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Video transcript

this video we're going to find the oxidation state of carbon in several different molecules an earlier video we've already seen the definition for oxidation state and also how to calculate it so let's start with methane and let's find the oxidation state of carbon in methane one approach is more of a general chemistry approach where we know that hydrogen usually has an oxidation state of +1 and we have 4 hydrogen's for a total of +4 the sum has to be equal to 0 so we know that carbons oxidation state must be -4 immediately since we have only one carbon here so let's go ahead and verify that with our dot structure so remember when we're using or we're calculating the oxidation state using dot structures we're thinking about bonding electrons and we know that each bond consists of two electrons so we need to put in the bonding electrons for all of our bonds next we think about the oxidation state for carbon we start with the number of valence electrons in the free atom or the number of valence electrons that carbon is supposed to have and we know carbons supposed to have four valence electrons so from that number we subtract the number of valence electrons in the bonded atom or the number of valence electrons carbon has in our drawing but now we now we need to think about these covalent bonds is being ionic and so the more electronegative atom is going to take all of the electrons in the bond so we need to think about electronegativity differences and we're comparing carbon to hydrogen so which is more electronegative we know that carbon is more electronegative than hydrogen so the two electrons in this bond here carbons going to take both of them so it's winner-takes-all carbons going to hog those electrons in this bond all right same for this next carbon hydrogen bond carbon is more electronegative so it takes those electrons and all the way around so we can see that carbon is now surrounded by eight electrons right let's count them up here 1 2 3 4 5 six seven eight so four minus H is equal to minus four so we already knew that minus four was going to be the oxidation state for carbon let's move on to let's move on to another molecule here so c2h4 this is ethene or ethylene what's the oxidation state of carbon in this molecule well hydrogen should be plus one and we have four of them for a total of plus four so the total for carbon should be minus 4 because that total has to sum to zero but this time we have two carbons so minus four divided by two gives us minus two each carbon should have an oxidation state of minus two let's verify that alright let's put in our bonding electrons and let's calculate the oxidation state of carbon by using our little formula here so you put in our bonding electrons and let's just pick one of the carbons to start off with let's say we're talking about the carbon on the right so we think about electronegativity differences and we know carbons more electronegative than hydrogen so carbon is going to steal those electrons and same thing over here our carbons more electronegative so carbon steels that those electrons - when we get to this double bond here between the two carbons we have these four electrons and now we're trying to compare the electronegativity of carbon to carbon and obviously that's the same electronegativity so with four electrons we're going to divide those electrons equally because both those carbons have the same value for the electronegativity and so we take those four electrons we divide them in half so we give two electrons to one carbon and two electrons to the other carbon so carbon normally has four valence electrons right so we're using our formula here for oxidation state oxidation state is equal to the number of valence electrons that carbon is supposed to have minus the number of valence electrons around carbon in our drawing so let's count them up after we've accounted for electronegativity that's one two three four five and six so four minus 6 gives us an oxidation State for carbon of minus two which is in agreement with what we calculated over here keep in mind that each carbon was supposed to have an oxidation state of minus two so if we look over here at the carbon on the left right and we just assign those electrons really quickly we can see that that would be the same calculation 4 minus 6 gives us minus 2 so each carbon has an oxidation state of minus 2 we move on to another molecule so ch2 oh this is formaldehyde and we know that oxygen usually has an oxidation state of minus 2 and we have one oxygen for a total of minus 2 down here hydrogen usually has an oxidation state of plus 1 we have two of them 4 plus 2 this total has to sum to 0 so carbons should have an oxidation state of 0 in this molecule let's go ahead and look at the dot structure right we put in we put in all of our bonding electrons so we can find the oxidation state easier and we think about electronegativity differences we know carbons more electronegative than hydrogen so carbon takes those electrons right same for this bond carbon takes those electrons but now now we're going to compare carbon to oxygen and we know oxygen is more electronegative than carbon so oxygen is going to take all of those electrons so all four of those electrons in that double bond so it's winner-takes-all when you're thinking about oxidation states alright so because oxygen is more electronegative it takes all of those electrons and now we can see that carbon is surrounded by four electrons so one two three four carbon is supposed to have four valence electrons around it and here we see it is surrounded by four once we take into account electronegativity so 4 minus 4 gives us an oxidation state of 0 so in the formaldehyde molecule carbon has an oxidation state of 0 which is what we predicted over here let's think about oxygen all right we said that oxygen would have an oxidation state of minus 2 so let's look at oxygen auctions right here and let's uh let's think about how many Vail electrons oxygen should have well because of its position on the periodic table oxygen should have six valence electrons how many electrons are around it now once we've accounted for electronegativity well here's one two three four five six seven and eight so six minus eight gives us an oxidation state for oxygen of minus two what about for hydrogen all right so hydrogen hydrogen should have one valence electron but there's zero electrons around it here because carbon is more electronegative than hydrogen so one minus zero gives us an oxidation state of plus one which is what we predicted over here and the same thing for this hydrogen so this this formula works for other atoms too alright let's do another example and this time we're doing formic acid so ch2o two oxygen should be minus two and we have two of them so we have minus four four totaling our oxygens hydrogen should be plus one and we have two of them for a total of plus two and now this total has to add up to equal zero so we know that carbons oxidation state should be plus two in the formic acid molecule here so let's go over and put in our bonding electrons alright so we put in all of our bonding electrons here and we think about the oxidation state of carbon we think about electronegativities so carbons more electronegative than hydrogen so carbon gets those two electrons in that bond but over here oxygen is more electronegative than carbon so oxygen takes those electrons the same thing above oxygens more electronegative than carbon so oxygen takes those electrons to so what's the oxidation state of carbon in formic acid well carbon is supposed to have four valence electrons and from that we subtract the number of electrons around carbon once we've accounted for electronegativity once we pretend like everything is ionic and that's only two electrons this time so four minus two gives us an oxidation state for carbon of plus two just like predicted over here so you can see every molecule we've done so far has had a different oxidation state for carbon carbon carbon is is unique right it has all these different oxidation states let's finally look at carbon dioxide so oxygen should have an oxidation state of minus 2 and we have two of them for a total of minus 4 so carbon must have an oxidation state of plus 4 so we look at our dot structure over here and we put in our bonding electrons and we think about electronegativity oxygen is more electronegative than carbon so oxygen is going to steal each option is going to steal the electrons in those bonds so let me redraw it here I'll show oxygen I'll show oxygen with all these electrons around it so the 4 electrons in that double bond here are all going to be on this oxygen on the left same thing for the oxygen on the right it has these lone pairs of electrons and it's more electronegative than carbon so it steals all of those electrons which leaves carbon with 0 electrons around it so define carbons oxidation state we know carbon should have 4 valence electrons and here right we have 0 electrons around carbon so 4 minus 0 gives us an oxidation state of plus 4 just as we predicted and if you wanted to do each oxygen alright oxygen should be should have 6 valence electrons and here we have 1 2 3 4 5 6 7 8 so six minus eight gives us minus two for the oxidation state of oxygen alright let's go back up to the beginning here alright let's look at all these different oxidation states that we've covered for carbons so we started off with an oxidation state for carbon of minus four we went to minus two and then we went to zero and then we went to plus two and then finally we ended up with plus four so carbon can have a range of oxidation states from minus 4 to plus 4 when you're talking about carbon with four bonds you can also have in between right it's possible of course for carbon to have an oxidation state of plus 3 I just and do an example like that in in this video