How to Calculate Oxidation Numbers Introduction

In this video we are going to learn how to

calculate

the

oxidation

numbers

of the different elements in a chemical compound. The

oxidation

numbers

are the numbers that I have written here above for each of the elements. Now if you want to learn more about what oxidation numbers are or why they are important watch my video called what are oxidation numbers? In this video, we're going to work through the process of how to figure out what these numbers are, so here are the rules that we're going to follow. to use now to

calculate

oxidation numbers, so you know that every teacher and textbook has their own version of these rules, but they all work more or less the same way, so if you learn mine you will still get the correct answer 100% of the time. time, even if they are a little different than your teachers, I'm going to talk about some of these rules right now and then present the rest as we work on practice problems.

The first is always an element in itself. has an oxidation number of 0, this is what I mean by there are many chemical compounds that have only one element, that element is not combined with any other element, that is what I call an element by itself, it is something like this like CL - it doesn't matter how many atoms of that element you have, as long as it's just that element and no others, so CL - an element by itself oxidation number of 0 sodium na by itself 0 s 6 just sulfur nothing else P for phosphorus and nothing else, zero, so that's probably the easiest rule here, it's always zero for the oxidation state if you have an element by itself without anything else.

The other rule here is about monatomic ions, these are ions that are made of just one and so on for monatomic ions. its oxidation number is the same as its ion charge, so for k+ here its oxidation number will be plus 1, for n 3 it will have an oxidation number of minus 3 and mg 2 plus here it will have an oxidation number of plus 2, Keep this in mind when we write oxidation numbers we write this sign first so plus minus and then the number after this is the opposite of how we write ionic charges so keep in mind the charge could be 2 plus in magnesium. but the oxidation number is plus 2.

Now, for the rest of these rules, we generally use more than one joint, so I'll just talk about them as we use them in example problems. Here is our first example of KCl. We want to calculate the oxidation numbers. of the elements in this compound, okay, so let's take a look at our rules here. K potassium is in this column of the periodic table, it is in group 1a, so there is a rule that elements in group 1a are always plus 1, so that is potassium Jacque's sedation number then we have CL here which is one of the halogens, generally minus 1 positive with oxygen, well, CL is certainly not with oxygen here, so we'll give it an oxidation number minus 1.

Now I want to use this to show you a third. rule, that's this rule here, the sum of the oxidation numbers of a neutral compound is equal to 0. KCl is a neutral compound here, it has no charge after, which means the sum of these oxidation numbers will be equal to 0 and that is definitely true. here plus 1 minus 1 equals 0 we will use this later when we do other examples, but note that the sum of the oxidation numbers of a neutral compound must always equal 0 mg or mg is in this column in the group of the periodic table Elements 2a and group 2a are always plus 2, which is why oxygen exists.

Here we have a rule for oxygen, it's usually minus 2, it's minus 1 in h2o2 peroxide, hydrogen peroxide is the most common peroxide and probably the only one you'll see, but anyway, this oxygen is definitely not in hydrogen peroxide , so it's fair to say that its oxidation number will be minus 2 plus 2 4 mg minus 2 4 oh and they add up to make 0 because this is a neutral carbon monoxide compound. Well, look, there's no rule for carbon here, so we'll just have to figure it out based on what we know. Okay, so we know that oxygen is generally minus 2, unless we're in a peroxide, it's definitely not a peroxide. then we can safely say that the oxidation number of the oxygens is minus 2.

Now let's use this other information that we know and that is that the sum of the oxidation numbers of a neutral compound must equal 0, so any number of oxidation of the carbons must be added with the oxygens to get 0 so we can calculate the oxidation number of the carbons must be plus 2 minus 2 of the oxygen gives us 0 nh3 ok and there is no information about it so just like in the last example we will have to calculate its oxidation number using what we do know here is that we have hydrates, there is a rule for hydrogen: hydrogen is plus 1 with non-metals and minus 1 with metals, so nitrogen is definitely a non-metal, which means that in this case hydrogen will have an oxidation number plus 1, ok, but there are 3 hydrogens, each of them has a plus 1, so what we have to do is multiply this plus 1 times 3 for the 3 hydrogens and that will give us more 3.

Now let's keep in mind this rule that the sum of oxidation numbers for a neutral compound must equal 0 NH 3 is definitely a neutral compound, we don't see any charge here , so any number of nitrogen here has to be added with plus three to make zero, which means the nitrogen here has to be minus three which is added there to make zero, so its oxidation number is minus three hydrogens is plus a CAC; so the calcium here is in this column of group 2a, so we know that its number is always plus two, we don't know what the number of carbons is, but we can work it out, what we do know is that the number of calcium plus any number of carbons must equal zero because this is a neutral compound, it has no charge, which means the total number on the carbon will be minus two, but there are two carbons here, so we have to divide this number by the two, so let's take this and divide it by two to get an oxidation number of minus one for each plus two for calcium, one for carbon. .2 one: look at this, it's an ion, so it's going to be a little bit different in this case, the sum of the oxidation numbers of a polyatomic ion is equal to the charge of the ion, so let's see how we're going to use that nitrogen.

I don't know what your oxidation number is going to be, but the oxygen here is generally negative two, so we have negative two here, but we have two oxygens, so negative 2 times 2 will give us negative four in total, whatever the nitrogens are. is when we combine it with negative four we need to get not zero but we need to get negative one because that's what gives us the ion charge number 2 1 minus, which means we're going to have a 3 plus 3 plus 3 minus 4 minus 1, which It means that the oxidation number here at night will be +3 hno3. Well, let's look at the rules.

We have hydrogen. Hydrogen here combines with non-metals, nitrogen and oxygen, which means its oxidation number will be plus one. The oxygen here is going to be minus two, but we have three oxygens, so we have to take this minus 2 times 3 to get minus 6. Now we have plus 1 of the hydrogen plus the nitrogen which is minus 6 has to give us 0. because here This is a neutral compound, that means that in this case the oxidation number of nitrogen will be plus 5 1 plus 5 minus 6 equals 0. Now I'm going to start talking about some more challenging examples that use some rules here that are a little less common ZN h 2 ok zinc here we don't know what its charge will be but hydrogen we have a rule for that now we have used two, the number of hydrogen is plus 1 because it has been with non-metals but here it is with a metal with zinc That means that its oxidation number will be minus 1, but we have 2 of these hydrogens, so it will be minus 1 multiplied by 2, which will give us minus 2, since this is a neutral compound.

Whatever zinc is plus minus 2, it has to give us 0. that means the oxidation number of zinc has to be plus 2 here, so we got plus 2 for zinc and minus 1 for hydrogen because hydrogen was with metal , so it has an oxidation number minus 1 BR or 3 1. This is a complicated question, but it is quite common, so bromine BR is one of the halogens. The rule for halogens is that they are generally minus 1, but they are positive with oxygen, so I'm not going to put minus 1 here because it is with oxygen, so let's go to oxygen instead of oxygen.

Minus 2 is usually not in peroxide, so let me put it here. Now we have three oxygens, so that will be multiplied by three to give us. Six, whatever bromine is. Six has to give us minus one because this whole thing. is a polyatomic ion that has a charge of minus one, which means the charge on bromine must be plus five plus five minus 6 gives us minus one, so this is an exception where halogens instead of having a number of oxidation of minus one have a positive number. oxidation number because they are paired with oxygen, keep it in mind clf3 okay, so here we have two halogens in the same compound, we have fluorine and chlorine, how are we going to solve this well?

This is one of those things that sometimes happens fluorine combines with another one of the halogens, first let's focus on this rule that fluorine is always always always minus one, so I'm going to put a minus one right there, multiply it times three because I have three of them and get minus three now chlorine the other halogen says these other halogens are usually minus one but they can't always be positive with oxygen they can also be positive with fluorine here so fluorine gives us minus three chlorine should be plus three so these can add together to give us zero as this is a neutral compound which means the oxidation number of chlorine must be plus three and it is plus three instead of minus 1 because it is combined with fluorine which It's always minus one no matter what, here's how to work out these rules to find out what the oxidation numbers of the elements in a composition are, if you want a little more practice, I'll have another video with more practice problems if you're a little shaky at this, practice more and more.

Problems are the best way to improve very quickly.