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Current time:0:00Total duration:13:29

Oxidation and reduction review from biological point-of-view

Video transcript

what I want to do in this video is review what we learned from our chemistry classes about oxidation oxidation and the opposite of oxidation reduction and then see how what we learned in our chemistry class relates to the way that a biologist or a biochemist might use these words and hopefully we'll see that they're the same thing so just as a bit of review if you watch the chemistry playlist oxidation you can do it and actually there's a famous mnemonic for it it's you know oil rig where the oil tells us that oxidation oxidation is losing and I put it in quotes because you're not necessarily losing the electrons so I'll show you what I mean is losing electrons this is what you should have learned in your chemistry class and then you also learned that reduction reduction is gaining and I'll put that in quotes as well is gaining is gaining electrons and I put that in quotes because you're not necessarily gaining the electrons you're more hogging it and the reason why it's called reduction is because if you are gaining electrons you're your notional charge if you really were gaining them is being reduced and the reason why this is called oxidizing is because you tend to lose electrons to oxygen although it doesn't have to be oxygen it could be any molecule that will hog electrons away from you and I think a nice example is is would be fair to kind of make this a little bit more concrete let's say I took some molecular hydrogen it's in a gaseous state and I were to combust that with some molecular oxygen this is what happened on the Hindenburg a fill a balloon full of hydrogen and you get a little bit of spark expose it to oxygen and you're going to have a big explosion but in the process for well for every mole of molecular oxygen if you have two moles of molecular hydrogen I'm just making sure the equation is balanced you're going to produce 2 moles of h2o plus a ton of heat this thing is really going to blow plus a lot of heat plus a lot of heat what I want to do I mean we could talk about the Hindenburg but really the whole reason why I even wrote this is I want to show you what is getting oxidized what is getting reduced so in this situation right here on the hydrogen the hydrogen molecular hydrogen just looks like this you have a hydrogen hydrogen bond there each sharing an electron with the other one so that they both can pretend that their one s orbital is completely filled so they're not losing electrons to each other they're not hogging electrons one from the other so we say that they have a neutral oxidative state they haven't gained or lost electrons these are sharing them the same thing is true for the molecular oxygen in here you actually have a double bond with the two oxygens but they're both oxygens so there's no reason why one would gain or lose electrons from the other but when you go on this side of the equation when you go on this side of the equation something interesting happens you have for every oxygen is connected to two hydrogen's two hydrogen's and the way to think about it is that oxygen is hogging each of these hydrogen's electrons so hydrogen has this one electron it's a valence shell you know the deal with most covalent bonding is hey I give you an electron you give me electron then we both have a complete pair but we know or hopefully we can review that oxygen is much more electronegative than hydrogen this is a little bit of glucose reminiscent is leftover from our from our from our cellular respiration video you can ignore it for now but I'm actually going to connect all this in a future video if we look at our periodic table if you remember from the chemistry playlist electronegativity increases as we go to the top right of the periodic table this these are the most electronegative elements over here these are the least electronegative and all electronegative means is likes to hog electrons let me write that electro electro negative just means likes to hog electrons odd electrons so even though oxygen and hydrogen are in a covalent bond in water they're sharing electrons oxygen is more electronegative much more electronegative than hydrogen so it's going to hog the electrons and actual if you take some elements on this side and you bond them with some guys over here these guys these are so much more electronegative than these left-hand elements that they'll actually just actually completely steal the electron just hog it for most of the time but when you talk about lycra negati it just means likes the electrons so when you look at this bond between hydrogen and oxygen we saw from the periodic table oxygen is a lot more electronegative so the electrons spend a lot more time on oxygen and we learned about a hydrogen bonding we learned that you know that creates a partial negative charge on that side of the water molecule as chart creates partial positive charges on this side and the electrons still show up around the hydrogen's every now and then but when you talk about oxidation and reduction you say look there's no partial charge if one guy is kind of hogging the electron more for the sake of oxidation states we're going to assume that he took the electron so for an oxidation state will assume that the oxygen and water takes the electron and we'll give him an oxidation state of 1 minus or the convention is you write the charge after the number four oxidation states so you don't confuse it with actual charges so this has a 1 minus because from an oxidation state point of view saying it's taking the electron it's gaining the electron that's why I put it in quotes because you're not really gaining it you're just gaining it most of the time you're hogging electrons and likewise this hydrogen or let me be careful this isn't you got one electron from this hydrogen and you got another electron from this hydrogen so instead of saying 1 minus it should be 2 minus it should be 2 - because he's hogging one electron from here and 1 electron from there and in general when oxygen is bonding with other non oxygen atoms or non oxygen elements it tends to have a 2 minus or a negative 2 oxidation state so this guy's 2 minus because he he's gained two electrons let me write that in quotes gained two electrons we know that it really didn't gain him that he's just hogging them these guys lost an electron each so this guy's oxidation state is going to be 1 plus and this guy's oxidation state is going to be 1 plus so you could say by combusting the hydrogen with the oxygen that the hydrogen's before they had a zero oxidation state each of these hydrogen's had a zero oxidation state now they have a one plus oxidation state because they lost their electrons when they bonded with the oxygen so we say that these hydrogen's have been oxidized we say that the hydrogen has been oxidized so due to this reaction hydrogen has been oxidized why has it been oxidized because before it was able to share its electrons very nicely but then it bonds with oxygen which will hog its electrons so the the hydrogen is losing its electrons to the oxygen so it's been oxidized similarly the oxygen due to this combustion reaction due to this reaction right here has been has been reduced has been reduced why has it been reduced here was just sharing electrons it wasn't losing or gaining it but here when it's bonded with much low an element with much lower electronegativity all of a sudden it can start hogging the electrons it gains electrons so this hypothetical charge is reduced by two it's been reduced by well I could write that down by two and if I wanted to actually account for all of the electrons cuz we're talking about losing electrons and gaining electrons we can write two half-reactions this should all be a little bit of review from your chemistry class but it never hurts to see this again I'm going to throw this in the biology playlist so that you biology people can can hopefully refresh your memory with with this stuff but we can write two half-reactions we could say that we started off with two two moles of molecular hydrogen and they have no oxidation states or they're neutral so I could write a zero there if I want and then I end up with on the other side I end up with two moles to moles of h2 but each of the hydrogen's now each of the hydrogen's have a plus one oxidation state they each have a plus one that's one plus oxidation state or another way to think about it is each of these there's four hydrogen's here right this is molecular hydrogen has two hydrogen's and we have two moles of this so there are four hydrogen's here each of the four hydrogen's lost an electron so I could write this so plus four electron that's the half reaction for hydrogen it lost four electrons so this is another way of saying that hydrogen hydrogen is oxidized because it lost electrons oil oxidation is losing and then the other half reaction if I were to write the water or sorry if I would write the oxygen so I'm starting with a mole of molecular oxygen and I'm adding to that four electrons right I can't make electrons out of nowhere I'm getting the electrons from the hydrogen I'm adding to the oxygen and so the half reaction on this side I end up with I end up with 2 moles well I could write it like this two moles of oxygen and each of them have an oxidation state of 2 minus so these are the half reactions and all this is showing is that the hydrogen over the course of this combustion reaction lost electrons and that the oxygen gained the electrons that the hydrogen lost so this tells us that oxygen oxygen is reduced now this is all fair and good and this is all a bit of review of what you've learned in chemistry class but now I'm going to make things even more confusing because I'm going to introduce you to how a biologist thinks about it so and it's not always the case sometimes the biologists will use the definition you learned in your chemistry class but a biologist or many times in many biology textbooks they'll say and this used to confuse me to no end really that oxidation oxidation is losing is losing hydrogen atoms hydrogen atoms and reduction reduction is gaining gaining hydrogen hydrogen atoms and you know at first when I got exposed to this I was like I learned it in in chemistry class they talked about electrons hydrogen atoms you know it's a proton and electron how does it relate and the reason why these two definitions is really the whole point of this video the reason why this definition is consistent with this one is be in but in the biological world in the biological world let me write this year in the biological world world hydrogen hydrogen is what tends to get swapped around and it tends to bond it tends to bond with carbon oxygen phosphorus nitrogen and if we look at the periodic table if we look at the periodic table and we see where hydrogen is and we see where carbon nitrogen oxygen and phosphorus and I really all of this other stuff is you see that all of the stuff that in biological systems hydrogen tends to bond with the things it tends to bond with are much much more electronegative so if a carbon is bonding with a hydrogen the carbon is hogging that electron and then if that hydrogen gets transferred to an oxygen along with the electron the carbon will lose it lost the hydrogen atom but it really lost the electron that it was hogging before and now the oxygen can hog that electron so these are really consistent definitions and the whole reason why I showed you this example because the biological definition kind of doesn't apply here I mean you could say well oxygen is definitely gaining hydrogen's in this reaction so we can definitely say that oxygen is being reduced still according to the biological definition but you can't really say that hydrogen is losing hydrogen's here right in this situation hydrogen is just losing electrons it's not losing itself I guess you could say it's losing itself because it's being taken over but the biological definition just comes from the same notion that when hydrogen bonds with most things in biological compounds it tends to give the electrons so if a carbon loses a hydrogen and gives it to an oxygen the carbon will lose that hydrogen's electron that it was able to hog and now the oxygen is hogging it so the carbon would be oxidized and the the oxygen would be reduced hope that doesn't confuse you in the next in the next video I'll show you a couple more examples and I have the whole reason why I'm doing this is to apply this to cellular respiration so that you don't get confused when people talk say that oh the NAD is being is being reduced when it picks up the hydrogen or being oxidized when it loses the hydrogen and so forth and so on I wanted to see that these are the same definitions that you learned in your chemistry class