Video instructions and help with filling out and completing How Form 2220 Compute

Instructions and Help about How Form 2220 Compute

In this video we're going to learn how to figure out the oxidation numbers for the different elements in a chemical compound the oxidation numbers are the numbers that I've written here above each one of the elements now if you want to learn more about what oxidation numbers are or why they're important check out my video called what are oxidation numbers in this video we're going to work through the process of how you figure out what these numbers are so here are the rules that we're going to use to figure out oxidation numbers now just so you know every teacher and textbook has their own version of these rules but they all work in pretty much the same way so if you learn mine you'll still get the answer right 100% of the time even if these are a little bit different from your teachers I'm going to talk through a few of these rules right now and then I'll introduce the rest just as we work through practice problems the first is this an element by itself always has an oxidation number of 0 here's what I mean by that there are a lot of chemical compounds that have just one element that element is not combined with any other elements that's what I'm calling an element by itself so that's something like CL - it doesn't matter how many atoms of that element you have just as long as it's only that element and none others so CL - an element by itself oxidation number of 0 sodium na by itself 0 s 6 just sulfur nothing else P for phosphorous 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 with nothing else the other rule here is about monatomic ions these are ions that are made of only one and so like this for monatomic ions their oxidation number is the same as their ion charge so for k+ here it's oxidation number is going to be plus 1 for n 3 - it'll have an oxidation number of minus 3 and mg 2 plus here is going to have an oxidation number of plus 2 now keep this in mind when we write oxidation numbers we write this the sign first so plus minus and then the number after this is the opposite of how we write ionic charges so just keep that in mind the charge might be 2 Plus on magnesium but the oxidation number is plus 2 now for the rest of these rules we usually use more than one together so I'll just talk about these as we use them in example problems here is our first example KCl we want to figure out the oxidation numbers of the elements in this compound ok so let's take a look at our rules here K potassium is in this column in the periodic table it's in group 1a so there's this rule that elements in group 1a are always plus 1 so that is potassium Jacque sedation number then we have CL over here that is one of the halogens halogens usually minus 1 positive with oxygen well CL certainly isn't with oxygen here so we'll give it a minus 1 oxidation number now I want to use this to show you a third rule that's this rule right here the sum of oxidation numbers for a neutral compound equals 0 KCl is a neutral compound here it doesn't have a charge after so that means that the sum of these oxidation numbers is going to equal 0 and that's definitely true here plus 1 minus 1 equals 0 we'll use this later on when we do other examples but just keep that in mind the sum of oxidation numbers for a neutral compound should always equal 0 mg o mg is in this column in the periodic table group 2a and group 2a elements are always plus 2 so there's that oxygen here we have a rule for oxygen it is usually minus 2 it is minus 1 in peroxide h2o2 hydrogen peroxide is the most common peroxide is probably the only one you'll ever see but anyway this oxygen is definitely not in hydrogen peroxide so it's fair to say that it's oxidation number will be minus 2 plus 2 4 mg minus 2 4 oh and they add together to make 0 because this is a neutral compound co carbon monoxide let's figure this one out ok so see here there isn't any rule for carbon so we'll have to figure it out based on what we do know okay so we do know oxygen oxygen is usually minus 2 unless we're in a peroxide definitely not a peroxide so we can safely say that oxygens oxidation number is minus 2 now let's use this other piece of information that we know and that's the sum of oxidation numbers for a neutral compound should equal 0 so whatever carbons oxidation number is should add together with oxygens to make 0 so we can figure out the carbons oxidation number should be plus 2 minus 2 from oxygen gives us 0 nh3 ok and there isn't any information about that so just like in the last example we'll have to figure out its oxidation number using what we do know here we've got hydrate there's a rule for hydrogen hydrogen is plus 1 with nonmetals and minus 1 with metals so nitrogen is definitely a nonmetal which means it in this case hydrogen is going to have a plus 1 oxidation number ok but there are 3 hydrogen's each one of them has a plus 1 so what we have to do is we have to multiply this plus 1 times 3 for the 3 hydrogen's and that's going to give us plus 3 now let's keep this rule in