Enthalpy of Reaction Bozeman Science English (auto-generated) English

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hi it's mr. Andersen and this is

chemistry essentials video 53 it's on

Hi. It's Mr. Andersen and this is chemistry essentials video 53. It's on the enthalpy

the enthalpy of reaction which is the amount of energy taken in or released

of reaction which is the amount of energy taken in or released during a chemical reaction

during a chemical reaction as bonds are broken and new bonds are formed and look no farther than nitroglycerin for a

as bonds are broken and new bonds are formed. And look no farther than nitroglycerin for

reaction that creates a huge amount of energy it was used by Alfred Nobel a

a reaction that creates a huge amount of energy. It was used by Alfred Nobel, a scientist and

scientist and explosives expert but it was highly unstable and his brother was killed in one explosion so he invented

explosives expert, but it was highly unstable and his brother was killed in one explosion.

dynamite which essentially stabilizes

So he invented dynamite which essentially stabilizes nitroglycerine by wrapping it in

nitroglycerin by wrapping it in some kind of cellulose and so on it was used not only in mining and construction but

some kind of a cellulose. And so it was used not only in mining and construction but it

it was also used in war leading some people to call Alfred Nobel the merchant

was also used in war, leading some people to call Alfred Nobel "The Merchant of Death",

of death which made him feel a little bit bad he put all of his money into a trust and then created the Nobel Prizes

which made him feel a little bit bad. He put all of his money into a trust and then created

which are famous to this day and so in

the Nobel Prizes which are famous to this day. And so in the enthalpy of reaction what

the enthalpy of reaction what we're looking at are the reactants and what are called the bond energy the amount of

we're looking at are the reactants and what are called the bond energy, the amount of

energy it takes to break the bonds in the reactants and then the amount of

energy it takes to break the bonds and the reactants. And then the amount of energy that

energy that is released in the products which is essentially the negative bond

is released in the products which is essentially the negative bond energy. And you can think

energy and you can think of putting these on a balance beam and if there's more reactant energy consumed we would

of putting these on a balance beam. And if there's more reactant energy consumed we would

call that an endothermic reaction if there's more release we call that an exothermic reaction now remember these

call that an endothermic reaction. If there's more released we call that an exothermic reaction.

reactions sit in their surroundings and

Now remember these reactions sit in their surroundings. And since energy can neither

since energy can neither be created nor destroyed in an endothermic reaction energy thermal energy is going to go

be created nor destroyed, in an endothermic reaction energy, thermal energy, is going

from the surroundings into the reaction that's going to cause that reaction to

to go from the surroundings into the reaction. That's going to cause that reaction to feel

feel cold and the opposite if we're looking at an exothermic reaction now if

cold. And the opposite if we're looking at an exothermic reaction. Now if we're looking

we're looking at the amount of enthalpy reaction there is Hess's law is something that kind of lays outside of

at the amount of enthalpy reaction there is, Hess's Law is something that kind of lays

what you should know in AP chemistry but there's a few ideas that you should

outside of what you should know in AP Chemistry. But there's a few ideas that you should really

really be able to apply and so if we look at an exothermic reaction like this thermite reaction we can look at the

be able to apply. And so if we look at an exothermic reaction, like this thermite reaction,

energy of the reactants and enter the energy of the products and we can see

we can look at the energy of the reactants and the energy of the products. And you can

that this is a downhill reaction in other words we're releasing energy and

see that this is a downhill reaction. In other words we're releasing energy. And that energy

that energy is called the enthalpy of reaction now it's given off from that

is called the enthalpy of reaction. Now it's given off from that reaction as heat into

reaction as heat into the surroundings if we were to actually measure it that's going to be the change in enthalpy which

the surroundings. If we were to actually measure it, that's going to be the change in enthalpy

is simply the enthalpy of the products minus the enthalpy of the reactants in

which is simply the enthalpy of the products minus the enthalpy of the reactants. In this

this case it would be neg 850 kilojoules per mole now if I were to

case it would be negative 850 kilojoules per mole. Now if I were to show you an enthalpy

show you an enthalpy diagram it would simply look like this an enthalpy diagram is look different than an energy

diagram it would simply look like this. An enthalpy diagram is different than an energy

diagram in that we simply put the enthalpy of the reactants and the enthalpy of the products since we're

diagram in that we simply put the enthalpy of the reactants and the enthalpy of the products.

subtracting the reactants from the products in other words since the

Since we're subtracting the reactants from the products, in other words since the products

products in this case is lower than the reactants we're going to get a negative value negative 850 kilojoules a mole now

in this case is lower than the reactants we're going to get a negative value, negative 850

what does that mean where is that energy

kilojoules per mole. Now what does that mean? Where does that energy go? It's going to go

go it's going to go as thermal energy to the surroundings now something interesting is that if we actually turn

as thermal energy to the surroundings. Now something interesting is that if we actually

that reaction around and make the reactants the products look what happens

turn that reaction around and make the reactants the products, look what happens to the change

to the change in enthalpy it stays the same the only thing that changes is

in enthalpy. It stays the same. The only thing that changes is going to be the negative value

going to be the negative value that was in the front and if we swap it around again it's going to be that negative

that was in the front. And if we swap it around again it's going to be that negative again.

again and so this is the first of Hess's law ideas that you should understand that is if we reverse the reaction then

And so this is the first of Hess's ideas that you should understand. That is if we reverse

we have to reverse the sign in that change in enthalpy if we look at an

the reaction then we have to reverse the sign in that change in enthalpy. If we were to

endothermic reaction in this case we have an uphill reaction where the

look at an endothermic reaction, in this case we have an uphill reaction where the products

products actually have more energy than the reactants and so this is going to be

actually have more energy than the reactants. And so this is going to be our enthalpy of

our enthalpy of reaction it's moving from the surroundings as thermal energy into that reaction itself we could

reaction. It's moving from the surroundings as thermal energy into that reaction itself.

measure it and we would find that it is a positive value now how do we measure

We could measure it and we would find that it is a positive value. Now how do we measure

that we're going to use a calorimeter remember we're going to have the

that? We're going to use a calorimeter remember. We're going to have the reaction take place

reaction take place surrounded by water we can measure the change in that heat

surrounded by water. We can measure the change in that heat of the water and we can measure

of the water and we can measure that enthalpy change it's a positive value or an uphill reaction and the reason why is

that enthalpy change. It's a positive value or an uphill reaction and the reason why is

that our products have a greater enthalpy than our reactants now we don't

that our products have a greater enthalpy than our reactants. Now we don't have to know

have to know what that energy is to begin or at the end we just have to know what that change is in order to measure

what that energy is to begin or at the end we just have to know what that change is in

enthalpy now what's interesting if we take a look at a reaction we find that

order to measure enthalpy. Now what's interesting, if we take a look at a reaction we find that

it's usually made up of a number of smaller reactions and so in this born-haber cycle what we're doing is

it's usually made up of a number of smaller reactions. And so in this Born-Haber cycle

taking lithium solid and combining it with fluorine gas and we're making

what we're doing is taking lithium solid and combining it with fluorine gas and we're making

lithium fluoride now if you look at that change in enthalpy so you it's very

lithium fluoride. Now if you look at that change in enthalpies, it's very small. And

small and it's going to be an exothermic reaction because it's negative but if we

it's going to be an exothermic reaction because it's negative. But if we look at all of the

look at all the steps of this reaction this would be an endothermic this would be endothermic this would be endothermic

steps of this reaction, this would be an endothermic. This would be endothermic. This would be endothermic.

and then we have two exothermic and so this is the next of

And then we have two exothermics. And so this is the next of Hess's law that you should

is law that you should be able to apply and that is that the enthalpy of reaction is equal to the sum of all of

be able to apply. And that is that the enthalpy of reaction is equal to the sum of all of

the reactions that make up that reaction so lots of times in chemistry you'll be

the reactions that make up that reaction. So lots of times in chemistry you'll be given

given a target reaction and then you're going to have parts of that reaction and

a target reaction and then you're going to have parts of that reaction. And your goal

your goal is to figure out the change in enthalpy of that whole reaction it's pretty simple algebraically to solve

is to figure out the change in enthalpy of that whole reaction. It's pretty simple algebraically

this as long as you remember those two parts of Hess's law that if we reverse

to solve this as long as you remember those two parts of Hess's Law, that if we reverse

the reaction we have to reverse the sign and then the sum of all the parts is

the reaction we have to reverse the sign. And then the sum of all the parts is equal

equal to the sum of the whole and so if I look at this equation right up here

to the sum of the whole. And so if I look at this equation right up here I've got my

I've got my carbon on the left side and so that's good but you can see here that

carbon on the left side. And so that's good. But you can see here that I have my carbon

I have my carbon monoxide on the left side and I eventually want it on the

monoxide on the left side and I eventually want it on the right side. You can see here

right side you can see here that I have two exothermic reactions and so the

that I have two exothermic reactions. And so the first thing that I could do is I could

first thing that I could do is I could simply switch this reaction around that's totally legal watch what happened

simply switch this reaction around. That's totally legal. Watch what happened to my delta

to my Delta H my Delta H now is a positive value and so I can change the

H. My delta H now is a positive value and so I can change the reaction around and then

reaction around and then I'm good to go if you look at this I've got carbon dioxide on the Left carbon dioxide on

I'm good to go. If you look at this I've got carbon dioxide on the left, carbon dioxide

the right side and so I can actually cross those off and now I'm kind of left

on the right side and so I can actually cross those off. And now I'm kind of left with almost

with almost just what I want there's one problem here I could move my carbon down

just what I want. There's one problem here. I could move my carbon down here. I could

here I could move my carbon monoxide here and I'm left with my oxygens well

move my carbon monoxide here. Now I'm left with my oxygens. Well on the left side I have

on the left side I have 102 on the right side I have a half of a note - and what

one O2. On the right side I have a half of an O2. And what am I looking for? A half of

am I looking for a half of o2 on the left side so what I can do is I can simply cross that off and now I've just

O2 on the left side. So what I can do is I can simply cross that off. And now I've just

got a half o2 on the left side and so now I've totally got my reaction all I

got a half O2 on the left side. And so now I've totally got my reaction. All I do now

do now is I simply add these up so algebraically I can add these up and

is I simply add these up. So algebraically I can add these up and this is going to be

this is going to be the Delta H of that whole reaction let's try a little bit

the delta H of that whole reaction. Let's try a little bit harder one. Now you could

harder one now you could pause the video at this point and you could try this on your own

pause the video at this point. And you could try this on your own. That would be a good

that'd be a good thing to do I'll wait for you okay so let's get at it so here's our target equation right down

thing to do. I'll wait for you. Okay. So let's get at it. So here's our target equation right

here and so you can see here that I have my n O on the left side my nitrogen

down here. And so you can see here that I have my NO on the left side. My nitrogen monoxide.

monoxide so that's good I'm going to leave that there but I really want is my oxygen all by

And so that's good. I'm going to leave that there. But what I really want is my oxygen

itself on the left side here so I'm going to take this and I'm going to turn

all by itself on the left side here. So I'm going to take this and I'm going to turn this

this equation around you can see that I got a negative value right here so that

equation around. You can see that I got a negative value right here. So that helps out

helps out a little bit you can see here that I have two O's O's on the left side

a little bit. You can see here that I have to have two ozones on the left side. And so

and so if I could get those two cancel out that's going to help me a little bit

if I could get those to cancel out that's going to help me a little bit. So I could

so I could switch that around so these are going to can in Seoul so that's good what am I

switch that around. So these are going to cancel. So that's good. What am I missing

missing then I got my nitrogen dioxide on the right side well you can see here

then? I've got my nitrogen dioxide on the right side. Well you can see here that I really

that I really want just one of these on the left side and so what I could do is

want just one of these on the left side. And so what I could do is I could take that whole

I could take that whole equation times a half well if I take that whole equation

equation times a half. Well if I take that whole equation times a half, watch what's

times a half watch what's going to happen to my Delta H I'm gonna have to take that times a half as well okay so

going to happen to my delta H. I'm going to have to take that times a half as well. Okay.

let's see how we're doing so far I got my Ozone's I can get rid of those now if

So let's see how we're doing so far. I've got my ozones. I can get rid of those. Now

we get to the oxygen this is really convenient you'll find this is lots of

if we get to the oxygen, this is really convenient. You'll find this is lots of times very convenient.

times very convenient on the right side now I've got 102 plus 1/2 so that's three halves of an o2 which is the same

On the right side now I've got one O2 plus a half. So that's three halves of an O2 which

that I have on the left side so I could cross out all my oxygens now I simply

is the same that I have on the left side. So I could cross out all my oxygens. Now I

add that up there's my target reaction and now I simply add up all my Delta HS

simply add that up. There's my target reaction. And now I simply add up all my delta Hs. And

and that's going to be the overall exothermic reaction that I'm looking at

that's going to be the overall exothermic reaction that I'm looking at. Okay. So did

okay so did you learn qualitatively and quantitatively patterns in enthalpy of

you learn qualitatively and quantitatively patterns in enthalpy of reaction? Again it's

reaction again it's simply a balancing act we look at the energy it takes to break the reactants the energy formed by

simply a balancing act. We look at the energy it takes to break the reactants. The energy

the products and whichever way that balance tips it's going to tell us what

formed by the products. And whichever way that balance tips, it's going to tell us what

kind of reaction it is and then we combine reactions to figure out the sum

kind of a reaction it is. And then we combine reactions to figure out the sum of all of

of all the parts and I hope that was helpful

the parts. And I hope that was helpful.