Learn about Senior Chemistry, Redox 7, in this comprehensive video by bannanaiscool.
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Rob Lederer: If somebody comes up to you and says, hey, balance that reaction, you know just right off the street, balance that reaction and you say, well, okay, I think that's a redox reaction, so I'm going to balance this and I'm going to make sure - not only do I have to balance it atomically, but you have to actually worry about these charges too. That equation actually can be kind of tricky to balance, if you don't have a technique called oxidation numbers to be able to solve it. Let me show you what these are all about. Oxidation numbers or oxidation states are numbers that we assign to various elements, kind of an informal type of charge that we give them in order to determine whether or not the gain or lose of electrons in chemical reactions. Now, I'm going to give you all the rules for oxidation and as we go ahead and just do them right now. So, I've got a whole bunch of chemicals here listed on board and we'll just go through them and assign oxidation numbers. Then you'll be able to give oxidation numbers for everything. Here we go. All elements in their natural state, the way they are found in nature, they have oxidation numbers equal to zero. So, if you lay out oxygen; oxygen in its natural state O2, so we put down underneath oxygen a zero; that's its oxidation state. What about hydrogen? H2-zero, because that's the way the element is found in nature. Alright, now here is H-positive. When hydrogen is not in its elemental state, it always has a plus one charge. Also, any type of free floating ion has as its oxidation number whatever charge it is. So therefore because that's a plus one, hydrogen is plus one and it's always plus one in a compound. Hydrogen is plus one. Now, here is SO4-two negative, and you've got to assign oxidation numbers to the sulphur and to the oxygen. Here is the rule for oxygen, when it's not in its elemental form, but in a compound or a kind of chemical, O is two negative. There is an exception, there is a couple about it, I'll tell you that in just a second. Just 99% of the time, O is two negative or negative two. So here is sulphur and oxygen and we give oxygen a negative two. We just write down what one oxygen is here, but there is four oxygen. You now get this; negative two times four is negative eight. But this polyatomic ions is - I only want to keep the two negative charge. So what is the Sulphur's charge has to be? Plus six, because plus six added to negative two times four, equals a total of two negative, that's how you do oxidation numbers in polyatomic ions. Now, hydrogen is always plus one, but oxygen here can't be minus two because look at it, minus two times two would be negative four, plus two would give you a two negative charge, but this hydrogen peroxide is zero. So oxygen in hydrogen peroxide is negative one, that's an exception. Oxygen is always negative two except when it's in hydrogen peroxide and, see OF-two; fluorine is more electronegative on the periodic table, I am telling you something you learn in grade ten. And there is only one element more electronegative than oxygen and it's fluorine. So, fluorine gets its normal charge of negative one, which means in this case, the total must be zero for this molecule; negative one times two, that's negative two total, oxygen has to be plus two in order to make a zero charge for that compound. No. There is couple of exceptions for oxygen, but 99% of time, oxygen is negative two in a compound, like this one, oxygen is negative two, so what does that make each potassium here to give a total of zero here? You with me? Plus one, plus one times two is going to be plus two and negative two, makes a zero charge. So, what about this one? Look, there is no oxygen or hydrogen in the compound, so you don't anything for sure, but when you see that, you break the chemicals down to their ions. K is normally positive and S is two negative on the periodic table. Those are their ionic charges. So, when you stuck break things down in the io