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Introduction to Thermochemistry and Enthaply

Introduction to Thermochemistry and Enthaply
the other day we talked about different types of energy and how we can convert between them so for example there's potential energy which is a type of energy and something has based on its position a car at the top of a hill for example has a lot of potential energy we can release the the brake on that car and push it rolling down the hill and eventually that cars you get more and more kinetic energy which is the type of energy that something has if it's moving there's also type of
introduction to thermochemistry and enthaply
energy called chemical energy which is stored in in the bonds between atoms and molecules gasoline for example has a lot of chemical energy in it and if we drop to a lit match into a big tank of gasoline we wouldn't want to be any near anywhere nearby but what we do is we convert a lot of that chemical energy in the gasoline into thermal energy now it's particularly important that you understand energy and how we can convert it so if you're a little bit shaky on this topic I really
suggest you go back and look at yesterday's lesson or the last lesson that we looked at which was called energy conservation and conversions take a look at that and once you feel really comfortable with it then then go ahead and come back to this but if you do feel pretty clear with energy and how we can move from one form to another let's go ahead and get started we talked about thermal energy for a second that's going to be the main topic of our evaru of our lesson today thermal
energy when it flows from one object to another we call it heat let's imagine that we've wrapped our cold hands around a hot cup of chocolate a hot cup of hot chocolate let's think about the way heat is going to flow heat is going to flow from the hot cup of hot chocolate into our cold hands and that's the way he always flows heat flows from a hot object to a cold object and the heat flow stops when the two objects are the same temperature so after the hot chocolate has
sufficiently cooled and it's worn my hands and they're the same temperature no heat flow occurs in because he always flows from hot to cold we would never see a situation like this imagine you wrapped cold hands around a steaming cup of coffee and found them getting colder and colder because he was getting sucked into the coffee I would never never have because he never flows from a cold thing to hot thing it always flows from an area of higher temperature to lower temperature now
thermal energy is particularly important in chemistry because after all what would chemistry be if it weren't for for huge explosions and big fires everything as a chemist I get to do stuff pretty much every day that would make my mother shudder so don't show her these videos she she come here the school she pulled me out of the room giving a stern talking-to in the hallway and probably let me probably make me quit my job so so make sure she doesn't see these but all joking aside um
safety is important to keep in mind I have a couple demonstrations planned here that are going to show us how thermal energy is is involved in some reactions and these are going to be pretty cool but I want to let you know that I've taken a lot of safety precautions these are not things that that you should do at home for example I'm going to be wearing goggles which are going to protect my eyes in case anything goes wrong and in addition I know what I'm doing I've taken a lot of
time to plan these demonstrations to make sure that they're they're going to work out safely and I've taken time as well to prepare the chemicals and so forth so that so that nothing goes wrong in addition I'm going to be handling a little bit of acid in this first demonstration somebody will be wearing gloves and on top of that I'm going to be performing a whole demonstration in this what's called a fume hood or behind this piece of plexiglass so that if anything does go
wrong which I'm not expecting it to the glass will protect me in this first demonstration which is one of my favorite to do I mixed a chemical called potassium chlorate together with some common table ship we're basically going to see how much thermal energy a small amount of sugar contains I mix these two compounds together they're both white grayish powders in this dish here and I'm going to just pour it out onto this onto this plate here spread it out so the more it is up is
introduction to thermochemistry and enthaply
exposed to the air that looks pretty good now just like that no reaction is happening but if I put just one drop of sulfuric acid on here we'll get the reaction to start let's get a little bit of sulfuric acid here and take a look at what's going to happen that's pretty cool isn't it as you can see when I added that sulfuric acid a tremendous amount of heat was released from this reaction this is what we call an exothermic reaction EXO is a Greek prefix that means out or out
of and so what I added that a little bit of sulfuric acid to the potassium chloride in the sugar a tremendous amount of energy came out of the reaction so an exothermic reaction is one which releases energy into the environment let's now take a look at another type of experiment that also deals with thermal energy but in a slightly different way we just saw an example of an exothermic reaction one that released a tremendous amount of heat into the environment let's take a look at another
type of experiment what I'm going to do here is add two compounds into this flask what I have here is some ammonium chloride pour that in I'm going to mix it with some of this barium hydroxide that I've just weighed out well let's get that in here that should be should be just about enough and what I'm going to do is I'm going to begin to mix this stuff and stir it together now difficult for you to see but this is becoming sort of more and more liquid as I stir around
there's another change that's taking place this is getting very very cold in my hand let me show you what I can do I'm going to put just a drop of water onto this board here and stick this bottle right on the board like that I'm just going to let it sit there for now we don't really have enough time for this to work so I already didn't want ahead of time just like a cooking show let me show you what eventually happens then as you can see the board is completely stuck to
this flask because the solution in here has gotten so cold that it's been able to freeze the water and freeze the board right to this flask this is what we call an endothermic reaction it's the opposite of an exothermic range in an endothermic reaction the reaction that's going on in this flask is sucking heat in from the environment endo is a greek prefix that means in or in - so it makes sense that an endothermic reaction is one that sucks heat in from the environment into the
reaction mixture so that's why an endothermic reaction often feels cool because it's sucking energy for example out of our hands now let's go back to the classroom review these concepts and see how we can use some mathematical and graphing techniques to understand these ideas in a little greater depth in order to look at exothermic and endothermic reactions and a little bit more depth we've got to define a few new terms the first of these are the idea of system and surroundings
they're not particularly tricky concepts but they pop up almost anytime we want to talk about thermal energy and thermal energy transferring between different things a system is anything that I choose to focus my attention on so for example if there's a reaction that I'm interested in in this beaker I can consider the vigor to be the system and everything else surrounding it the whole rest of the universe is considered this roundings if I'm interested in a particular cow or field
I can consider that cow to be my system and everything else that universe is a surroundings well how does it relate to the reactions that we looked at at an example for of an exothermic reaction for you I said that it created a huge amount of heat that it let out into the environment this is true but a more precise way to define an exothermic reaction would have been to say that an exothermic reaction releases heat from a system into the surroundings and these red arrows here show the direction
introduction to thermochemistry and enthaply
of heat transfer in an exothermic reaction in the demonstration that I did the system was that little pile of chemicals that I add on a plate and the surroundings were where everything else but most noticeably the the air close by because I could actually feel that air getting hot because of the heat being transferred to it from the system on the other hand an endothermic reaction is one in which thermal energy is transferred from the surroundings into the system the second demonstration that I
did the flask was able to freeze the water those that it was placed on because it was pulling heat energy out of that water because the water is part of the surroundings and into the reaction it was going on in the flask let's now take a look at how we can use some graphing techniques to to better understand the ideas of exothermic and endothermic reactions but before we do there's another vocabulary order that we have to define this is the term enthalpy enthalpy is one of those terms
that life like dating or going out it's a lot easier to understand than it is to define a textbook would apply say that enthalpy has to do with the heat content of a particular system what enthalpy isn't so much about whether something is hot or not or what its temperature is although that is slightly important it's more about its potential to create heat it's about a systems it's about the amount of energy that's in a particular system so for example a log has a lot more
enthalpy in it because it's able to produce the more than for example ashes on at the same time a heat pack that we haven't yet used has more of a potential to create heat than that heat pack after it's after it's released its heating after it's finished with a reaction so let's look at a graph of enthalpy and see how it changes during a particular reaction let's first look at an exothermic reaction here's my graph and I'm going to put F of P on my y-axis here
we often use a term H the letter H to denote enthalpy and enthalpy increases as we go up the y-axis so burning a log would be a really good example of an exothermic reaction we said that relatively speaking a log because it has a high potential to create heat has a pretty high enthalpy so let's put it up here after we've burned that log we get a variety of products and we get carbon dioxide and water and ashes because combustion of a log is is rarely complete so that's why we get the
ashes these don't have as much of a potential to create heat as the log did so they have a lower enthalpy so I'm going to mark them down here here we have co2 water and ashes in science we're often concerned about the change that has taken place during a process the difference between the beginning in the end and scientists have a very fancy term for this they like to use the Greek letter Delta to talk about the change in this particular reaction I'm interested in the Delta H or
the change in enthalpy that takes place from the beginning to the end of the reaction now the reactants had a pretty high enthalpy the products have a pretty low angle P so let's look at how this changes enthalpy in this case doesn't go up it goes down so I can say that this has a negative Delta H for an exothermic reaction because it's giving off heat into the environment if this is a tricky concept for you let's use an analogy with money let's imagine that we're really
rich so we have a high starting value for the amount of money that we've got and then we go and we spend a lot of money putting that energy into our environment the same way an exothermic reaction releases heat into it into its environment so we go when we spend money on everything that we see we we buy sunglasses on the street we buy Louis Vuitton handbags you buy newspapers we give money to everybody who's asking for spare change because we're giving out all this money what happens
is the amount of money that we end up with is lower than the amount that we started with so because of that we could say that our Delta money the change in the amount of money that we have from the beginning to the end of the process is a negative value same thing happens with you it's a negative change in enthalpy value because all that heat was released to the environment now let's take a look at a graph of an endothermic reaction we'll set this up the same way put my my enthalpy
value right there an endothermic reaction is isn't there in in many ways the opposite of an exothermic reaction a good analogy for an endothermic reaction would be someone who goes and tries to steal as much money as they can so they're pulling money out of the environment and they're holding on to it they're keeping it for themselves so somebody who starts out without much money but then then steals it or takes it from their environment ends up with more energy with sorry with
more money at the end than they had at the beginning an example of that chemically might for example be a cold pack a cold pack feels cold because there's a reaction taking place in it that's pulling in energy from the surroundings that cold pack happens to be placed on your skin it feels cold because it's pulling heat out of your skin a cold pack starts with a relatively low enthalpy so we'll put it down here cold pack but during the process of an endothermic reactions we said
thermal energy is being pulled in from the surroundings and then a cold pack the chemicals in that cold pack hold on to that heat energy that they pulled in so because of that they raise the amount of enthalpy that they have in because they're pulling it in from the environment so at the end of the reaction a cold pack has more heat energy inside of it that it had when it started so we'll put used cold pack up here again we're interested in the change in enthalpy that Delta H of this
reaction that goes up or whether goes down in this case as we can see our products that we end up with have more enthalpy than our reactants so the change in enthalpy from beginning to end or the Delta H of this reaction has a positive value endothermic reactions have a positive Delta H and exothermic reactions have a negative Delta H be sure that you be sure that you understand this and you understand the graphs and the terms before you go on this is really important stuff and we're going
to keep coming back to it for about the next week or so so if you're a little bit shaky on what you've seen so far just go ahead and rewind this on and watch it again if you're set let's go and finish the last thing that we're going to talk about today