Now for this experiment, this is a constant enthalpy experiment for the Joule-Thomson experiment, this is equal to zero.
对于这个实验,焦耳-汤姆逊实验,是一个焓不变的实验,焓变化等于0,所以我可以。
But we've defined the enthalpy of those elements in their stable state at room temperature and pressure as zero, right?
但我们已经定义了这些,元素处于它们室温和常压下,最稳定的状态时的焓为零,对吧?
Anyway, if you go to your Periodic Table you will find that on the one side you can get what is called the enthalpy of atomization.
如果你看元素周期表,你会在这一侧发现,这是原子化焓。
And the bigger lesson from that is that entropy, unlike energy u or enthalpy H, we could define an absolutely number for it.
热力学第三定律的一个更重要的推论是,与内能和自由焓不同,我们可以给上定义一个绝对的数值。
Something I can measure and I can tabulate, and when I turn my dT knob here I know what's going to happen to the enthalpy.
----热容,我们可以用实验,测出热容并成表,当我知道这里的变量。
And this is the reason why enthalpy was invented and why it's so important. Because we want to know this. So this for a finite change.
这也就是为什么焓要被提出,而且如此重要的原因,因为我们想要知道这些。
These are physical knobs that are available us as experimentalists. And so when we turn these knobs on our system, we want to know how the enthalpy is changing for that system.
这些都是我们做实验可,以得到的物理变量,所以当我们在系统中,改变这些量的时候。
So we're just subtracting, in effect, zero, right, from the enthalpy of the product, but of course it's important have that established because the heat of formation is something you could measure, right?
所以从效果上说,我们只是,从生成物的焓中减去了零,但,是确立这一点很重要,因为生成热是,你能测量的东西,对吧?
You know, if you want to measure the potential energy of something in a gravitational field, you have to define the zero somewhere, right, because it's arbitrary. You can set it anywhere you want. It's the same with enthalpy.
你知道,如果你想测量重力场,中某种东西的势能,你需要,在某处定义一个零点,对吧?,因为势能的值是任意的,你可以把它设定在。
Even though the signs of the enthalpy changed, and the signs of the entropy changed because it's a combination of the two that matters.
即使自由焓的符号发生变化,熵的符号也会发生变化,因为这两部分都起作用。
It's the constant enthalpy process that we're looking at. This we can do experiments on.
是一个焓不变的过程,这个量我们可以做实验。
The cart could be a little ways up the hill, and in this case, it's enthalpy, but again, there would be lots of accessible states.
在这种状态下,小车会稍稍向山坡上移动,在现在考虑的情况下,自由焓决定一切,但是同样有很多可能的状态。
And now, what is this saying, the equilibrium state is the one with the lowest possible enthalpy.
这时,上面的结论表明,平衡态是自由焓最小的态。
What we've been talking about, the Joule-Thomson experiment, constant enthalpy process?
首先,刚才说的,那些有什么问题吗?,焦耳-汤姆逊实验,等焓过程?
The constraint is the constraint of the experiment and the constraint of the experiment is that the enthalpy is constant. So the constraints we have here, is the constant enthalpy.
这个实验的限制,就是焓是常量,所以这里的限制是焓不变,我们考虑的这个过程。
And our heat of reaction or enthalpy of reaction is defined as the enthalpy at constant pressure.
我们的反应热,或反映,的焓被定义为恒压,等温。
What we define as zero is the enthalpy of every element in its natural state at room temperature and ambient pressure.
我们将零点定义为每种元素,在室温和正常大气压下,在其自然状态下的焓。
There's a balance here between ordinary energy or enthalpy and entropy.
通常所说的能量,或者自由焓和熵之间存在一个平衡。
You just change volume to pressure and basically you're looking at enthalpy under a constant -- anything that's done at a constant volume path with energy, there's the same thing happening under constant pressure path for enthalpy.
可以看到这就是把体积换成了压强,一般我们都是在一种恒定状态下,考虑焓的,任何在恒容条件下,能伴随能量变化的东西,也在恒压条件下伴随焓同样地变化,所以你可以经常。
OK, so we ended up last time, we talked about Joule-Thomson expansion, which is an irreversible expansion through a nozzle, through a porous plug, constant enthalpy expansion.
上节课,我们讨论了焦耳-汤姆逊,膨胀过程,也就是气体,通过毛细管。
It's a state function. If we know the enthalpy of the products minus the enthalpy of the reactants, right. It's a state function.
如果我们知道生成物的焓减,去反应物的焓的值,它是个态函数,原则上我们可以做到。
This is equal to zero. So this irreversible process this Joule-Thomson process, is a constant enthalpy process. Delta h for this process is equal to zero.
等于0。所以这个不可逆过程,也就是焦耳-汤姆逊过程,是一个等焓过程。
But if the system is in equilibrium, none of those states has lower enthalpy.
但是如果系统处于平衡态,所有其他的态都不可能具有更低的自由焓。
Delta H of formation means the enthalpy of this compound minus the enthalpy of its constituent elements in their most stable state at room temperature and pressure.
生成焓就是这种组分,的焓减去它的所有组成元素,在室温和常压下处于,其最稳定的状态时的焓。
It's already in the lowest enthalpy state.
系统已经处于自由焓最低的状态。
That was a constant enthalpy change.
这个过程是一个自由焓不变的过程。
And this is where enthalpy comes in.
这时我们及需要引入焓的概念了。
Enthalpy is just u plus p V.
你想要的任何地方。
That's by definition of enthalpy.
这就是由定义焓得到的。
That's why we created enthalpy.
也就是我们为什么定义焓的原因。
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