| 21.9 |
| The Third Law of Thermodynamics |
| In the previous section you might have wondered why entropy was not one of the functions in connection with standard states. The reason is not hard to guess .The absolute values of entropy can be ascertained. This follows from the third law of thermodynamics. Entropy is related to disorder or randomness. As the temperature of any system increases, the system changes from solid to liquid and then to gas. |
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| In the gaseous state, the molecules move randomly with respect to each other and there is a great deal of disorder. As the temperature is reduced to very low values, the system generally cools and gets arranged in a definite order. In the case of perfect crystals, there is indeed perfect order and the entropy of such a state is zero. |
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| This is the content of the third law of thermodynamics, which was formulated by Nernst, Planck and others. G.N.Lewis and M Randall stated the third law of thermodynamics as follows. “If the entropy of each element in some crystalline state be taken as zero at the absolute zero of temperature, then every substance has a finite positive entropy; but at the absolute zero of temperature, the entropy may become zero and does so in the case of perfectly crystalline substances”. |
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| The value of zero for entropy refers to perfect order. Many substances do not condense into perfect crystals even at the absolute zero of temperature. For example, CO at 0 K does not have perfect order. Each CO molecule can be placed as CO or as OC, because both arrangements are energetically equally favorable. The value of entropy can be obtained by the Boltzmann formula, S = kB ln W, where W is the number of possible arrangements of the molecules. Since each molecule can be arranged in two ways, NA molecules can be arranged in 2NA ways. Here NA is the Avogadro number. Thus the molar entropy of CO at 0K is NA kB. ln 2 or R ln 2, which is about 1.4 cals/deg/mole. This has been verified experimentally. |
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