| 21.8 |
| Standard States |
| In the case of thermodynamic functions such as energy, enthalpy and free energies, only the difference of these functions between two states of the system can be measured. It is not possible to ascertain the absolute values of these functions. In order to get around this problem, standard states for each substance are defined and the values of these functions in other states are measured with respect to these standard states. The standard state values are represented as Uo , Ho , Ao and Go. |
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For solids and liquids, the standard state at each temperature is the state at a pressure of 1 bar. For gases, the standard state at each temperature is the (hypothetical) ideal gas state at a pressure of 1 bar. The changes in thermodynamic functions between the standard states and other states can be calculated using the equations (21.14), (21.19), (21.23) and (21.24). These four equations are equivalent to one another and are referred to as the fundamental equations of thermodynamics. They contain the combined statements of the first and the second laws of thermodynamics. |
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