Reference State and Values of Chemical Potentials

Reference Level Up to now, what we have been missing in order to make concrete predictions are the p values of the substances we have been dealing with. The chemical potential can be assigned an absolute zero value, just as temperature can. In principle, the absolute values could be used, but they are enormous. It would mean that in order to work with the tiny differences in potentials common in chemical and biological reactions, at least 11 digits would be necessary (the ratio between the potential differences and the absolute values is around one to one billion ). This alone would lead to numbers that are much too unwieldy, not to mention that the absolute values are not known accurately enough for this to be feasible. 

But there is a simple way out of this dilemma. The heights of mountains are not referred to the geocenter but to the sea level (Fig. 4.3). Everyday temperatures are not referred to absolute zero, but are given as Celsius temperatures based upon the freezing point of water. (The zero point of Daniel Gabriel Fahrenheit s original scale was determined by placing the thermometer in brine, here a mixture of ice, water, and ammonium chloride.) 

Elements Used for Zero Levels Next, we will turn to the question of what reference states are suitable for measuring potential differences. It is useful to refer to the conventional basic substances in chemistry, the elements, as long as we limit the transformations of substances to chemical reactions in the broadest sense and exclude nuclear reactions. The values of the chemical potentials of substances are related to the chemical potentials of the elements they are composed of and can be determined experimentally by means of chemical reactions. Because it is not possible to transform one element into another by chemical means, the values of the various elements themselves are not related to each other. This means that in principle one could arbitrarily determine the reference level for each basic substance, i.e., for every element. Because in the case of chemical reactions the elements are preserved, i.e., an equal number of chemical symbols appears on 

N appears two times oti the left as well as on the right side of the conversion formula H, however, appears six times. Therefore, if the chemical potential of a substance is increased by a fixed, although arbitrary summand (say 1,000 kG, as shown above in the third line) for every H appearing in its content formula, this added value cancels when we compute the potential difference and we end up with the same result as in the second line above. The same holds for nitrogen. This means that the reference level for any element could in principle be chosen arbitrarily as mentioned earlier. For the sake of simplicity, the value 0 is assigned to the chemical potential of all elements. 

Eor elements E such as H, N, O, Cl, etc., which, at standard conditions, usually appear as diatomic gases, 1 mol E simply means 2 2 and yu(E) correspond- 

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