oxidation reduction reactions aka redox reaction is going to be the topic in this lesson and we'll start by defining oxidation and reduction we'll find out that these are just electron transfer reactions one species is going to lose electrons one species is going to gain electrons and we associate each of those names with one of those we're going to find out that determining oxidation states will be pivotal here because that's what we're going to use to determine who gains and who loses electrons so we'll go through those rules for assigning oxidation states and then finally we'll finish this lesson off talking about single replacement reactions a type of redox reaction and we're going to learn about the activity series which is going to help us determine whether or not these single replacement reactions actually happen or not the word we use is whether or not they are spontaneous or not my name is chad and welcome to chad's prep where my goal is to take the stress out of learning science this lesson is part of my new general chemistry playlist i'm releasing these lessons several a week throughout the school year so if you want to be notified every time i post a new one subscribe to the channel and click the bell notification now in addition to high school and college science prep we also do mcat dat and oat prep as well you can find those courses at chatsprep.com all right let's dive into this here so we've got to start with an example of a redox reaction here or an oxidation reduction reaction uh the one i've given here and we've got to define some terms here which are going to be oxidation and reduction and oxidation is the loss of electrons reduction is the gain of electrons and we have some lovely little mnemonics here uh to help us remember those and oxidation is the loss of electrons reduction is the gain of electrons so you got oil rig my personal favorite is leo the lion says ger it's just more fun to say for one but also it includes the letter e which stands for electrons lose electrons oxidation gain electrons reduction whereas here we're just saying oxidation is loss well loss of electrons reduction is gain well gain of electrons but here it's incorporated that we're talking about electrons in the mnemonics that's going to be important here so if we take a look here one of the first rules we'll learn about oxidation states that any element in its elemental form not a charged form but elemental form is in the zero oxidation state so in this case that's going to correspond here to both zinc on this side and copper on this side and we'll also find out that from monatomic ions like the copper and zinc ions here their oxidation state corresponds to their charge here so in this case the oxidation state would be plus two and in this case the oxidation state would also be plus two and so we're going to look at for is the change in oxidation state as evidence that electrons have either been lost or gained and so in this case you gotta think about this a little bit different than you normally think of losing or gaining something if you lose money out of your bank account then the value of your bank account goes down if you gain money of your bank account then the value of your bank account goes up but when you lose electrons electrons are negatively charged and if you lose negative charges your charge or oxidation state actually goes up and when you gain electrons gain negative charges your oxidation state or charge goes down and so it seems a little bit backwards but that's where that terminology comes from so just keep that straight so if we take a look at what happens with zinc here zinc is going from zero to plus two it's getting more positive and so we might wanna think well did he lose negative charges or gain negative charges well again if you gain negative charges you're not gonna get more positive so in this case he must have lost negative charges and we would say that zinc here got oxidized now on the other hand if we take a look at copper copper is going from copper two plus to copper in the elemental form so from the plus two oxidation state to the zero state and now we can see that he has gained two negative charges two electrons and so we say that copper two plus here has been reduced and if you notice that's why they actually call it reduced because it's your oxidation state that's getting reduced your oxidation state is getting lower so cool whenever you've got an oxidation reduction reaction you're always going to have at least one oxidation and one reduction technically you can have more but you're not likely to encounter them just yet in this chapter i will warn you uh we will have an entire chapter on electrochemistry which is going to be really all centered around oxidation reduction reactions but at the very end of this playlist towards the end of the gen chem 2 topic so this chapter is really just this really this lesson in this chapter is just going to introduce uh the topic here of oxidation reduction reactions now on the study guide here i have provided some rules for determining oxidation states and when we cover this again in the electrochemistry chapter at the end of the playlist i'll give a little more robust set of rules here but for now this is a pretty adequate set of rules and you follow these rules in order and the first rule just says that if you have an element in its elemental form it is in the zero oxidation state and so in this case these are the only two elements that are on the board so everything else up here is either a compound or a polyatomic ion and so these are the two elements so one's monatomic one's diatomic but notice there's no charge on either one of them and that's the evidence that you've got an element in its elemental form and they're in the zero oxidation state that is rule number one now rule number two talks about monatomic ions so not polyatomic ions but monatomic ions which implies you have an ionic compound like nacl here and these are both monatomic ions not polyatomic ions and when you've got monatomic ions they just get their normal ionic charge now key thing you're gonna have to realize though is that if you've got transition metals or at least non-group one and group two metals that can take on variable oxidation states variable charges you'll have to figure them out based on what anion they're with so but in this case sodium is from group one and in an ionic compound they're always plus one is ions chloride being a halogen so they're always minus one as monatomic ions and voila there you go so monatomic ions just based on where they're located on the periodic table or again if we had a transition metal like if we had copper let's say and you had cu cl2 so copper can sometimes be plus one and plus two and it depends on context now i don't expect you to know that it's either plus one or plus two for most students but what you could figure out though is that up chlorine as a monatomic ion the chloride ion is minus one and there's two of them for a total of minus two and therefore the copper must be plus two to balance and that's what i mean that you'll have to figure out the transition metals oxidation state based on what anion they're with and how many and stuff so all right we'll move on to sodium bicarbonate here and so in this case sodium is still a monatomic ion and we still have one of them and he's still going to be plus one but now we've got bicarbonate and that's going to be a little bit of a pain in the button so the way this works uh after you look at elements in their elemental form which obviously we have a compound here so none of those and after we look for monatomic ions we got sodium he's one of those then you're going to start looking for some of the regular non-metals if you will and so some of the ones i put on your sheet is oxygen is usually minus two unless it's part of a peroxide in which case it's minus one we've got hydrogen which is typically plus one when it's combined with any combination of non-metals and when it's with only metals that's when it ends up being minus one um and so actually we can assign him now so hydrogen's with some combination of nonmetals had this just been sodium hydride and with hydrogen with a metal only that's when it would have been minus one but if it's present with any other non-metals in the compound it's going to be plus one all right and we got fluorine is going to be minus one always in a compound when it's not in its elemental form and then halogens are usually minus one except when they're part of the oxoanions with oxygen then they're going to be something else all right so we said that oxygen when he's not part of a um peroxide is minus two we'll go from there and then one thing you should note we're going to keep a tally here there's three of these negative twos that's a total of negative six there's just one of the hydrogens that's plus one so it's plus one total and there's only one of the sodiums that's plus one total and the idea then the last rule just says that for a neutral compound all of the oxidation states have to total up to zero and again that factors in not just the oxidation individual oxidation state per atom but for how many you actually have in the molecule or in the compound as i should say in this case and so in this case carbon we can see has to balance out the rest of this we got more negatives than positive so he's going to need to be positive and in this case we can kind of see that he's going to need to be positive 4. and so this is kind of the approach we take assign you know any monatomic ions in the compound then go and look for the regulars like oxygen hydrogen halogens and then anybody that's left over is going to have to balance out the rest of the charge now you could also make an algebra equation out of this you could just say sodium plus hydrogen plus carbon plus 3 times whatever oxygen is has to add up to zero and so in this case we figured out that sodium was plus one hydrogen was plus one carbon we didn't know yet he's the last one to assign and then oxygen was minus two each and if this all has to add up to zero then we can figure out algebraically solving for this is kind of like it's x if you will solving for carbon it would indeed come out to plus four okay let's get a little more challenging here so here we've got carbon dioxide and in this case there's no mild atomic ions because it's not an ionic compound here so uh definitely nothing in their elemental form because it's a compound and no monatomic ions so then we look for the regulars so oxygen hydrogen halogen and we've got oxygen so we're going to make oxygen negative 2. cool and carbon's just going to have to balance the rest of this out this is a neutral compound so it's going to have to balance it out to zero and that's going to make carbon here since there's two of these negative two options a total of negative four carbon's going to have to be plus four to balance that out okay this is hydrogen peroxide i want to point this out because it is a per oxide and this is that case where the peroxide where oxygen is minus one instead of minus two so in a compound oxygen's almost always minus two but peroxide is the most notable exception where it's minus one now how do you recognize peroxide because a lot of students memorize it when you have two oxygens well we just had two oxygens so and this is not often spelled out all that well but the way this works so oxygen uh well in this case hydrogen's irregular but oxygen is two but uh the way you're gonna recognize it as a peroxide is one it does have two oxygens and it's gonna be with either hydrogen or a group one metal is the most common by far and so here it's with hydrogen two oxygens with hydrogen great that's a peroxide and so in this case that's going to make oxygen minus one and we said that hydrogen here is always plus one when it's with non-metals and so this always would be the other way to figure it out if hydrogen is always plus one with non-metals well to balance this out then the auction would have had to have been minus one not minus two for this to balance out to zero total all right i want to make sure we did one example of an ion rather than a neutral compound and the big difference here is that instead of all the individual oxidation states summing to 0 in this case for sulfate ion it's going to sum to negative 2. all right so nobody in their elemental form because it's a polyatomic ion not an element uh and in this case we've got no monatomic ions it's all you know everything here is just a simple uh uh or simply part of a polyatomic ion uh so then we look for the regulars and oxygen is the regular here and is going to be negative two so and in this case there's four of them which means we get a total of negative eight now one thing to note you might get a question on a test that says what's the oxidation state of oxygen and oxidation state is always defined per individual atom and so in this case the right answer would be negative two not negative eight so there's a total of negative eight charge here but oxidation state per oxygen is still negative two per oxygen that's again how oxidation state is defined and so then finally sulfur is going to have to balance the rest of this out but again if we were balancing it out to zero well then we could see he needs to be plus eight but we don't need to balance it out to zero now we need to balance it out to negative two in which case we can see that sulfur is going to have to be plus six instead because six minus eight is negative two and once again you could make an algebra equation out of this and you could say sulfur plus four times whatever oxygen is doesn't add up to zero now but adds up to negative two and we already figured out again that oxygen was negative two and four times negative two is negative eight and you could solve it algebraically and get sulfur big positive six in that fashion as well all right so now we want to spend just a couple of minutes identifying these oxidation reduction reactions aka again redox reactions and question i'm giving you here is which of these four is or are redox reactions so well the hallmark again of a redox reaction is that somebody loses electrons somebody gains electrons they are electron transfer reactions and the hallmark of you know having species both lose and gain electrons is a change in oxidation state and so one way you could approach this is you could just start assigning oxidation states for all the elements and all the compounds in all four of these reactions uh but that's going to be kind of a slow and laborious process uh not the fastest if you're trying to do this in a hurry and in a pinch on an exam so we're going to go through some shortcuts a little bit of how you might pick out some of these redox reactions and how you might eliminate some of these as not being redox and so first thing i'll tell you is this look for elements in their elemental form that's the first thing so like if we look at the first one here we've got both magnesium and oxygen both in their elemental form and odds are they're not going to be in the elemental form on the other side in fact they're not now they're part of a compound and if you're in the elemental form on one side well that means you're in the zero oxidation state and if you're not in the elemental form of the other side well that's gonna typically almost always mean you're not in the zero oxidation state and so we don't even have to assign oxidation states but we've got some that start out as zero and end up not zero that's a change and that is a redox reaction so yes this is a redox reaction and if you want to figure it out so in this case you're going from zero in magnesium's case to plus two and he got oxidized where's oxygen is going from zero to minus two and he got reduced all right so the next one here we don't have any elements in their elemental form so we'll skip it the next one we don't have any elements in their elemental form so we'll skip it and the last one here we've got copper in his elemental form so he's in the zero oxidation state and silver over here both in their elemental forms and so oh copper's in his elemental form here he's part of a compound here not in his elemental form he changed don't have to figure out that he's plus two and went from zero to plus two and got oxidized all i have to see is that he's zero and now he's not zero that's a change and this is going to be a redox reaction now we can take this a step further so it turns out that this you might recognize as a single replacement reaction we're going to spend a little more time talking about those in just a little bit but single replacement reactions are a type of redox reaction so they are always going to be redox reactions so if you see this even if you didn't recognize that you had stuff in the zero oxidation state would be like oh that's single replacement that's redox great so not all redox reactions are single replacement but all single replacements are redox all right so so far so good we instead of having to assign all the oxidation states in these two at least we just quickly figured out that yes they are redux because we had elements in their elemental form in the reaction all right now for the other two you might recognize what kind of reactions these are this first one is a decomposition reaction going from one species to two species so and then this next one you might recognize as a double replacement reaction so we're cations and energy trading partner and if you recall that was actually the object of the last lesson and so in this lesson we're talking about redox reactions in the last lesson we talked about double replacement reactions and the reason they're taught in separate lessons is because they're separate double replacement reactions are never redox and so in this case once you recognize it as double replacement you'd be like oh no that's not redox so you could take it that far if you actually went through to assign things what you'd find out is that here silver is plus one and here silver is still plus one here chlorines minus one and here chlorine's still minus one here hydrogen's plus one and hydrogen is still plus one and as long as the polyatomic ion doesn't change it's nitrate and it's nitrate so you could figure out that nitrogen oxygen are gonna be the same but you could also go a step further and figure out that oxygen's negative two and nitrogen's plus five and that is still true over here but notice that's a lot of work so and i've got a lot of practice so i can do it quickly but it's a lot of work assigning all those oxidation states but again if you recognize it as a double replacement you can be like oh yeah those are never redox all right now i don't have a rule for this decomposition reaction for you so um a lot of decompositions tend to not be redox but i don't know that i can say within you know any kind of absolute uh definitive statement that they're never redox i just don't know that that's true so if you go and assign oxidation states in this case you've got no elements of their elemental form but you do have uh monatomic ions and here calcium is a monatomic ion and it's a monatomic ion being in group two is plus two still a monatomic ion right there so it's still plus two so on the other side you also got oxygen as a monatomic ion and he's minus two uh being two elements away from the uh the noble gases all right for the rest of these uh you wanna assign the regulars now either in a molecular compound or as part of a polyatomic ion and again the regulars are oxygen which is typically minus two like he is here as well as here uh and if we had any hydrogen or halogens we'd be starting to look for those as well but we don't and now in both these compounds we only have one element left and this one's gonna be a little easier you have two oxygens that are negative two each for a total of negative four so carbon's gonna have to be plus four and over here we've got three oxygens that are negative two each for a total of negative six the calcium's plus two we need an additional plus four to balance this out to zero and so in this case nobody changed calcium stayed plus two oxygen stayed minus two and carbon state plus four and with no changes in oxidation states this is not a redox reaction cool so you're going to want to get very proficient at assigning oxidation states and this is one of the reasons why not only can you expect on the exam on this section to just simply have to find the oxidation state of a certain element for a certain question but you might also have to identify when a reaction either is or is not a redox reaction so now we've got to spend just a few minutes on single replacement reactions which again are a type of redox reaction and before we discuss these single replacement reactions i just want to point out what is often provided in this context which we call the activity series which is on the study guide there and the way the activity series works is they put the most active metals or in this case what we call the most reactive in an oxidation reaction at the top of the list and the least reactive at the bottom and so we like to say that they get more easily oxidized as they go up the list so if i gave you say lithium going to lithium plus one plus an electron versus potassium going to potassium plus one plus an electron and i said which of these reactions is more easily going to happen well in this case again you went from zero to plus one those are both oxidations in this case i can see that lithium is more easily oxidized than potassium and so this first reaction would be the more likely one the easier one to happen if you will and so this is going to become useful when we start talking about these single replacement reactions in just a second all right so once you're given this activity series again you're given it for a purpose but you might just get a question on your exam that just says hey which of the following is the most active metal or which of the following is the most easily oxidized and either case you're picking the one that's highest on the list all right so that's your activity series we typically don't make students memorize this this is almost always something that students are provided with if they're going to need it all right so we've got three single replacement reactions here uh and if we look we got uh in this case they call it single replacement if you look because here we've got a happy couple h and cl and here magnesium is all alone but magnesium and chlorine are going to end up together and then hydrogen is going to end up all alone and so magnesium is replacing or displacing hydrogen in being with chlorine and so that's where it gets its name single replacement single displacement and if you look what's nice is there's another way to look at this that might make it easier to remember but it's not going to be quite as helpful as from an understanding perspective but it's the way i like to look at this initially and i like to say you know magnesium thinks chlorine is so cute and magnesium would like to be with chlorine but hydrogens with chlorine right now so magnesium is going to replace chlorine in this relationship and leave hydrogen all alone it's kind of sad so here's the deal though the only way this is going to happen the only way magnesium is going to replace hydrogen in this relationship is if magnesium is better looking than hydrogen so and the best looking metals or elements because hydrogen's not a metal the best looking elements are at the top of the list now one thing to note i didn't have room i didn't space this well enough to get the entire activity series there was a couple of metals left off the bottom but this will work for our purposes but in this case magnesium is a better looking metal than hydrogen and so yes this reaction is spontaneous we'd say it will happen so when you drop a solid piece of magnesium and aqueous hydrochloric acid what you'll find is that the solution starts to bubble as a gas is produced in this case that hydrogen gas and that magnesium solid dissolves away into the solution turning into part of the aqueous part of that solution cool so it's spontaneous all right so if you look at what's really going on in the real appropriate perspective here is if you look magnesium starts out in the zero oxidation state and ends up plus two whereas hydrogen starts off in the plus one oxidation state and ends up at zero and so the question is who's getting oxidized who's getting reduced well again the one getting reduced is the one where the oxidation state gets reduced and that's hydrogen going from plus one down to zero that's a gain of electrons gain of negative charges so whereas magnesium is the one being oxidized and that's the one we're really going to focus on because that's what the activity series is going to give us some details about and so the way this works and the way you should formally theoretically understand this is that the species that gets oxidized needs to be the species that is more easily oxidized and so in this case out of magnesium and hydrogen that are both either gaining or losing electrons it is the magnesium that is getting oxidized so therefore magnesium has to be higher than hydra on the list because the species can dioxized has to be the species that is more easily oxidized all right so if we look at the next one here so again i like to think that the one that starts out all alone has to be better looking to displace the other one or replace the other one and so has to be better looking for the reaction to happen in this case is copper better looking than zinc well here's copper down here here's zinc up here and actually zinc's better looking copper is not better looking and so this reaction we can write it all day long but it's not actually going to happen and we would say that it is non-spontaneous and so in this case if you take a solid piece of copper and put it in an aqueous solution of zinc nitrate it will just sink to the bottom and sit there this reaction as written does not happen cool now again the way you're supposed to look at this is you're supposed to look and say okay copper goes from the zero oxidation state to plus two and that means he's the species getting oxidized and so he's got to be higher on the list than the species getting reduced for this to happen and it's zinc going from plus two to zero that's obvious reduction and once again though the species that's getting oxidized is not the one that's more easily oxidized and that's why this reaction is non-spontaneous why it's not going to happen all right one more so this one the spectator ions have been eliminated like in the this reaction right here the nitrate ions are actually spectator ions and wouldn't appear in the net ionic equation and this one up here the chloride ions are spectator ions and wouldn't appear in the net ionic equation and if the spectator ions are left out it's hard to see why they even named it single replacement or single displacement here uh and remembering that the best looking metal you know uh is higher on the list doesn't actually help you because there's no happy couple anywhere on here and so you actually do need to understand it from an oxidation perspective and say okay who's getting oxidized and typically that's the one that starts off in the elemental form in the zero oxidation state that's going to be getting oxidized as is the case right here and so manganese is going from zero to plus two he is the species getting oxidized whereas the nickel that's getting reduced and going from plus two to zero and again the species getting oxidized has to be the species that's more easily oxidized and higher up on the activity series so what we're saying is that for this reaction to be spontaneous manganese manganese not magnesium manganese has to be higher on the list than nickel and in this case manganese is right here nickel's right here manganese is indeed higher on the list than nickel and this reaction is going to happen it is going to be spontaneous cool so quickly you know in a pinch if we're not showing the net ionic equation but just the complete molecular equations then by all means so whoever's doing the replacing whoever starts out all alone has to be higher on the list life is good however you really do want to understand it from an oxidation perspective and again whichever one is getting oxidized has to be the ones more easily oxidized and higher on the activity series now if you found this lesson helpful a like and a share goes a long way to making sure other students get to see it and if you're looking for a lot of practice or final exam reviews and practice final exams check out my general chemistry master course i'll leave a link in the description a free trial is available happy studying