Enthalpy | Thermodynamics | Chemistry | Khan Academy

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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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Chemistry on Khan Academy: Did you know that everything is made out of chemicals? Chemistry is the study of matter: its composition, properties, and reactivity. This material roughly covers a first-year high school or college course, and a good understanding of algebra is helpful.
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Опубликовано:

17 окт 2024

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Комментарии : 283

@eragon2121 8 лет назад

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

@rozito2086 5 лет назад

There should be a compilation of sal saying it

@lusandasithole5704 4 года назад

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

@Shadow12aven 3 года назад

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

@lili011 12 лет назад

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 3 года назад

Truth

@chaijackleng4486 7 лет назад

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

@whogavehimafork 6 лет назад

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

@sirchomsky784 2 года назад

Tag me

@virupannamedikinal Год назад

Include me

@benjaminrickdonaldson 8 месяцев назад

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

@ahmedmalik-nc1ky 10 дней назад

@@benjaminrickdonaldsonhaha true they even my teacher did the same. And i asked him several times.he told , consider it as heat and I was like 🤷‍♂️ if it was that simple I wouldn't stuggle for hours at home

@mariannecasastorres3930 3 года назад

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

@a-levelking8610 2 года назад

RU-vid asked me to give feedback about your comment lol

@zeinabfarhat1920 3 года назад

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

@wasswa6 12 лет назад

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 лет назад

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

@rubendragan6514 4 года назад

13:35

@addy7464 4 года назад

Thats too nerdy even for a nerd like me

@NotoriousB3 7 лет назад

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

@831candygurl 11 лет назад

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

@tope2 15 лет назад

way better than college explanation thank you so much

@ashhleyz 8 лет назад

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 лет назад

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

@luisambriz4673 7 лет назад

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 7 лет назад

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 лет назад

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

@peytonstein8616 6 лет назад

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.

@madass888 13 лет назад

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.

@ull893 12 лет назад

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..!!

@candiengng4063 5 лет назад

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

@c310188 13 лет назад

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

@akhilr123456789 12 лет назад

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

@purpleworld5025 Месяц назад

Why is there need of Enthalpy when there are already functions like Q and U to represent energy

@mragren 13 лет назад

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

@kicsiqki 12 лет назад

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!

@justincrowder8621 2 года назад

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

@Islam_Challenges1 4 месяца назад

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

@Amphibianman94 12 лет назад

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

@JonAndKatyAnderson 7 лет назад

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 6 лет назад

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

@jcamargo2005 Месяц назад

It is simpler to me if you consider Δ(PV), in parenthesis. Then Δ(PV) = (ΔP)V + (ΔV)P. The second term cancels out with W, and ΔH=Q in constant pressure conditions

@adamsalinas82 6 лет назад

You made this understandable (amazing)

@theguardianaj 11 лет назад

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

@manjula4872 4 года назад

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

@EECCtv 14 лет назад

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

@modlexstudies2245 Год назад

@kevinbaughman5042 12 лет назад

you are literally the best teacher ever...

@lulabell7253 13 лет назад

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.

@shauryaverma2705 Год назад

Great video love you sal

@dhruw9935 2 года назад

I wonder how awesome the teacher was who taught Sal.

@cihattekne7265 Год назад

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

@ThisizRio Год назад

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???????

@tangycheezexists 13 лет назад

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.

@dashabaliar2942 9 лет назад

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

@austinehl 12 лет назад

already feel better about my exam, thanks!

@SciFiFactory 4 года назад

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 года назад

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

@prateeksharma6756 2 года назад

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.

@wvadam 14 лет назад

does this work for constant volume as well?

@sdfzhy61 11 лет назад

wow...THANKS SO MUCH

@ummehoneybisma2065 Год назад

14 years!!!! thats huge

@wc7vdaner 12 лет назад

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

@PawanKumar-hx5zi 6 лет назад

Awesome video, thank you for the effort

@jacobvandijk6525 Год назад

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

@jacobvandijk6525 Год назад

@@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.

@paulcandiago9649 3 года назад

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.

@owlbabu 13 лет назад

EXCELLENT! Thank you!

@kirkmetkirk 10 лет назад

"good old P-V diagram" nice :)

@truth3137 7 лет назад

Make a video on bond energy

@265HITMAN265 12 лет назад

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

@vayar8908 5 лет назад

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

@spxtfxre1 7 лет назад

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.

@廣東歌世一 7 лет назад

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

@Eta_Carinae__ 2 года назад

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.

@stritax9478 2 года назад

This was awesome

@prakasheragon 13 лет назад

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!!

@dayaropmay6455 7 лет назад

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

@Aleu931 6 лет назад

I was about to ask this question

@spectrum792 5 лет назад

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

@youdontknowme8818 5 лет назад

@@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 лет назад

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 лет назад

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

@SaucyNight 11 лет назад

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 лет назад

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

@brandonbennett4970 2 года назад

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.

@raplopez4258 3 года назад

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...

@RandomGuy-nl4yq 4 года назад

Best thing avialable on u tube

@Rei-fm9yw 4 года назад

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

@fionawonnacott3654 12 лет назад

you help me soooo much thanks

@sandrejev 14 лет назад

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

@vivekchaudharyiitguwahati3511 5 лет назад

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??

@antoniovianaaa 7 лет назад

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

@louyclare 13 лет назад

@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.

@Morain42 12 лет назад

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.

@jozefinagramatikova8278 6 лет назад

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!

@MrRaji81 8 лет назад

Thank you very much..

@menglimarrero4296 3 года назад

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

@sophie3howl 12 лет назад

So, to make enthalpy equals to the amount of heat added we make the pressure constant so that in the calculation we can cancel out W and PV, right? Can you give me an example In real life, in which this condition really applies? Let's say in Energy Conversion thing, like the Heat Pump Cycle or Refrigeration... Really appreciate it, Sir!

@GHRepresent 13 лет назад

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.

@varathan1662 7 лет назад

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

@allielikes17 11 лет назад

you have been my savior for my chem test tomorrow

@divyeshparadava5629 8 лет назад

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 ).

@92thomas92 12 лет назад

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

@damgnj 8 лет назад

FINALLY I KNOW WHAT IS THE ENTHALPY!

@ahmarrehan1044 Год назад

Great teacher

@Nanomachines5on 12 лет назад

Thank you!!! This makes so much more sense to me now!

@ACRemote-z8x 4 года назад

Thank you so much

@ishaaqahamed3888 4 года назад

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 6 месяцев назад

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 6 месяцев назад

@@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 6 месяцев назад

@@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 😁

@AdamThomas671 8 лет назад

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

@muniqloved 11 лет назад

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 лет назад

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

@CFD_Mechanic 12 лет назад

love it

@lokeshjangir3089 2 года назад

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

@hahatrolledbabe 13 лет назад

wait but doesn't delta V and V also have to be the same? Because if they're different that doesnt cancel out the P(deltaV) with (deltaP)V?

@metrolady72 15 лет назад

i love these videos. thanks

@عباديعيد 17 дней назад

thanks

@asusvilla 11 лет назад

wow which software is that to sketch?

@rscottalex 14 лет назад

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?

@reimannx33 6 лет назад

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

@DerToasti 7 лет назад

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

@superroydude 6 лет назад

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

@DerToasti 6 лет назад

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

@ProfFeinman Год назад

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

@junkmana 12 лет назад

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?

@shylildude 10 лет назад

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

@as456d1 10 лет назад

if the entropy is close to zero, what does this say about the enthalpy- would it be exothermic, endothermic or something totally different?

@agent475816 10 лет назад

even if delta H is = to -.00000000000001 it is exothermic. lol if it is plain out zero, you're doing something wrong. All reactions either release or absorb energy.

@pedrocarvalho6587 4 года назад

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

@0utKast10S 11 лет назад

excellent. thank you

@vivekchaudharyiitguwahati3511 5 лет назад

if heat is added to the system and system is doing equal work on surrounding then heat is converted to work and it's heat content will be constant after many cycles