5. Therefore, our oxidation state on phosphorus is plus 5. So whenever you encounter a problem that asks you to identify the oxidation state or sometimes referred to as the oxidation number on a given element within a compound. The first thing you want to look at are your hydrogens and oxygens. If your hydrogen is there and it's attached to the to a non-metal, it has a plus one charge.
If it is attached to a metal, it has a minus one charge. Oxygen usually will have a minus two charge unless it's a peroxide or superoxide. And then your group one and group two elements will have an oxidation state that is the same as their charge, which is plus one for group one, plus two for group two. There are several other rules for assigning oxidation states, and you can find them in my PowerPoint presentation for this chapter.