Second law of thermodynamics | Chemical Processes | MCAT | Khan Academy

Second law of thermodynamics | Chemical Processes | MCAT | Khan Academy

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– Let’s talk about the
Second Law of Thermodynamics. This law is weird. There’s about 10 different
ways to state it, which is one reason why it’s weird. Let’s start with one of the
most common ways to state it, which is, if you’ve got a
cold object and a hot object, heat will never be seen
to flow spontaneously from a colder object to a hotter object. So if you have these two sitting together, maybe an ice cube and
a hot piece of metal, and you make them touch, heat’s going to flow between them, but we know what’s gonna happen. The heat’s gonna flow from the hot object to the cold object, and never the other way. At least, not spontaneously. You can force heat from a cold object to a hot object, like we
do with a refrigerator or a freezer, but that’s
using a heat pump. And those refrigerators
and freezers are doing work to force that heat from the
cold region into the hot region. It won’t do it spontaneously by itself. You’ve got to force it to do it. So what the second law says,
or at least one version of it, is that that process will never be seen to happen in reverse. The heat will never be seen to flow from the cold object to the hot object. Now, you might be thinking, “Duh. “Do we really need a law to tell us that?” But it’s not so obvious,
because you can still conserve energy and momentum and all the other rules of
physics and laws of physics by allowing heat to flow from the cold object to the hot object. In other words, let’s say the cold object started with 10 Joules of thermal energy and the hot object started with … It’s hotter, so let’s just say it has 30 Joules of thermal energy. You could imagine five Joules of energy going from the cold
object into the hot object which would leave you
with five Joules of energy for the cold object, 35
Joules of thermal energy for the hot object. You still have 40, just
like you did before. You didn’t break the law
of conservation of energy. It’s just, energy won’t go that way. So why? Why is thermal energy never seen to flow from the cold
object to the hot object, even though it satisfies every
other known law of physics besides the second law? Well, before we answer that question, I think it’d be useful to talk about an alternate version of the second law, which looks something like this. The total disorder will
never be seen to decrease. What do I mean by “disorder”? Imagine you had a room and
there were blue spheres. And they’re bouncing around wildly. So these all have some
velocity and random directions. And when they strike a wall or each other, they lose no energy. So they keep bouncing around like crazy. And then there’s another
section of the room with red spheres, and these are also bouncing around randomly. They lose no energy. They
keep doing their thing. Except, there’s a divider in this room that doesn’t allow the red spheres to go onto the blue spheres’
side, and vice versa. These can’t mix up. So right now, this is an ordered state because the reds are
separated from the blues. So we say that this state has a certain amount of order to it. But let’s imagine we removed the divider. Now what’s gonna happen? Well, you’ll see these things mix up. This blue sphere will move over here, and it’ll bounce onto this side. This red sphere will go over here. They’ll just keep getting mixed up. And at some given moment,
you might find the spheres in some configuration like this. They’re still bouncing around, but now they’re all mixed up and we say that this state has a
higher amount of disorder. This is not ordered. We say that this is more disordered, which supports the second law. The second law says, if you let things do what they wanna do spontaneously, your system will go from
a more ordered state to a more disordered state. And you’ll never see it go the other way. We can stand in this room and wait. But you’re probably never gonna see the blue spheres line
up all on the left side and the right spheres
line up on the right side. With 12 total spheres, maybe
if you wait long enough, a really long time, you might catch it where all the red spheres are on one side and blues are on the other. But image this. Imagine
now, instead of six reds and six blues, there’s
100 reds, 1,000 reds, maybe 10 to the 23d and
Avogadro’s number of reds, and now they’re all mixed up. The odds of ever seeing them
get back to this ordered state are basically zero. The probability isn’t exactly zero, but the probability is very, very low that you would ever see a disordered state with that many number of particles reassemble themselves
into an ordered state. So we kind of just know
that from experience and what we’ve seen in
our day-to-day lives. But you still might be
wondering, “How come? “How come we never see a disordered state “go to an ordered state?” Well, it basically has
to do with counting. If you were to count all the possible ways of lining up the reds
over here on this side and the blues on the left-hand side, there’d be a lot of combinations that would satisfy that condition. I mean, you could swap
this red with that red, and this red with that red, all on the right-hand side. All these reds could get swapped around. And these blues, as well. They can get swapped around
on the left-hand side. You get a large number of variations that would satisfy the condition of blues on the left, reds on the right. But now I want you to ask yourself, how many possibilities
are there for having blues and reds spread out
through the whole room? Well, you could probably
convince yourself, there’s more. And it turns out, there’ll be a lot more. Now this red doesn’t have to just maintain its position on the
right-hand side somewhere. This red can get swapped
out anywhere over here. I can swap a red with this blue, and this blue with this red, and this red with this red,
and this blue with this blue. I can move them all over. Now that these spheres have the whole room through which they can mix, the amount of states that
will have blues and reds mixed throughout the whole room will vastly outnumber the amount of states that have just reds on one side and just blues on the other side. And this simple idea is the basis for the Second Law of Thermodynamics. Roughly speaking, the
Second Law of Thermodynamics holds because there are so
many more disordered states than there are ordered states. Now, I’m gonna tell you something
that you might not like. This particular disordered
state that I have drawn, this exact one, is just as likely as this exact ordered state. In other words, if I get rid
of the barrier over here, if you came in, you’d be just as likely to find the room in
this exact configuration as you were to find it in
this exact configuration. These two exact states are equally likely, which sounds weird. It makes you think, “Well,
you’re just as likely “to find an ordered state
than a disordered state.” But no. This particular
state is just as likely as this other particular state. But there are so many more mixed-up states than there are separated states. Even though any particular
state is just as likely, since the mixed-up states vastly outnumber the separated states, if
you pick one at random, it’s gonna be a mixed-up state because there are so many more of them. Imagine putting these all into a hat. Imagine writing down all the possible configurations of states,
ordered, disordered, in between. You put them all into a hat,
you pull one out randomly, any particular state is just as likely. But since there’s so many
more disordered states, you pick one out randomly, it’s probably gonna be mixed up. And if there’s a large
number of particles, you’re almost certain to find it mixed up. So to help us keep these ideas straight, we need some different terms. Physicists came up with a couple terms. One is a macrostate. And a macrostate is basically saying, okay, the particle are mixed up. That’s one possible macrostate. And we could be more precise. We can say, the reds and the blues can be anywhere within the box. Another possible macrostate would be to say that the particles are separated, that is to say, reds are on this side, anywhere on that side,
but on the right side, and blues are on the left side,
anywhere on the left side. These terms are referring to a macrostate, an overall description
of what you would see. Now, there’s another term, a microstate. And a microstate is a
precise, exact description of the nitty-gritty details of what every particle is doing within there. If I just tell you, “The
particles are mixed up,” you’re not gonna know
exactly where they are. Similarly, if I just tell
you, “They’re separated,” you’re not gonna know
exactly where they are. You’ll know they’ll be
on the right-hand side, the red ones will, but you won’t know. Maybe this red ones moves down here, maybe this red one moves up here. The microstate is an exact description. This red one’s right here,
going a particular speed. This blue one’s right here,
going a particular speed. If you specify the exact location, blue right here, blue right there, going that fast, red right here, what you’re describing
to me is a microstate. And so the second law, another way of thinking about it, there are more microstates
for a disordered macrostate than there are micorstates
for an ordered macrostate. And that’s why we see systems
go from order to disorder. It’s really just a statistical result of counting up the
possible number of states. You might be wondering,
what does this have to do with heat going from hot to cold, all this talk about
microstates and macrostates? Well, it’s not just position
that can get disordered. It’s velocities that can get disordered, energy that can get disordered, and that’s more of like
what’s happening up here. The positions of the hot molecules aren’t necessarily moving
over into the cold range. But the energy over here
is getting dissipated into the cold area. So image it this way. Let’s get rid of all this. And imagine you had a
room with a gas in it, but this gas was kind of weird. At this particular moment,
all the gas molecules on the right-hand side
were moving really fast, and all the gas molecules
on the left-hand side were moving really slow. So the room was separated
into a cold region and a hot region, just
like this energy is. This is ordered, or at
least, somewhat ordered. It’s more ordered than it’s going to be. If you wait a while,
this is all gonna mix up. You’re gonna have some
fast-moving particles over here, some slow ones over here. It’s all gonna be blended together. And so, what would you say
if you were standing in here? At first, you’d feel cold because these particles
don’t have a lot of energy. Then you start feeling warmer and warmer. You’d say heat is flowing over to the left because you feel faster-moving particles striking your body. And so you’d rightly
say that heat is moving from the right of this room
to the left of this room. It flows from the hot to the cold. And that’s what’s happening up here. Heat flows from the hot to the cold. You might object. These
are solids, I said, copper and an ice cube. A copper atom’s not gonna make it over into the cold ice cube. But the energy is gonna move. So you can make the
same argument over here. Don’t allow these, let’s say
these are the copper atoms moving around fast, or at least
jiggling in place rapidly. When they bump into the
slower-moving water molecules in the ice cube, they’re gonna
give those water molecules some of their energy. And this energy’s gonna become mixed up. The energy will become disordered. It will go from this ordered state, where the high energy is over
here and low energy’s here, to a disordered state where the energy’s distributed somewhat evenly. So essentially what I’m saying is, if you consider the macrostate, where the hot molecules are separated from the cold molecules,
there will be less microstates that satisfy that condition than there will be microstates
that satisfy the condition for a macrostate where
the energy is mixed up and you’re just as likely to
find a fast-moving particle on the left as you are on the right. This will have vastly more microstates, many more possible ways of
making up a mixed-up state than there are microstates
that create a separated state. I mean, there’s gonna be a lot. I’m talking a lot of microstates that satisfy this condition
for this macrostate, separated. But there will be so many more microstates for the mixed-up
case, this dominates. That’s why you always see heat flow from a hot object to a cold object, just because it’s statistically inevitable with the large number of
particles that you have here. There are so many more
ways of heat flowing from hot to cold than
there are from cold to hot, statistically speaking, you just never see it go the other way. Energy will always, at
least spontaneously, if you let it do what it wants to, energy’s always going to
dissipate and evenly distribute. That’s why it goes from
the hot to the cold. This energy’s trying to get mixed up, just because statistically, there are so many more ways for that to happen. Now, I need to tell you
that there’s actually a scientific term for
the amount of disorder, and we call it the entropy. Physicists use the letter
S to denote the entropy. And if you wanna know the
formula for the entropy, you could look on Bolzmann’s grave. This is Ludwig Bolzmann. He’s got it on his gravestone.
How awesome is that? The entropy S is k, Bolzmann’s
constant, times log. This is actually natural log of W. And W is the number of microstates for a particular macrostate. So you got some configuration,
you wanna know the entropy? Just look at what macrostate it’s in, count up how many microstates are there for that macrostate, take log of it, multiply by Bolzmann’s constant, that gives you the entropy. And there’s a term for this W. It’s called the Multiplicity, because it’s determining the multitude of microstates that satisfy the conditions for a particular macrostate. Now, entropy is cool. Entropy is weird. Entropy is somewhat mysterious and still, probably, has secrets
for us to unlock here. I don’t have time to go
into all of them here, but if you read up on it,
entropy has a role to play in the fate of the universe, the beginning of the
universe, the arrow of time, maybe our perception, all
kinds of facets of physics that are extremely interesting. And entropy, you always find
this guy lurking around. And one place you always find entropy is in the Second Law of Thermodynamics, because it allows us a third
way to state the second law, which is that the total
entropy of a closed system will always be seen to increase. Technically, if it’s a reversible process, the entropy could stay the same. But honestly, for all
real-world processes, the entropy’s gonna increase
for a closed system, which is to say that
the disorder increases.

77 thoughts on “Second law of thermodynamics | Chemical Processes | MCAT | Khan Academy

  • Thomas Bradley Post author

    Great video!

  • June Jeon Post author

    Good explanation! Thx

  • Gubrist Jarosch Post author


  • Aadi Malik Post author

    The way sal kahn and his team describes is just loving!

  • q0m0p Post author

    It says it works this way, but why it does, it's a mystery of the universe. Rightly so x)

  • Tonya Bigham Post author


  • Tonya Bigham Post author


  • Maira Jalil Post author

    wow…u are so good

  • Maira Jalil Post author

    wow…u are so good

  • Albert Einstein Post author

    But that means that there always has to a chance that energy from a colder object moves to a warmer object, a small chance, but still a chance. Which must mean that there always is a chance that alot of heat in the air just comes togheter really close which provides enough heat to ignite the air around it, which means that air can combust which means that alot of these combustions can form fire in the form of a human. Imagine a fire-human walking on the streets :3

  • violinsheets Post author

    Umm… what does this have to do with "The entropy of an isolated system increases in an irreversible process and remains unchanged in a reversible process."?

  • Jonathan Somer Post author

    The last equation and remark, together seam weird to me.

    The equation seams constant. Whereas he says Entropy rises.

    K is a constant, W is a constant.. what changes?

  • Amit Post author

    I also saw the video about gibbs free energy and spontaneity from khanacademy where the speaker was very good in explaining it, he spoke slowly and throughly explained stuff. You are speaking so fast and there are so any macro and micro states that you are just confusing people. I think you need paraphrase things properly, slowly and in order. No wonder this video has less views, Khan academy, thank you for all the great videos.

  • Incrue Post author

    Who are you? But you explain things very well, i see why Sal hired you

  • alexzracer Post author

    Man he stated that hot energy goes to cold energy toomany times.

  • Rahul Soni Post author


  • Robin Francis Post author

    Move heat from a cold object to a hot object using a refrigerator or heay-pump??…..a refrigerator of the vapour compression type has an evaporator containing a saturated refrigerant or 2 phase mix, which is at a lower temperature than the product!!….hence the transfer of heat energy!!……this is not 'cold to hot'…..I will stand corrected should anyone wish to point to my error!

  • Sun Kaur Post author

    what he is saying


    speak what is required,and shut the fuck up wen its not necessary to speak

  • MrSottobanco Post author

    Can you explain this? During winter, when I stick my tongue on a metal pole it gets stuck. The heat from my tongue flows into the pole. After some time, hypothermia sets in. But that is a different matter. Yet I feel a sharp painful cold before my body becomes numb. Isn't this "information" cold flowing back into the hotter object?

  • Samuel Ferrer Post author

    What do you call "ordered"? If you have a finite number of states, as you can always consider, then the power set is equivalent to the natural set. Now divide the power set in two sets, the one labeled with odd numbers, and the one labeled with even numbers. Those sets are equivalent. If one of them is labeled "ordered" and the other one "disordered", then order and disorder happens with the same probability. I think the right way to state it is: under certain conditions (natural spontaneous conditions) some configurations are favored over others. We happen to observe the favored ( disordered ) more frequently … and that is because of the limitation of our methods of observation that disturb the conditions …

  • zsolt tildy Post author

    ahh, S=k*ln1S=k*0

    so when theres only 1 macrostate entropy is 0. the most ordered things can be!

  • arixii Post author

    you mentioned that energy from the cold object will not be transferred to the warmer object but when you drew up the disordered/mixed up diagram you drew the cold energy migrating all over the place. Did I misunderstand something? I thought the cold energy doesn't move.

  • Aer Dun Post author

    This law needs to be reevaluated by the scientific community because it doesn't make sense for the two same constituents of the system not to exert equal forces on one another. When something acts upon something, there's always an equal force back in reverse!

  • ItsAnmolHimself Post author

    WHERE IS SAL !!!

  • Rajni Sabharwal Post author

    very nice…amazing. .thnx a lot khan academy

  • Murtaza Abdulhussein Post author

    Hello,I have a question, you said disorder will always increase, but when crystals are formed the order is increasing? Is this because this isn't a closed system?energy has to be inputted?

  • Anupam Chauhan Post author


  • MattsGotIssues Post author

    Heat= Excited Particles= High Velocity
    Cold= Depressed Particles= Low Velocity

    Energy from excited particles pass to the depressed particles. Since the total amount of energy can not increase spontaneously, the energy becomes divided? So the depressed particles speed up the same amount that the excited particles slow down until they all carry equal energy?

  • Divan Cronje Post author

    So only statistically speaking entropy will increase, BUT there is a small (very small) chance that entropy can decrease??

  • Roy Post author

    what about an object cooled to absolute zero, the molecules would stop and the speed of light would decrease, at this point the second law of thermodynamics would be redundant and the energy movement would then only be observable as sub atomic particles, which would then be able to operate outside the bounds of the second law of thermodynamics.

  • Samrat Banik Post author

    awesome description

  • Nicko Toedebusch Post author

    This helped so much really appreciate itπŸ‘ŒπŸΌ

  • Count Fleet Post author

    Why is Saul Goodman narrating this

  • Weirdo Science Post author

    Whats your name ??????????

  • Weirdo Science Post author

    Your lecture is awesome )))

  • Darshan Kothari Post author

    would it be more appropriate to say the "net" flow of energy from an ordered macrostate to a disordered macrostate?

  • Arnold Christian Post author

    I have a question. Once the entire system has achieved a steady temperature, would that mean that it would now be an ordered system? since every circle is red in the system. Do you know what i mean? lol

  • Kalyani Mehta Post author

    Really helpful

  • Krispijn Vanderpoorten Post author

    The order one is just like a rubix cube.

  • Rama Salem Post author

    Thank you so much!!

  • HSpartaL Post author

    I came from muse-unsustainable

  • Zhou Chen Post author

    very clear explanation

  • Asha joshi Post author

    But, The theory of very large set says that even the events with very less probability will occur given enough time.Since heat flows all around us isnt it possible that in one of the case the heat fliws from colder object to hotter objecy spontaneous

  • Johnny Dekker Post author

    Can this refute evolution because it's basically saying that things go from better to worse when evolution states that only better is ahead? 3:00–7:00

  • Mohammad Khan Post author

    This is amazing , why so many dislikes

  • Diptendu Roy Chowdhury Post author

    What this video lacks is the concept of a state in a State Machine

  • Werner Heisenberg Post author

    Best video I've seen describing this, just thought I'd let ya' know.

  • Thomaz Neto Post author

    I don't know if thos is right , but if the chance of the red balls and blue balls become ordered again isn't zero , so is there a chance of this system to become ordered again ? And if that happens the disorder of the system haven't decreased?

  • ginger nut Post author

    Wait so if you put an icecube and a hot block of copper together the chance of the cube being hot and the coper being cold is the same as the cube just melting?

  • TT Smasher Post author

    This law is stupid, it’s too obvious to be a law. If you tell me that it’s a good way to look at things, that’s fine, but make it a law, that sounds ridiculous to my ears

  • Sebastian Lukito Post author

    This video doesn't help me at all hahahaha. I would rather try to understand the equation and finish my college with B grade than suffer at this silly explanation.

  • Kaustubh Pandey Post author

    so if i did that for a infinite times will i get the other way around a lot of times

  • No Va Post author

    I always thought it was entropy of an isolated system that is greater or equal to 0 (not a closed system which can exchange heat but not matter with surroundings)

  • Akshay Chopra Post author

    Well lets say cold water slowed the hot molecules, coldness moves from colder body to hoter body, it can never move from hotter body to colder body

  • Cameron Reedy Post author

    he sounds a bit like mr poopybutthole. But sick vid man! Very very useful

  • Michael VPS Post author

    " Fewer microstates" – Stannis

  • khalil Post author

    Till now, best explanation I've seen

  • Nilaya Deshpande Post author

    Cold doesn't exist it means absence of heat which is not possible stated by thermodynamics

  • Abb Al Post author

    ur the best!

  • DeadDarkness Gameplay Post author

    I have a question, in the beginning, you mentioned that heat is not seen to flow from cold to hot, maybe you might refer to net heat transfer? I just have a thought that more heat flows from hot to cold than cold to hot, but none of the values be 0

  • sauronfupoc , Post author

    The order state describes an ordered universe so there is a disorder cause you describe a closed state

  • sauronfupoc , Post author

    A disordered state describes a messy room

  • Cherif Post author

    too much fluff with little substance!

  • Vikram Venkatesh Post author


  • Life after 40 Post author

    Absolutely fantastic explanation, thank you πŸ™‚ x

  • Radostin Vasilev Post author

    That's a good explained

  • joshua lynn Post author

    Refrigerator isnt forcing cold into hot. The evaporator is absorbing heat energy from the air volume in the refrigerator. As the air temp drops below that of the food heat is transferred from the food into the air where it is used to facilitate the evaporation of the refrigerant material. Therefore it is still in compliance with the 2nd law. (I'm a self educated hvacr tech)

  • joshua lynn Post author

    5:30 discussing the probability of all the balls being separated by color. Yes there is less probability of that 1 state vs the probability of all other states combined. That's basic math. But there should be the same probability for each individual state to exist therefore it is as likely that they will segregate by color as any other single state.

  • RAD RED Post author

    Why is this video too slow even if I watch in 2x

  • Around the world Post author

    What u mean

  • BeenThere&LovedIt Post author

    I don't understand the statement 'the first law of thermo doesn't account for the direction of spontaneous processes.' I don't see why not.

  • Deborah E. Post author

    This conflicts with the current belief of a vacuum of outer space next to the gas of the earth’s atmosphere. The firmament division is supported with this law. Space is fake.

  • Zheng Youtong Post author

    Does it violate the definition of entropy change: dS = dQ/T? For the total system described in the video, dQ = 0 and dS should be zero. However the S increases.

  • Pragya verma Post author

    I just realised that I have wasted 13 minutes of my life on this Video 😌. This Whole video doesn't make any Sense πŸ˜“

  • T H I C C . O X Y G E N Post author

    I AM ConFuSiON

  • Carlos Castanheiro Post author

    I think its because the molecules in a hot substance are already moving faster and the ones in the ice cube which are almost still get overwhelmed by the power of higher energy fields. Because energy is shared between the two states the one with a higher frequency of vibration is going to be moving with much more energy and literally overwhelming the cold ones. In a glass of water with an ice cube, the water "gives" its heat. Like a crowd of people running is going to overwhelm a small group of people and make them run. Higher frequency and in higher numbers means higher energy and more overwhelming power. Just like an atom bomb, more atoms, more reactions, more megatons , more energy dispersion, more destruction. So basically if you put an ice cube in water, the ice cube appears to drain heat energy from the water, but thats just the higher numbers in both energy and frequency invading the spaces in the ice cube because the energy is confined and compacted in the limited space of the glass. And if you drop a tiny bit of water on a big chunk of ice, the water will eventually freeze as it runs out of energy. There are 4 Forces in physics, strong nuclear force, weak nuclear force, electro magnetic and gravity, but theres also dark energy and dark matter and theres also two other Forces yet to be discovered. Lets call them planks, planks are hypothetical, they are the smallest particles in the universe and make up the very fabric of space. Imagine empty space has no empty space, its all filled with tiny planks, now imagine two types of planks, one attracts, the other repulses. Lets call them negative and positive planks. Now imagine these planks as the two extra Forces in the universe. Its because of them that we have physical matter, they are what makes up all the Forces in the universe, they make up the frequency that create strings, in string theory, and therefore make up all particles and therefore make up all molecules. Now this where science ends and science fiction begins. We can only hypothesize on this, but imagine these planks are connected and they obviously connect everything else. This is what they call the zero point energy field and what Einstein called space-time in his theory of relativity. Now if youre familiar with Bell's experiment on quantum entanglement, you could hypothesize that the basis of quantum entanglement communication is actually just interaction with the zero point energy field, which literally connects everything. So when we find a way to interact with planks we can communicate instantly between galaxies. And if Alcubierre's equation is right, we can also learn how to manipulate gravity by rearranging planks and contract and expand space. Just like Bob Lazar said, we could travel between solar systems instantly. Now we just need two things, a chemical element to generate a gravitational field around the space ship, thats a stabilized version of ununpentium or something else, element 115 as they call it. And a ship equiped with a reactor capable of generating the gravitational field around the space ship and also equiped with the means to generate a distortion around the ship. Basically it contracts space in front of the ship and expands it behind the ship, making it move forward without breaking any rules of thermodynamics, because the ship is inside a gravitational field, literally an indestructible force field and the laws of inertia do not apply to it. So this is how aliens travel. Lol, wtf did I just wrote?

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