This course is designed to cover subjects in advanced high school chemistry courses, correlating to the standard topics as established by the American Chemical Society. This course is a precursor to the Advanced Chemistry Coursera course. Areas that are covered include atomic structure, periodic trends, compounds, reactions and stoichiometry, bonding, and thermochemistry.
7.03 Enthalpy
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来自 University of Kentucky 的课程
Chemistry
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从本节课中
Week 7
In this unit, we will learn about thermochemistry, which is the study of the thermal energy transfer (heat) in chemical reactions. We will learn how these measurements of heat are made via calorimetry. Then, learn to calculate heat transfer using various methods.
与讲师见面
Dr. Allison SoultLecturer
Chemistry
Dr. Kim WoodrumSr. Lecturer
Chemistry
The learning objective number three, we are going to define enthalpy.
We're going to see the connection of enthalpy and
heat that we learned about in the previous learning objective.
We're going to see the connections of the sign convention of
enthalpy in endothermic and exothermic reactions.
So, let's begin by defining enthalpy.
The definition seems kind of strange, it's an equation.
The definition of enthalpy is E plus PV.
Now, the symbol for enthalpy is a capital H.
Okay? So that's enthalpy.
It is an energy, and if you were to take the total amount of
internal energy and add to it the pressure and the volume, you would have enthalpy.
Now, in the same way we can't know the total amount of internal energy,.
We cannot know the amount of enthalpy of a substance.
You just cannot obtain that value.
But we can certainly measure a change in enthalpy, delta H.
So, we know that the Greek letter delta means change,
and the change of anything is final minus initial.
So when we're talking about delta H, delta H would be H final minus H initial.
The problem with that equation is, we cannot know the H final and
the H initial, so we cannot determine delta H this way.
But we will find that we have lots of ways we can determine delta H in as we proceed.
But for now let's just think about if you could, which we can
conceive of doing a final minus initial, how would, what would the signs be?
Okay, let's look at our two reactions.
We've seen these before, okay?
Let's imagine for just a moment, that along this axis on the left-hand side.
We have, we are representing an increase
in enthalpy, this H, okay?
And so, in our diagram on the left,
this is the amount of H that the products have and
this the amount of H, the reactants have.
Well, if you were to determine the delta H, okay?
It would simply be the products minus the reactants.
That would be the final minus the initial.
So in our picture on the left, the final is here, that's your products.
Your initial is here, that's your reactant.
So this is how much, even though we don't know how much that is,
we can know that one's bigger than the other.
So here is the H final, here is the n, H initial up top.
And if we wanted to change, we do products minus n, reactants.
Well the products have a low value, so
this would be small in this picture on the left.
The reactants have a large value in comparison.
So, what would be the sign of that?
A small number minus a large number.
When you are dealing with an exothermic reaction,
the delta H will always be negative.
Now this is like what we saw with q.
Q was negative for an exothermic reaction, so
there's the connection we're seeing there.
Okay, on the diagram on the right, on the other hand, let's change the pen color so
that we can keep track of it versus the other.
Over here, here is my products.
So this would be I mean, my reactants.
This would be the initial, and
this would be amount of final, which is your products.
So the final, or the products is a large number, is high.
The reactants in comparison is a small number.
So, if we did final large number minus initial,
which is the small number, what would be the sign?
Well the delta H is greater than zero, that is an endothermic process.
So, we saw that q had a convention the same way, that if you were giving if
you were absorbing heat, okay, we used q for that earlier, and that's endothermic.
Now there's a connection, between enthalpy change and
q, we're going to see it her in a little bit.
But we're definitely seeing a connection in terms of the sign.
So, we use the same term of exothermic and
endothermic for the delta H that we used for q.
This is a demonstration of an endothermic reaction.
In an endothermic reaction,
you have to supply heat in order to make the reaction progress.
And sometimes you'd have to actually add a heat source but this reaction can
take enough heat from the surroundings to run and so you feel it getting cold.
We're going to attempt to freeze this flask to the board with a couple drops of
water because it gets that cold.
These are two dry chemicals that I will mix together.
I have barium hydroxide and ammonium chloride, and
it's kind of unusual to have two dry ingredients that will actually react well.
[SOUND] When this reaction proceeds, it produces some ammonia gas,
which I'll be able to smell.
It also release some water that was hydrated in the barium hydroxide.
And it'll start getting a little slushy.
When it gets slushy, I'm going to add the drops of
water to the board, and set the flask down, and then we'll wait a while.
I'm going to add a couple drops of water.
This flash has got a indention on the bottom, so
I want to try to freeze it on either side of the flask.
And now we'll wait.
So, if all works well, this flask, when I pick it up, will be frozen to the board.
All right, so we have our endothermic reaction.
The demonstration you're about to see will be taking place outside, and
you're going to see why here in a little bit.
But it involves two substances.
One is iron oxide, which is basically rust in a powdered form, and
the other one is powdered aluminum.
When these two substances are mixed together,
we won't have anything happen until we light them.
And we don't want to light it with a match or
something that we have to approach closely to.
So we have a secondary reaction, and
this reaction will have glycerin and some potassium permanganate.
As those substances are added together, it'll give a delay, which will allow,
which will allow Dr. Saltz to back away from the reaction.
And then smoke will billow out and the reaction will take place.
And we'll talk more about the reaction after you get a chance to see it.
[MUSIC]
We just examined the thermite reaction, then did a demonstration of that reaction.
My question for you is, is that reaction endothermic or is it exothermic?
And what is the sign of delta H?
Well certainly when it gave off, it was so
hot that the iron that's produced is molten.
We know that, that's an exothermic reaction.
Exothermic reactions have negative signs associated with them.
You examine a demonstration of the freezing of a flask to a board.
That reaction is?
Choose your answer.
Did you say endothermic and positive?
Then you are correct.
It absorbed the thermal energy from the surroundings,
from the water that was on that board.
It dropped the temperature of that water down below it's freezing point and
it froze it to the board and that is a positive delta H.
Now let's at this reaction.
It is giving me a delta H and it has a sign, so
I should be able to determine whether it's endothermic or exothermic.
It is also, if we look at the reaction, you could determine what's happening
to the amount of gas present, and by looking at the amount of gas present,
you could determine whether expanding or contracting.
With that you could answer the question.
Well, if you selected number two, that's the correct answer.
I know it's exothermic because of this.
I know because I started with one and a half moles of gas.
And I finished with one mole of gas, that it is going to contract and
we have a delta V that's negative.
If deltaV is negative, it is getting smaller.
We are doing work by the surroundings on the system, and work is positive.
Now we're seeing connections between work and heat, I mean heat and enthalpy change.
So, let's see what the real definition of connection here is.
If you have a reaction that takes place in a constant pressure environment,
so it's open to the environment, so whatever the external pressure is,
it stays the same during the course of the reaction.
If that happens, by definition, the enthalpy change,
which is delta H is synonymous with the heat change, which is q.
So we could write it this way, q is equal to delta H at constant pressure.
And so that is the connection between enthalpy change and heat.
They are not always synonymous, but as we work through this chapter, I will very be,
often be talking about heat and enthalpy change with the same statement, okay?
When I'm talking about heat and
I'm talking about enthalpy change, talking about the same thing because we're
going to be assuming that we're running under a constant pressure environment.
And this is no doubt where the H symbol came from.
Why did they use H for enthalpy?
Well, it is a very, is very connected to the word heat.
So there's our definition of enthalpy change, from the begin, I mean,
of enthalpy.
And we've got what enthalpy change is, and we have our sine convention of
endothermic being positive and exothermic being negative for this section.
And that's the end of learning objective number three.
