let's talk about these five major types of chemical reactions we'll look at examples of each and learn how to tell them apart so that you can look at a chemical reaction and classify it figure out what type it is the first reaction we're going to talk about is the synthesis reaction these are sometimes called combination reactions now synthesis is just a fancy word that means making and that's exactly what happens in a synthesis reaction a compound is made from simpler materials here's an example carbon comes together with oxygen gas to make carbon dioxide CO2 what we make in this reaction is more complex than the two simple things that we start with here's another example we take sodium and chlorine gas cl2 and that makes sodium chloride NAC what we end up with is more complex than the simple things we started with now a quick word about the equations that I'm using in this lesson some of these equations I'm going to be talking about like this one right here are unbalanced so there might not be exactly the same number of atoms on both sides of the equation now normally it's really important to balance equations but when we're going to be learning about the different types of reactions the balancing numbers can be a little bit distracting so here I just want you to focus on the elements and how they're rearranging or combining with each other in different ways that being said a synthesis reac is one where we start with simple materials and put them together to make something more complex if we want to represent a synthesis reaction more generally or more generically we could say that it looks kind of like this we have a and b combining to make a here A and B are different elements or they're different compounds coming together to make something more complex so that's a synthesis reaction let's move on a decomposition reaction is kind of the opposite of a synthesis reaction in a synthesis reaction we put things together in a decomposition reaction a compound is broken down into simpler compounds or all the way down to the elements that make it up so in this example here we have water H2O and it's breaking down into hydrogen and oxygen gas these are the elements that make it up now in a decomposition reaction you don't have to break things down all the way down to their basic elements you can also break them down just into simpler compounds for example here we have ca3 calcium carbonate and that gets broken down to two simpler compounds CAO and CO2 it's not like we're taking this and breaking it down into just calcium and just carbon and just oxygen but still because these are simpler compounds it is also a decomposition reaction so if we wanted to come up with sort of a generalized way to write a decomposition reaction we could could write it like this AB breaking apart into a plus b where AB is some kind of compound and a and b are simpler compounds or elements okay combustion reactions combustion is basically a fancy word for burning and when something Burns what happens is that a compound containing carbon and hydrogen and sometimes oxygen combines with oxygen gas to produce carbon dioxide and water here in my example CH4 which is the chemical formula for methane that's a type of natural gas combines with oxygen and it forms carbon dioxide and water H2O now we can start with different things in a combustion reaction and as it says here the compound usually contains carbon and hydrogen so here is another example of a combustion reaction this one starts with C3 h8 which is the chemical formula for propane another type of natural gas and just like with this reaction we combine c38 with O2 and this gives us carbon dioxide and water so these two reactions are essentially identical except for the number of carbons and hydrogens in the compound that we start with it turns out that a lot of things that we burn like natural gas diesel gasoline are really really similar and they only really differ in the number of car carbons and hydrogens that are in the molecules that make them up so combustion reactions for a wide variety of compounds look pretty similar now as this definition says sometimes we have oxygen in the compound that we're burning here is one example of a combustion reaction that has oxygen in it this is the chemical formula for ethanol or ethyl alcohol and you can see that just like these it has lots of carbons and hydrogens except it also has oxygen as well but that's no big deal because it combusts just like the other two by combining with oxygen and making CO2 and H2O so if we wanted to come up with a general way to write the formula for a combustion reaction it might look a little bit like this we start out with something that has carbon and hydrogen in it and we can have different numbers of carbons and hydrogen so that's why I put this X and Y here because the number of carbons and hydrogens varies and it doesn't really matter either some times the compound has oxygen in it which is why I put the oxygen here in parenthesis we take this compound it combines with oxygen and it produces carbon dioxide and water so this here is the generic General equation for combustion reaction single replacement reactions break my heart and in a minute you'll see why so in a single replacement reaction what happens is it one element that starts out by itself replaces another element in a compound kicking it out and here's an example to show you what I mean we start out with iron Fe which is this element that's by itself and iron combines with ccl2 which is copper chloride okay so copper and chloride are paired up here but what happens is iron kicks out the copper the CU so the CU ends up by itself and the Fe the iron takes the place of that compound so now the iron and the CL they are now paired up now I like to use a dance analogy to explain this and it reminds me of something that happened all the time in high school here's what's going on we have a dancing couple the purple and the green and they're so happy dancing together or at least the purple guy is pretty happy dancing and then red comes along and red is like hey purple I'm so much cooler get out of the way I want to dance with green and so poor purple gets booted out and red ends up dancing with green purple ends up all by himself standing up against the wall pretending to text pretending to play a game on his cell phone but you know he's really actually sad CU he's just been booted from this dancing couple so you'll see that this is exactly what's going on in the single replacement reaction right Fe iron is like red here coming up to a dancing couple of Cu and cl it boots out cu cu ends up by itself and then red Iron takes the place that CU had and iron ends up paired with cl here's one more example of a single replacement reaction you can see how this works CU in this case is the red character and CU goes to a dancing couple of AG silver and nitrate ag3 CU boots out AG so AG ends up by itself and CU takes AG's place by pairing up with NO3 so that is how a single replacement reaction happens and a generalized reaction for that would look like a which is the element that starts out by itself plus BC that's the dancing couple and then that gives us B by itself which is this element that got booted out and then a and c ending up paired together so that's a single replacement reaction so finally here's the double replacement reaction now I should mention that single and double replacement reactions are sometimes also called single displacement and double displacement just in case your teacher at textbook uses a different term for them okay so double replacement reactions are not nearly as heartbreaking as single replacement reactions here's why because in a double replacement reaction what happens is the positive and negative ions in two compounds just switch places nobody gets kicked out in a double replacement reaction in a double replacement reaction it's just like you have two different pairs of dancing couples and the red which used to be with a gray ends up with a green and the purple which used to be with a green ends up with a gray nobody gets kicked out nobody sat up against the wall with their cell phones we're just switching dancing partners so here is a chemical equation that shows a double replacement reaction we start out with ba and cl together and then na and S so4 together and they just switch places so ba ends up with s so4 there it is and na ends up with cl there it is right there all of these compounds are ionic which means that we can break them down into the positive and negative ions that they're made up of so here the positive ions are in purple and the negative ions are in green and as you can see the positive and the negative just switch places so ba2 plus and cl minus were initially paired up but then ba goes and it gets switched it finds a new dancing partner it has to find the other negative ion right so the other negative ion here is s so42 minus so ba2 plus and S so42 minus end up together making baso4 and then sodium na1 plus has to find the other negative ion which here is CL min us and they end up paired up over here na1 plus and cl1 minus making NAC here's another example of a double replacement reaction okay I'll break this down into its ions right away and we get this we start with k+ and BR minus paired together and ag1 plus and n31 minus paired together and then they just switch the positive and negative so k+ goes and finds the other negative ion which here is n31 minus k+ and n31 minus end up together and ag1 plus silver looks for the other negative ion which is br1 minus and ag1 plus and br1 minus end up paired up together making AG BR so that is a double replacement reaction and if we wanted to come up with a general or generic way to explain it we could use this reaction here where we have AB where A and B are paired up plus CD where C and D are paired up and then they switch Partners to give us a d and BC so that is a double replacement reaction so these are our five major types of chemical reactions in synthesis simple things combin together to make something more complex in decomposition something complex breaks apart into simpler pieces in combustion a compound that contains carbon hydrogen and sometimes oxygen comes together with oxygen gas to make carbon dioxide and water single replacement and double replacement are our two Dance Floor reactions in single replacement an element that's by itself combines with two elements that are paired up it kicks one of those elements out so that element ends up on its own and then that element takes its place in double replacement it's like two dancing couples where the partners just trade places A and B and C and D start out paired together and then A and D end up together and B and C end up together so those are the major types of chemical reactions in the next video we'll do some practice problems so you can look at a bunch of different reactions and figure out what type they are