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It takes the right combination of wind,temperature,pressure and humidity to create even a weak tornado.
VOA: special.2010.05.11
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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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are all functions of state and parameters that we can control like temperature and pressure.
公式里面的全部都是态函数,我们控制态函数的参数比如温度或者压强。
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Whereas under these conditions, these quantities, if you look at free energy change, for example at constant temperature and pressure, H you can still calculate H.
但是，在这些条件下，这些物理量，如果我们考察自由能的变化,例如在恒定的温度和压强下，我们仍然可以计算。
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Hydrogen gas it's in its most stable state, right at room temperature and pressure.
和常压下氢气是氢元素，最稳定的状态，这个小“0“
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It's a state function, so we're at constant temperature and pressure, and now we want to consider some chemical change or a phase transition or you name it.
这就是态函数，我们处于恒定的温度和压强之下，然后考虑某些化学变化或者相变,或者你想考虑的东西。
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There's a volume, there's a temperature, than the pressure here. There's other volume, temperature and pressure here, corresponding to this system here.
温度等状态函数有本质区别，这个状态有一组，确定的体积，温度与压强。
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And I didn't specify the conditions, but if we were to do this under ordinary chemical conditions of some, you'd say room temperature and pressure, right, they all happen spontaneously.
我并没有特别说明反应条件，但是如果我们在通常的化学条件下,实现这些化学反应，比如在室温和大气压下下，他们都是自发的。
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Now in this particular reaction, I've got hydrogen gas, iron solid. Those already are elements in their most stable forms at room temperature and pressure.
特定的反应中，我有氢气,固态铁，这些都已经是,处于室温和常压下,最稳定的元素单质。
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And this volume, temperature and pressure doesn't care how you got there. It is what it is.
另一个状态,也有一组确定的体积。
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Because so much of what we do in chemistry does take place with constant temperature and pressure.
因为化学中我们所做的很多东西,都是在恒定的温度和压强下进行的。
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There's our condition for equilibrium at constant temperature and pressure.
这就是我们在,恒定温度和压强下的平衡条件。
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Similarly for G as a function of temperature and pressure, I can go through the same procedure.
同样的G作为,温度和压强的函数，我可以采用相同的步骤。
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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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And where does that happen, At what temperature and pressure and so forth.
在什么温度，和压强下。
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And maybe if it some well-defined temperature and pressure.
这两个态可以具备确定的,温度和压强。
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Iron as an element is a solid. That's it's most stable state at room temperature and pressure, right, and so on. And then we can figure out heats of formation.
铁单质是固态,这是他在室温和常压下,最稳定的状态，然后我们,可以算出生成热，现在这个。
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And the triple point of water is 16K that temperature and pressure - there's a unique temperature and pressure where water exists in equilibrium between the liquid phase, the vapor phase, and the solid phase.
与水的三相点,我们定义它为273。,水的三相点,是一个特定的温度和压强,可以使水的液态，气态和固态三相共同达到平衡，三相点是液态。
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pV Also A plus pV and G is minimized at equilibrium with constant temperature and pressure.
同时等于亥姆赫兹自由能A加上,同时在恒定的温度和压强下。
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So this unique temperature and unique pressure defines a triple point everywhere, and that's a great reference point.
这样，无论在何处,三相点都具有相同的温度和压强,十分适合来作参考点。
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On the other hand, temperature, volume and pressure are variables that are much easier in the lab to keep constant.
另一方面，温度，体积和压强,在实验室中比较容易保持恒定。
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We could just collect a bunch of data. For a material .What's the volume it occupies at some pressure and temperature?
对一种物质我们可以得到一系列测量数据,在给定的温度和气压下,它的体积是什么？
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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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And the equation of state, pressure versus volume at constant temperature, is going to have some form, let's just draw it in there like that.
系统的态函数，恒温下压强比体积，变化曲线，就像这样。
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For instance, the pressure and the temperature, or the volume and the pressure.
比如压强和温度，或体积和压强。
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And so, again, we see a temperature increase, and we know the work, and the temperature increase, it's a constant pressure thing.
好，我们看到温度升高了，然后我们有做功量和温度的升高量，这是一个恒定压力下的值。
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So, all I want to do now is look at the derivatives of the free energies with respect to temperature and volume and pressure.
我现在所要做的一切就是,考察自由能对,温度，体积和压强的偏导数。
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And you can find these compressibility factors in tables. If you want to know the compressibility factors for water, for steam, at a certain pressure and temperature, you go to a table and you find it.
各种气体的压缩系数,想知道水或者水蒸气,在某个温度和压强下的，压缩系数，查表就行了，这是实际气体状态方程的。
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We know how the volume and temperature vary with respect to each other at constant pressure.
知道在恒定压强下,体积如何随着温度变化。
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OK, you use the ideal gas law, etc., then you get a relationship that connects the pressure and the temperature, like here we got a relationship that connected the temperatures and the volumes together.
我们会得到，一个联系初末态,的压强和温度的，关系式,就像这个联系过程中。
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