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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。
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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？
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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。
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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。
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That's the important message, delta H of formation of the products, minus delta H of formation of the reactants.
去反应物的生成焓，这,是个重要的信息，生成物的生成焓,减去反应物的生成焓。
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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。
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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？
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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。
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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。
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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。
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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。
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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？
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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。
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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。
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So, I just want to write a few examples down with a few values for delta u or delta H or delta S, and see whether we can get any clues from what we see.
我先写一些例子,例子包含一些内能，自由焓和熵的变化数值，然后看从中,我们能不能得到一些启示。
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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。
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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。
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And from that, then you can look at enthalpies of reaction for countless numbers of reactions, right.
很多很多种化合物,利用这些你就可以研究,不计其数的反应的反应焓。
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OK, but in order to relate turning these physical knob to this quantity here, which we don't have a very good feel for, we've got to have a feel for the slopes.
热量是怎么进一步改变的,但是为了把这些物量同我们，不是很理解的焓联系起来，我们对微分已经有了一定的了解。
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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。
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And I want to make sure everybody figured out that it really was an irreversible expansion.
或者多孔塞的,不可逆的等焓过程。
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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。
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Because eventually they will tell us maybe things about how heat is changing further on.
我们需要知道焓在系统中怎样变化,因为最终它们或许会告诉我们。
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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。
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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。
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So now we have a different sign for delta H and it still happens spontaneously.
现在自由焓的变化的符号不一样了,但是这个过程仍然自发的进行。
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Do we calculate, you know, delta S, delta u, delta H?
是否要计算熵的变化，内能的变化，自由焓的变化？
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In other words, we choose a convention for the zero of entropy, so that then we can write entropies of products and reactants always referring to the same standard state.
换句话说，我们选择了一个,焓零点的约定，这样我们,就可以总是相对于同样的标准,状态写出生成物和反应物的焓。
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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。
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It's already in the lowest enthalpy state.
系统已经处于自由焓最低的状态。
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