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Today’s episode dives into the HOW of enthalpy. How we calculate it, and how we determine it experimentally…even if our determinations here at Crash Course Chemistry are somewhat shoddy.
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Transcript Provided by YouTube:
00:00
Hydrochloric Acid: every chemist’s frenemy, as terribly dangerous as it is terribly useful.
00:06
It’ll burn your skin, your eyes, even your mucus membranes if you breathe in its fumes for too long.
00:10
But HCL as an acid gives up its hydrogen pretty easily,
00:13
which makes it good for making things like fertilizers and dyes and even table salt.
00:17
Then, there’s sodium hydroxide, another substance that I wouldn’t wish to be on my worst foe,
00:22
although I’m glad we have it.
00:24
You may know it as lye, an extremely caustic substance that’s used for
00:27
everything from clearing clogged pipes to purifying drinking water.
00:31
It’s a base. It readily accept the protons that acids release.
00:35
So what do you think will happen when I mix solutions of these two things together?
00:38
Will they just cancel each other out and do nothing, or will they explode, or maybe they’ll travel through time?
00:43
Well, if you’ve been paying attention, you already know what’s going to happen.
00:45
They’re going to undergo a neutralization reaction, which we’ve talked about before.
00:49
These two potentially deadly substances will form harmless salt and water.
00:54
But the reaction will also have an effect that you can actually feel.
00:58
It will release heat, and not just a little heat.
01:01
Mixing concentrated acids and bases releases so much heat that it can result in an explosion.
01:06
But I will show you how to produce a safe, but noticeable amount of heat with this reaction.
01:11
To me, the coolest part of this is where the heat actually comes from.
01:14
The energy used to exist as part of chemical bonds in the acid and the base.
01:18
Just like a ball at the top of a hill, the molecules always move towards a lower energy state if they can,
01:24
and that’s just what they’ll do.
01:26
High energy bonds will break and lower energy bonds will form.
01:29
The change in energy between those states you can actually feel the effects of, and that’s pretty dang cool.
01:35
And what’s even more awesome, if you ask me,
01:37
is that we can actually figure out exactly how much heat will be released by this reaction.
01:42
[Theme Music]
01:52
Remember that measuring heat change is closely related to enthalpy,
01:55
which we defined as the internal energy of a system
01:58
plus the energy it uses to push the surroundings back and make room for its own pressure and volume.
02:03
And in a constant pressure, like we have here at the surface of the Earth,
02:06
that works out to be exactly the same as the heat that’s absorbed or released by a reaction.
02:11
Naturally, it can be very useful to know how much heat a chemical reaction absorbs or releases.
02:15
In addition to the exothermic hand-warmers that we have out there,
02:17
there are also endothermic chemical ice packs for treating injuries.
02:20
The ability to calculate change in enthalpy is also what tells pilots how far the fuel
02:24
in an airplane’s tank will allow it to fly,
02:27
which I personally am very interested in making sure they get right.
02:30
One of the ways we can calculate the change in enthalpy of a system is with Hess’s Law,
02:34
which you’ll recall states that the total enthalpy change for a chemical reaction doesn’t
02:38
depend on the pathway it takes,
02:40
but only on its initial and final states.
02:43
It’s often expressed in terms of Standard Enthalpy of Formation,
02:46
that is, the amount of heat lost or gained when one mole of a compound is formed from its elements.
02:51
That’s how we figured out exactly how much heat my hand-warmers release.
02:55
But that’s not the only way that Hess’s Law can be used.
02:57
The law itself says nothing about the standard enthalpy of formation.
03:00
Any way that we can figure out the change of heat between the products and the reactants
03:03
will work just as well, and that’s where calorimetry comes in.
03:07
Calorimetry is the science of measuring the change in heat associated with a chemical reaction.
03:12
And this may look like a plastic bottle inside a koozie, but it’s actually a calorimeter.
03:17
A calorimeter can be fancy and an expensive piece of hardware, or it can be simple.
03:21
But no matter what it looks like, it’s basically an insulated container that contains a thermometer.
03:26
And it can be made out of stainless steel or Styrofoam cups,
03:29
but there really are no fundamental differences in how they work.
03:32
And you know the general setup by now:
03:34
the chemicals in the calorimeter make up the thermodynamic system
03:37
and everything else is the surroundings.
03:39
The insulation minimizes the amount of heat that leaks in or out of the system,
03:43
so that we can be fairly confident that any heat transfer is part of the system, not the surroundings.
03:48
The thermometer tracks the temperature change, which is part of the calculation we have to do.
03:52
And there’s usually some way to stir the solution to make sure that the reaction occurs fully.
03:57
Alright everybody, safety first, though I really should be wearing gloves…
04:01
I’m gonna put 100 mL, also 100 grams, of HCL’s one mole of HCL solution into my calorimeter here…
04:10
(mumbling)
04:11
And now I’m going to put the same amount of sodium hydroxide solution.
04:17
Before I do the reaction, I have to know our starting temperature,
04:20
so I’m going to stick my thermometer in there and wait for a second to see what it does.
04:24
It should be room temperature, it’s been in the room for a long time.
04:27
So, we are currently at, like, 20.8 degrees Celsius.
04:35
So that’s like, 294 Kelvin, and now I shall add my 100 mL of sodium hydroxide.
04:45
The temperature, unsurprisingly, is rising very rapidly.
04:47
And I’m doing something right now that you should never ever do, which is stir with the thermometer,
04:52
because if this happens in schools across the world,
04:55
then there will be a million billion broken thermometers and the stuff inside these thermometers is not good.
04:59
So never do what I’m doing.
05:00
All right, the temperature should be stable by now, we have 28 point like 2 degrees Celsius.
05:08
Now there’s a simple formula that allows us to calculate the heat change of a reaction,
05:12
simply by measuring the change in temperature that occurs in a calorimeter.
05:15
It says that the change in heat equals the specific heat capacity of the substance
05:19
times its total mass times the change in temperature.
05:22
Let’s examine the parts of this.
05:24
First of all, the heat change in the calorimeter is normally represented by a lowercase “q,”
05:28
but it can also be represented by change in enthalpy, or delta H,
05:32
because remember that constant pressure (delta H) equals q,
05:35
and constant pressure is almost always a good assumption for the duration of an experiment,
05:39
or at least as long as we stay at the surface of the earth.
05:42
For reasons that will become clear later, we’ll sure delta H to represent the heat change for this experiment.
05:47
Specific heat capacity, represented by a lowercase “s”,
05:51
is the amount of heat required tp raise the temperature of one mass unit,
05:56
like a gram or kilogram, of a substance by 1 degree Celsius.
05:59
So it turns out that different amounts of heat create different temperature changes,
06:04
like metals get hot really easily and cool down really easily.
06:08
Others like water require a lot of thermal energy to raise the temperature,
06:12
and therefore have to release a lot of heat to cool down.
06:15
I’m always wondering though, like, what does that really mean?
06:18
Like, physically in the molecules, shouldn’t heat raise the temperature of all substances equally?
06:23
And why does water in particular have such a high specific heat capacity?
06:27
Heat energy can do a lot of things besides just increase temperatures.
06:30
Temperature, or the speed at which molecules bounce around,
06:33
is just one way that atoms or molecules can absorb energy.
06:36
Heat energy can also be absorbed by the breaking and formation of bonds between molecules,
06:41
and as we’ll learn in another episode, the extremely high specific heat capacity of water
06:45
is due to the breaking and formation of hydrogen bonds that are associated with relatively small changes in temperature.
06:51
And how do we know the specific heat capacity?
06:53
Well, I am happy to report that some noble chemists have worked hard to determine the
06:57
specific heat capacities of hundreds of substances so that we don’t have to.
07:02
We just have to look up the numbers in a table.
07:04
Okay, so specific heat capacity times mass times the change in temperature.
07:07
The mass is important because the more mass of a substance we have, the more chemical bonds are present,
07:12
and because energy is contained in chemical bonds,
07:15
they have a big effect on how much energy we’re able to absorb or release.
07:19
And finally, there’s the change in temperature.
07:21
When doing calorimetry, we calculate a change in heat by measuring a change in temperature,
07:25
but as we’ve said a billion times before, heat and temperature are not the same thing.
07:30
But please do not think that this thing is measuring heat because it’s not!
07:34
It’s just that luckily, in this specific case, they are related by our handy little calorimeter formula.
07:40
Now you might not have noticed, but we are right at the interface between chemistry and physics here.
07:45
Each science could claim ownership over this phenomenon,
07:48
but the truth is humans made up the difference between chemistry and physics anyway.
07:53
Thermodynamics, the study of heat, energy, and work, doesn’t care about our little rules.
07:58
Thermodynamics itself makes the rules of the universe. It is the ultimate law.
08:02
So now you know, even though you might not have cared, but you should!
08:06
Because it’s good! It’s all wiggly-wobbly bondy-wondy.
08:09
All right! Enough talk, let’s get out there, actually do some math here.
08:12
Remember that the formula is delta H, s, m, delta T.
08:15
The solutions we’re using here are so dilute that almost all of their mass consists of water.
08:20
Therefore, we can use the specific heat capacity of water.
08:23
If we look that up on our table, we’ll see that it is 4.184 Joules per gram degrees Celsius;
08:27
I used 100 grams of each chemical for a total mass of 200 grams.
08:31
And finally, we need the temperature change.
08:33
If you remember, the temperature rose from 294.0 Kelvin to 301.4 Kelvin;
08:37
the difference between these two is 7.4 Kelvin.
08:40
It’s a positive value because the temperature increased.
08:43
Cancel out all the appropriate units and then bang on the calculator to get a final release of 6192.32 J,
08:50
or 6.2 kiloJoules of heat from the reaction.
08:53
Because this formula is based on temperature change,
08:56
and since the temperature increased, we end up with a positive result.
09:00
But most importantly, it tells us the magnitude of the change in heat energy.
09:04
So, I wonder how that compares to the amount we would predict using Hess’s Law
09:09
and the standard enthalpy of formation?
09:12
Remember that we can look up the standard enthalpy of formation for all the products
09:15
and reactants in the back of a textbook or online,
09:18
the chemical reaction between hydrochloric acid and sodium hydroxide produces liquid water and sodium chloride.
09:23
The standard enthalpy of formation for hydrochloric acid is -167.2 kJ per mole;
09:29
for sodium hydroxide it’s -469.15 kJ per mole;
09:34
for liquid water it’s -285.8; and for sodium chloride it’s -407.27.
09:40
I’m not gonna do the mole calculations on-screen,
09:42
but trust me when I say that we used 0.100 mole of HCL and the amount of NaOH.
09:48
Because everything in the equation balances out, it’s just a 1:1:1:1 ratio,
09:52
we can assume that they all have the same amount of each product as well.
09:56
If we plug these into Hess’s Law and do the calculation,
09:58
we found that the change in heat or enthalpy of the reaction is -5.67 kJ.
10:03
The system is releasing or losing energy, so the number is negative,
10:08
but again it’s really the magnitude that we wanna know.
10:10
So there you go, the calorimetry formula gave an absolute enthalpy change of 6.2 kJ,
10:15
while Hess’s Law gives a change of 5.67 kJ.
10:19
So, why the difference?
10:20
Well, the greatest factor is probably that we used the specific heat capacity of pure water
10:25
instead of the salt water that we actually created.
10:28
We also didn’t include the heat capacity of our calorimeter itself.
10:31
The calorimeter walls and the thermometer were heated too,
10:34
resulting in some of the produced heat not being accounted for.
10:37
The insulation of the calorimeter is obviously a bit light,
10:40
which allowed some heat to escape entirely and that’s another major factor.
10:43
Even so, I’d say we did pretty well, the important thing is that it showed us what we needed to see,
10:47
even though it was just a little plastic bottle in a koozie.
10:49
For a quick simple method, the calorimeter got us pretty close to the calculated value.
10:53
If we were calculating the amount of a particular fuel we would need to travel to Mars,
10:57
or inventing a cold pack that won’t give you frostbite,
10:59
we’d wanna use a more sophisticated system and work more carefully, but this was pretty cool for our purposes.
11:05
Thanks for watching this episode of Crash Course.
11:07
If you paid attention, you learned that we don’t necessarily have to use standard enthalpies
11:11
of formation to solve Hess’s Law,
11:13
you learned what a calorimeter is,
11:14
that calorimetry is another way to investigate heat changes in chemical reactions,
11:19
and that specific heat capacity tells us
11:20
how much heat energy affects the molecules in a substance without changing its temperature.
11:25
And finally, you learned some potential sources of error related to calorimetry.
11:28
The episode of Crash Course Chemistry was written by Edi Gonzalez.
11:32
The script was edited by Blake de Pastino and our chemistry consultant was Dr. Heiko Langner.
11:37
It was filmed, edited, and directed by Nicholas Jenkins.
11:39
Our script supervisor was Caitlin Hofmeister, and our sound designer is Michael Aranda.
11:43
And, of course, our graphics team is Thought Cafe.
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This post was previously published on YouTube.
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Photo credit: Screenshot from video