Redox reactions Oxidation reduction (or redox) reactions
- We now know what an oxidation state or and oxidation number is and what it means when things are oxidized
- or reduced and lets see how that actually happens in reactions so what we're going to study is this video
- is oxidation/reduction reactions and all that is a reaction where somebody is being oxidized, which means
- electroons are being taken away from them, and someone's being reduced, which means they are being handed
- electrons or they are taking electrons away from someone else, and sometimes this has been termed, because
- you have the red in reduction and you have the ox in oxidation an they switch them around and they call i
- it a redox reaction, and it sounds like a very fancy chemistry term but it just means a reaction where
- something is getting oxidized and if something is getting oxidized, something else is getting reduced,
- so lets study a bunch of them. So right here I actually have combustion, this is actually methane, and
- you know, methane is a fuel because it used to make a moter powered on methane, right there, its a hydrocarbon,
- most fuels that we use are hydrocarbons, most fuels that we use are hydrocarbons which just means carbons ,
- bonded in a bunch of different ways to hydrogens and if you add enough heat for the reaction to happen,
- so its activation energy if we put it in there with some oxygen around, its going to combust, its going
- to produce carbon dioxide and water, and i didnt put it here, and even more heat than ou put into it
- so this is an endothermic reaction, it produces more heat than you put into it and ill do a lot more
- on that in future videos, about endothermic and exothermic reactions, but anyway, we care about the oxidation
- and the reduction, so lets see if anyone is getting oxidized or anyone is getting reduced here, so, lets
- look at their oxidation numbers, or their states, here carbon is bonded to four hydrogens, who's giving,
- who's taking? the carbon, lets go to our periodic table, carbon is here, hydrogen is here, carbon is
- more electronecative, its kind of, its adjacent to the three musketeers of electronegativity, these guys
- are the most electronegative, we always ignore the halogens, not the halogens sorry, the noble gasses
- because they pretty much dont react at all, they are pretty much happy with their 8 valence electrons,
- these guys love to gain electrons, they're small molecules, their outermost shell is close to the nucleus
- and they're so close to becoming noble they just love hogging electrons. carbon's almost there, carbon
- is much further right on the periodic table than hydrogen, so if you have carbon bonding with hydrogen,
- carbon is going to be the electron hog of this situation, lets go down here, so if carbon is the electron
- hog, and hydrogen is having its electrond taken away from it, remember this is all kind of hypothetical,
- its more partial is the reality, but if you had to pick or choose, hydrogen is going to lose an electron
- each, so its going to have an oxidation state of plus one for hydrogen, you have four hydrogens each
- giving up an electron, so the carbon must be taking four electrons, so it's oxidation number is minus
- 4, its taken 4 electrons so its charge will go down by 4, so that's why its negative, fair enough, now
- whats the oxidation state of this oxygen right there, well its just bonded to itself, there's no reason
- to believe that one oxygen should be able to take any electrons from another oxygen, so it has a zero
- oxidation state, its not hogging more than its fair share of electrons that it was initially kind of
- born with, now after the combustion occurs, what are the oxidation numbers, well I have oxygen bonded
- with carbon, two oxygens bonded with carbon, now we know oxygen, i mean oxidation was something named
- after oxygen, it is one of the most electronegative, almost anything bonded with oxygen is going to be
- giving up its electrons, we also know that oxygen likes to take two electrons because if it takes one,
- two electrons, then it starts having a, it takes one it gets here, it takes two it gets here, it has
- 8 valence electrons, so it's typical oxidation number is negative two, so, carbon, in this situation,
- each oxygen is going to have a minus two oxidation number, each carbon dioxide molecule is neutral, so
- everything has to add up to zero, so the oxygens, you have two of them, each one of them with a minus
- two for a total of minus four, so carbon must be plus four, plus four, which means that it has given
- up four electrons, right, and really it only has four electrons to give up, it has one, two, three, four
- valence electrons, in its second shell, which is its reactive shell, so its oxidation number is plus
- four. Now lets look at the water, we've done that a bunch, where the hydrogens each give up and electron
- and they have and oxidation number of one, each oxygen takes two electrons, there is only one of them,
- so its minus two, so whats going on here? Whats getting oxidized and whats getting reduced? You have
- the carbon, which went from an oxidation number of minus four to an oxidation number of plus four, so
- let me just draw whats happening to the carbon, carbon goes from minus four which means its hogging four
- electrons to a situtation where its having four electrons being hogged from it, its kind of giving away
- four electrons, so somewhere in this process, this guy must have given away eight electrons, this is
- a difference of eight electrons, so plus eight electrons, so what's happened to carbon, carbon has been...,
- has it been oxidized or has it been reduced, its charge had gone up, so it hasn't been reduced, another
- way you could say electrons have been taken away from it so it has been oxidized, oxidized, carbon had
- been oxidized, now lets look at the oxygen, oxygen, over here you have four oxygen molecules, I'll just
- write four oxygen molecules, and they all have an oxidation state of zero, because from an oxidation
- number point of view they are neutral, and on this side we have two, no we have four oxygen molecules,
- and what's there oxidation state? They're all minus two, they're all minus two, so oxygen, each of these
- oxygens must have gained an electron, actually, let me rewrite this reaction, let me erase a little bit
- of it, so each of these, actually even better let me just move this over, much better, move that over
- there you go, and move that over, now let me fill in that so asthetically its pleasing, there you go,
- alright, so we have four oxygens witha zero oxidation state turning into four oxygens with a minus two
- oxidation state, so each of those four oxigens took two electrons, there's four of them, so we must have
- we must have gained eight electrons, so what happened to oxygen, its oxidation number went sown, or its
- hypothetical charge went down, it was reduced, its charge was reduced, reduced, ocygen was reduced, what
- was it reduced by? It was reduced by the carbon. What was the carbon oxidized by? It was oxidized by
- the oxygen. Which oxygen tends to do, it oxidizing things, what is the oxidizing agent, well its the
- thing that did the oxidizing. So oxygen is the oxidizing agent. What is the reducing agent? What is the
- thing that did the reducing? It is the reducing agent, this is the carbon. you see in this redox reaction,
- carbon was oxidized was oxidized, it gave electrons from this state to that state, hypothetically, and
- oxygen was reduced, its charge was reduced by a total of eight electrons but two for each oxygen and
- so its oxidation number went down. Lets do a couple more, and each of these things i wrote, just to do
- a side note, this is called a half reaction because here I'm writing just what happened to the carbon
- , and here I'm just what happened to the oxygen. And I did ignore something, I did ignore the hydrogen,
- and I encourage you to do something like this with the hydrogen, but you'll see the hydrogen was neither
- oxidized or reduced, on both sides of this equation, all of the hydrogens had a plus one oxidation state.
- Let's do another one, this is another combustion situation, this is what happened to the hindenburg,
- they filled a balloon with hydrogen because they wanted it to float because hydrogen is a very light
- gas, unfortunately there must have been a spark, and in the presence of oxygen, and it combusted. and
- actually, this is for rocket fuel as well, if you have liquid hydrogen and, actually i don't think they
- have liquid hydrogen. Well, I don't know enough about rocket science, I'll have to do another video on
- that in the future, but lets look at the oxxidation states, well lets look at the oxidation state of
- hydrogen here in its elemental form, its zero, two hydrogens bonded to each other no reason why they
- should hog or be hogged by another hydrogen, two oxygen molecules or atoms bonded to each other, once
- again zero oxidation state, once they combust and form water, what's their oxidation states? Weve done
- this multiple times. This oxygen has a minus two oxidation state, each hydrogen has a plus one, so lets
- write the half reactions. We have two hydrogens that are just happy as they are; neutral state. And they
- end up with, being, no sorry, two H twos, they turn into two hydrogens, two hydrogen molecules cuz there's
- actually four hydrogen atoms,with a plus one oxidation state, so they must have each given away one electron,
- now hhow many total hydrogens are there? There are four. So we must have given away four electrons. Plus
- four electrons, and this is the half reaction for hydrogen. Now lets do the same thing for our oxygen,
- we have some oxygen here, on the left hand side it has a neutral oxidation state, and then it ends up
- with a , we end up with two oxygens on the write hand side, I can write it, I'll write it like this.Two
- O each with a minus two oxidation state so eac of these oxygens must have gotten two electrons, so it
- got two electrons each or it gained four electrons. So what was oxidized? Oxidized means electrons were
- taken away from you. the hydrogen was oxidized. This was oxidized, oxidized by oxygen. What was reduced?
- The oxygen was reduced by the hydrogen, and if you actually add these two reactions up, if you make this
- the left hand side of your equation and you make this your right hand sade of the equation, you can say
- "Ok" Let's remove the electrons from both sides and you'll end up with your original reaction. What was,
- and just to make sure our terminology is right, what's our oxidizing agent? It's the thing that did
- the oxidizing, it's the oxygen. What's the reducing agent? It's the hydrogen. Ket's do one more. So here
- I have iron, plus some Hydrochloric acid, let's say this is in an aqueous solution, and you end up with
- iron (II) chloride plus some hydrogen. So let's do some oxidation numbers, and I'll do it fast this time.
- iron is just by itself it's got a zero oxidation state. Hydrogen with chloride, chlorine's a halogen.
- haloge, these guys love to take electrons-they love to take one electron, they typically have a minus
- one oxidation state, so the chlorine is going to have minus one, the hydrogen is going to be plus one.
- Add them together you get to a neutral compound, fair enough. now you go on this side, what is chlorine's
- oxidation state? Well once again it always likes to take one electron. So this is minus one, but I have
- two chlorines here, this is a neutral compound so the iron oxidation state must be plus two. Plus two.
- What about the hydrogen? Well now it
- is jus in its elemental form, so it's going to be neutral and have a zero oxidation state. So what was
- oxidized? Our iron, our iron went from neutral to being, to having two electrons taken away from it which
- gave it a positive charge, so plus two electrons got taken away, so this was oxidized. This is oxidized.
- What about the hydrogen? The hydrogen went from two hydrogens with a plus one oxidation state and essentially
- they went to two hydrogens with a neutral state, so two electrons must have been added to the hydrogens.
- two electrons were added, their charge was reduced, so they were reduced. And what were they reduced
- by? They were reduced by the iron. What was the iron oxidized by? It was oxidized by the hydrogen. What
- about the chlorine, the chlorine has a minus one oxidation number here, it has a minus one oxidation
- number here, it was neutral relative to the redox reaction, anyway I think you get the point now, and
- you can do this with a bunch of reactions, but this will give you a little more insight of actually whats
- going on and who's gaining and who's losing electrons and in the future it will also help us think about
- alot of the structures of molecules. Anyway, see you in the next video.
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At 5:31, how is the moon large enough to block the sun? Isn't the sun way larger?
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