Enthalpy | Thermodynamics | Chemistry

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Enthalpy | Thermodynamics | Chemistry | Khan Academy

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15 жыл бұрын

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Understanding why enthalpy can be viewed as "heat content" in a constant pressure system. Created by Sal Khan.
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Пікірлер: 281

@eragon2121 7 жыл бұрын

"Let me do this in a different color" = Sal's catchphrase.

@rozito2086 5 жыл бұрын

There should be a compilation of sal saying it

@lusandasithole5704 4 жыл бұрын

Another one: " let me use a Magenta colour!" I love it when he says that. 😆

@Shadow12aven 3 жыл бұрын

Khan "Half of the video is me changing colours" Academy. lol.

@mariannecasastorres3930 3 жыл бұрын

"It's almost kinda too obvious for me to explain" Oh Sal. You overestimate me 🥲

@a-levelking8610 2 жыл бұрын

KZfaq asked me to give feedback about your comment lol

@chaijackleng4486 6 жыл бұрын

40 pages of textbook and five hours of lectures can't make me understand enthalpy, Khan did it in just 15 mins.

@whogavehimafork 5 жыл бұрын

Yep, my thermo book tends to not explain concepts very well, rather it focuses almost entirely on the mathematics.

@sirchomsky784 2 жыл бұрын

Tag me

@virupannamedikinal Жыл бұрын

Include me

@benjaminrickdonaldson 6 ай бұрын

Those books were probably explaining other stuff around enthalpy assuming you already knew what enthalpy is.

@lili011 12 жыл бұрын

You have no idea how much appreciation I have for your videos. I'm practically self-teaching myself by watching your videos and I swear, I'm learning more than I'm lectured in class.

@debashreekonwar061 2 жыл бұрын

Truth

@wasswa6 12 жыл бұрын

i would appreciate it if you could lend me your head for a day for my exams, will return to you ASAP. thanks

@mpiana3 6 жыл бұрын

"if I'm sitting at the beach and I have my chemistry set" lol

@rubendragan6514 4 жыл бұрын

13:35

@addy7464 4 жыл бұрын

Thats too nerdy even for a nerd like me

@zeinabfarhat1920 2 жыл бұрын

12 years later and you're videos are teaching us better than anything !

@NotoriousB3 7 жыл бұрын

Sal takes his chemistry set to the beach. What a nerd

@ashhleyz 8 жыл бұрын

what I don't understand is why we need to know all of this. like why do we need to know HOW we get what equals to delta H.

@Rammsteinmaiky1 7 жыл бұрын

You don't have to, if you don't want.

@luisambriz4673 7 жыл бұрын

I think the biggest misconception about school is what we learn is important it is. But what is the most important thing about all these subjects is how you learn, not what you learn. Through learning and going through the processes you learn how to think differently and abstractly rather than the old way of 2+2=4

@NotEvenPerfect 6 жыл бұрын

Hey, let's hope you are not a chemist for NASA now, since if you don't know why it only applies at constant pressure, your spaceshuttles will explode

@victorianunez7113 6 жыл бұрын

Honestly same... I just want to get into nursing school. 😂

@peytonstein8616 6 жыл бұрын

Its less about learning the specific lesson, and more about teaching the learning process. And putting you in situations where you have to learn and think for yourself.

@831candygurl 11 жыл бұрын

Thank you for helping me not fail chemistry. Much love. -Julianne

@ull893 12 жыл бұрын

He may sound funny, but he can explain things in such clear and simple manner only because he understood the subject very well and not just text book learning. When you think he teaches almost every subject with such mastery, he is AMAZING..!!

@mragren 13 жыл бұрын

"So I´m sitting at the beach and I have my chemistry set". A perfectly reasonable scenario haha. Love these videos.

@tope2 14 жыл бұрын

way better than college explanation thank you so much

@manjula4872 4 жыл бұрын

I am from India I am just 12 and I need to learn all these but khan bro done this very easy : )

@candiengng4063 5 жыл бұрын

Understanding state function is vital important in studying Thermodynamic. This are also the key to understand entropy and enthropy.

@JonAndKatyAnderson 7 жыл бұрын

at 10:30 to 11:45 -- If pressure is constant, then delta P is zero. if you have a dP*V, you can't "factor" it to P*dV. dP means "change in pressure", so if you posit that pressure is constant, then dP= P2 - P1 = 0, because P2 = P1. Then you have dP/P = 0 = dV/V, which means that volume is also constant. What am I missing here?

@JayaSaputra 5 жыл бұрын

It's d(PV)= dP*V + P*dV Since dP =0, then d(PV) can be reduced to only P*dV Cmiiw

@EECCtv 14 жыл бұрын

Your 10 minute videos teach me more stuff than a 3 hour lesson with my chemistry teacher

@modlexstudies2245 Жыл бұрын

@madass888 13 жыл бұрын

You can rewrite dpv to: dpv = vdp + pdv. Which gives dH = Q + vdp. More compact and you can see immediately what happens when p is constant.

@dayaropmay6455 6 жыл бұрын

In the textbook I have.. the first law of thermodynamics is U = q + w not q-w

@Aleu931 5 жыл бұрын

I was about to ask this question

@spectrum792 5 жыл бұрын

@Prachi Sharma what's the difference between work done on vs by the system?

@youdontknowme8818 5 жыл бұрын

@@spectrum792 (considering pressure-volume work) Imagine there is a cylinder filled with gas with a piston. If you apply pressure on the piston, the gas compresses which implies work is done on the system If the gas expands moving the piston upwards it is said that work is done by the system

@guillaumethemapler 5 жыл бұрын

This is because in your book they consider every change in energy from te systems point of view. It is de convention in chemistry. I personaly think its less confusing. A negative q means there is heat taken from the system. A negative w means the work is done by the system. So it loses energy. This energy is now on the surroundings. That is why a negative Delta H is a exotherm reaction. Heath is coming out of the system so the energy content of the system decreases.

@jed2112 5 жыл бұрын

It changes btween chimestry and phy .the point of view is not the same(by/on)

@kicsiqki 12 жыл бұрын

I tried reading the P. W. Atkins book both in English and in my native language and after 50 pages I was so lost I had no frickin idea what it was talking about. thanks for explaining these things!

@akhilr123456789 11 жыл бұрын

you are a gift by god!!! he will bless u sal!!

@dashabaliar2942 9 жыл бұрын

Thank You very much. I'm studying for my biophys exam and I was getting pretty helpless until I watched your tutorial..:)

@theguardianaj 11 жыл бұрын

MOTHER OF GOD I understand it! And just in time for tomorrow's chem test!! Thank you!

@c310188 13 жыл бұрын

That's what i ever wanted! Someone who DOES know how to teach Thermodynamic's law... Thank's God a lot!

@Amphibianman94 11 жыл бұрын

Thanks, don't know what I'd do academically without the internet!

@kirkmetkirk 10 жыл бұрын

"good old P-V diagram" nice :)

@kevinbaughman5042 11 жыл бұрын

you are literally the best teacher ever...

@adamsalinas82 6 жыл бұрын

You made this understandable (amazing)

@tangycheezexists 13 жыл бұрын

Thank you kind sir, although it's a little difficult for me to understand since I'm only in high school chem, this helped a lot, nonetheless.

@lokeshjangir3089 2 жыл бұрын

fun fact: in india their is different sign conventions in physics and chemistry of work done by the system .

@cihattekne7265 Жыл бұрын

ı am so happy because ı didnt need to listen the lecture from indian engilish. :D

@austinehl 12 жыл бұрын

already feel better about my exam, thanks!

@justincrowder8621 Жыл бұрын

My textbook shows deltaU = q + w. You're saying deltaU = q - w. Which is correct?

@Katy_panda22 2 ай бұрын

Yes, that's right. It's just a terminological thing, so some books assume the opposite

@ummehoneybisma2065 10 ай бұрын

14 years!!!! thats huge

@antoniovianaaa 6 жыл бұрын

Thank you, i thought i wouldn't sleep tonight. This doubt of enthalpy definition was really bothering me.

@truth3137 6 жыл бұрын

Make a video on bond energy

@dhruw9935 2 жыл бұрын

I wonder how awesome the teacher was who taught Sal.

@lulabell7253 12 жыл бұрын

he makes it seem so easy...if there is anyone out there that has time can you tutor me..I have in exam next week and if i dont pass it i fail the class. this is the only class that i dont understand.

@wc7vdaner 12 жыл бұрын

i spent all night trying to get this. it took this video for me to actually get it

@JohnSmith-ed1sr 8 жыл бұрын

This video did absolutely nothing to tell us what enthalpy is. but only what enthalpy is during constant pressure

@HitanshuTheBoy 5 жыл бұрын

correct!

@spxtfxre1 6 жыл бұрын

Why don't you get a separate branch for the Indian curriculum for Chemistry like you have for Physics and Mathematics?? I would be really glad if you take this into consideration.

@92thomas92 12 жыл бұрын

"good ol' PV diagram" lol this guy cracks me up, not to mention knows how to explain chemistry topics

@divyeshparadava5629 8 жыл бұрын

I am still confused , if the enthalpy is state function then why it should increase after completion of cycle( as said in video that heat content is first 4 and then 6 ).

@prateeksharma6756 Жыл бұрын

Wow The best video explanation of not just enthalpy but why we need it and why it's helpful, you won't get this quality education if you paid for it.

@sdfzhy61 11 жыл бұрын

wow...THANKS SO MUCH

@louyclare 13 жыл бұрын

@bagrasweetheart No, it will not work if V is constant. That is because work is equal to pressure times the change in volume. If the volume is held constant, then a change in pressure will not do any work. Remember, the mechanical definition of work is force times displacement. If you push against a brick wall you technically do no work.

@ThisizRio 10 ай бұрын

hello, I have a question! >>>>> so enthalpy is a state variable that defines/expresses the heat content of that state at a constant pressure? which means that at p1 water at some point in the liquid state will have a heat content or enthalpy with a certain value no mater the route it got there>> what are the different possible routes that we can get there? that's what I don't understand, any materials scientists out there???????

@vayar8908 4 жыл бұрын

Thank you so, so much! I finally understand this after many years))

@265HITMAN265 12 жыл бұрын

compressing something while keeping the internal temperature in the system the same at all times

@jacobvandijk6525 Жыл бұрын

@ 10:07 d(p.V) = dp . V + p . dV; at constant pressure (dp = 0), thus d(p.V) = p . dV

@jacobvandijk6525 10 ай бұрын

@@Knipper2000 The non-delta values are the initial values (V1 and p1). Here (0:18), your case of variable pressure and constant temperature is explained. After a full cycle, d(p.V) = 0.

@PawanKumar-hx5zi 5 жыл бұрын

Awesome video, thank you for the effort

@varathan1662 7 жыл бұрын

what happens to enthalpy when pressure of a gas is reduced (joule thomson)

@prakasheragon 13 жыл бұрын

you say when that heat content would need to be incremented by 2 everytime you go around the path. but wouldn't the equivalent amount of energy be used up by work? as work is equal to heat in closed path? so the heat content should remain the same ryt? someone please answer this!!

@SaucyNight 10 жыл бұрын

I am a little confused here, deltaPV=PdeltaV say that Pressure is constant at 5 atm and volume is going to change from 6 to 7 liters. wouldnt delta Pressure equal zero if there is no change? then you would end up with zero on the left since deltaP=0 and you would end up with 5x1 on the right because deltaV=1 and P= 5 ?

@ghizlanegzili1695 10 жыл бұрын

deltaPV is delta(PV) not delta(P)*V I think this will clear things up for you! :)

@DerToasti 6 жыл бұрын

how can we just assume that pressure is constant? it clearly can't be if the volume is changing. see pv=nrt.

@superroydude 6 жыл бұрын

The Volume increase is proportional to the increase in temperature. So, overall The pressure stays the same. P=nRT/V

@DerToasti 6 жыл бұрын

is that true for both an open and a closed beaker? is the open beaker the nice case where the formulas work just fine?

@jonson1474 4 жыл бұрын

I know it's an old video, but aren't you missing a very important assumption? I mean, enthalpy is equals to heat at constant pressure IF the energy you give to the system is all heat, because if there's work involved that's also included in enthalpy (except of course the expansion work). As a more general rule I'd say that at constant pressure enthalpy = heat + work other than expansion.

@benjaminmcintosh857 2 жыл бұрын

if work is involved then for the pressure to remain constant there must also be some heat transfer, so the constant pressure assumption is sufficient

@owlbabu 12 жыл бұрын

EXCELLENT! Thank you!

@raplopez4258 2 жыл бұрын

11:05 , why isn't it: ∆PV = (Pf - Pi)V Say P is *constant* , and P = 5 (5 - 5)V = 0 Or another way, why is there an xf and xi if there is no ∆ associated with them: ∆5x = 5x - 5x = 0 or ∆PV = ∆5V = 5(f)V - 5(i)V = 0 I assume then, that the ∆ refers to the entire "group" of variables that are multiplied together, and not just the one variable...

@AdamThomas671 8 жыл бұрын

meanwhile 49 other people do not believe that enthalpy is the heat content in a constant pressure system

@jozefinagramatikova8278 6 жыл бұрын

Thank you very much for explaining this! I needed this to solve one problem. I've been looking for this for a while. You made my day!

@damgnj 7 жыл бұрын

FINALLY I KNOW WHAT IS THE ENTHALPY!

@paulcandiago9649 3 жыл бұрын

Sir of Khan Academy, congratulation, you solved a problem of Energy between to conditions of matter, but zero explanation about Enthalpy: how it got there in the first place? Or if you want X, Y, Z didattical introduction and explanation to the the word Enthalpy as defined by the Physics. Then an example defintetly will help.

@shylildude 9 жыл бұрын

is change in internal energy equal to the heat added when volume is constant?

@Morain42 11 жыл бұрын

it can be both, the work done 'w' is negative if the environment does work on the system, and positive if the system does work on the environment.

@sandrejev 13 жыл бұрын

When you say deltaPV it would be nice if you would add brackets around delta(PV). Otherwise it reads like (deltaP)*(V)

@wvadam 13 жыл бұрын

does this work for constant volume as well?

@reimannx33 6 жыл бұрын

God said ..let there be light ..rest is bada bing , bada bong

@GHRepresent 13 жыл бұрын

why does it look like we are counting pressure volume work twice ? Internal energy includes PV work. Why do you add it again when defining enthalpy.

@RandomGuy-nl4yq 4 жыл бұрын

Best thing avialable on u tube

@stritax9478 Жыл бұрын

This was awesome

@shauryaverma2705 Жыл бұрын

Great video love you sal

@allielikes17 10 жыл бұрын

you have been my savior for my chem test tomorrow

@menglimarrero4296 2 жыл бұрын

Ive just learned from this video Delta pv=P delta V

@rscottalex 14 жыл бұрын

Ah, midway thru the equation has delta p on one side and assumes constant pressure. Doesn't that make delta p 0 and then the whole equation falls apart?

@samuelvasans8296 4 жыл бұрын

Did anyone hear the sound of a wave at 13:52 when he said sea level?

@user-hu2sc6po3z 7 жыл бұрын

How about the volume is constant instead of constant pressure? Still get same calculation or other results?

@Rei-fm9yw 3 жыл бұрын

You need to put PV in parenthesis. delta(PV)

@brandonbennett4970 2 жыл бұрын

So is enthalpy only defined for isobaric processes? I dont see how it would be useful variable to define with any other process, as the work and P(delta V) terms wont cancel out nicely.

@ad2181 15 жыл бұрын

Sal, The Thermo concepts where deveolped (1800s) to extract the max work from a steam engine. They can be applied today when looking at alternate energy. Personal I don't believe in this hype. Energy at a high Temperature is not free and that's whats required to keep this PC and my lifesytle running on demand.

@Eta_Carinae__ 2 жыл бұрын

If /del/ H = Q, and /del/ PV = P/del/V, doesn't that make Q a valid state variable, and consequently imply all paths are constrained to a single curve? Because I feel like the solution space for constant P sounds to restrictive, since that's only a line instead of a curve.

@searay2621 8 жыл бұрын

Tyler Dewitt all the way for videos.

@pedrocarvalho6587 4 жыл бұрын

If the pressure is constant then the change in pressure is zero? Why did you write that deltaPV= PdeltaV?

@KristhAJ 12 жыл бұрын

One question... the work is positive? No, negative....

@Be1smaht 13 жыл бұрын

@Yohannesiscool its because one is work done ON the system and one is work done BY the system. Work done BY the system should be positive ( STILL CHECK THIS ) I think and work done ON THe system is negative. u should google work done by vs. work done on...

@muniqloved 11 жыл бұрын

Delta p = change in pressure. since there is no change in pressure, as it is constant through out, delta p = 0. hope it helps.

@bhaskarjanu525 5 жыл бұрын

Its ∆(PV). IF Pressure is constant then P∆V

@smashadams9811 12 жыл бұрын

Examkrackers type humor lol. I love it.

@junkmana 12 жыл бұрын

If the system does net work, where does the extra energy to do the net work come from? FOr example, if a work is done on the system from 2-1 and then the system does work from 1-2 with a different path. We have a net work done by the system, but the system should have energy equal to the work done on it. So how can the system do extra work that is more than work applied to it?

@Maibes 12 жыл бұрын

hahaha, he's such a good teacher. what a good thing.

@celestialjellyfish 12 жыл бұрын

@ccrunnerx92x W(by system) = -W(on system) which is Newton's Third Law of Motion (you are assuming the system obeys the assumption s of Ideal Gases, so particle collisions are elastic). He is using the work done BY the system, therefore it is negative. You can substitute in the work done ON the gas and hence it would be... dU=Q+W :)

@vivekchaudharyiitguwahati3511 5 жыл бұрын

it was explained in the vedio that after each cycle heat content gonna increase in the system, it is right??? then how can we get work from system??

@ProfFeinman Жыл бұрын

DeltaP = 0 if constant P. You switch from the cycle to a general state. It is, in any case, too fast.

@Emma-kw1nx 3 жыл бұрын

I cant believe this video is 11 years old

@Saiixx 10 жыл бұрын

holy shit this was a great explanation. totally explained my thermo class on enthalpy thanks! The whole reasoning for state properties was huge

@divyanshveersingh9680 4 жыл бұрын

But how is internal energy a state variable when it is the addition of a state variale and a non-state variable?

@karenchai5956 8 жыл бұрын

Hi I need help with defining enthalpy. In chemistry we learnt that enthalpy = (q / mol of limiting reactants ) x coefficient My question is that why enthalpy based on limiting reagent? Why is it not based on the TOTAL reacted reactant? For example 2A+B -> C Enthalpy = q/ (mole A+B)

@SciFiFactory 4 жыл бұрын

I am extremely confused at 10:10 So when you write "delta P V" you actually mean "delta (P*V)" ? Because otherwise a constant P would result in "delta P=0" and then you would have "0*V = P*delta(V)" , which would not be true, right? Am I missing something ? Does the first delta apply to both P and V or just to P?

@clarencegreen3071 2 жыл бұрын

delta (pV) = p delta V + V delta p There's a bit of sloppy notation in the video.

@ishaaqahamed3888 3 жыл бұрын

Can someone help me through this dilemma? Work is defined as the (W = F * s) when a body moves a straight distance under a constant force. However, since for a curved path of varying force, we break down the curve to infinitesimal segments where the Force is locally constant and the distance (ds) is essentially straight. Thus, for this tiny line segment, we apply the Work concept and we have dW = Fds. Summing up all the dW would give us the total Work. This is why dW = Fds (and never dW = Fds + sdF). I have the following problems: Under what circumstances is Enthalpy explicitly defined for a system? How can we say that dH = du + PdV + VdP when we never say dW = Fds + sdF?

@varunarora1256 4 ай бұрын

I know it's pretty late and you might've left these roads a lot behind in your life. But, if you still wanna know (of course if don't already), it's actually d(PV) and not P.dV or V.dP as it is in the work done formula dW= F.ds d(PV) expands in accordance with the chain rule of differentiation which gives us PdV+VdP. Either of these terms can get zero in an isobaric or isochoric process.

@ishaaqahamed3888 4 ай бұрын

@@varunarora1256 Its been a long journey. I graduated since and currently a prospective doctoral student. And yes, I did figure it out eventually... But thank you for taking the time to reply. 😆🥰

@varunarora1256 4 ай бұрын

@@ishaaqahamed3888 ohh it's crazy how much one does in just three years 😌 Btw, a doctorate in what subject if I may ask? Is it chemistry related 😁