Datums and reference states

Both of these examples used h, not u. That is true for most but not all simple energy balance problems. It is not true, for example, for the combustion process in the piston and cylinder of your auto engine. 

Furthermore, this book is about equilibrium, mostly for mixtures. The steam tables and other such tables are for pure substances. Few if any such detailed tables exist for mixtures. We will see that much of the rest of this book is devoted to ways of measuring and/or estimating the thermodynamic properties of mixtures. Once we know those properties, we can carry out the material and energy balances on which to base our chemical plant designs. These estimating methods generally begin with the assumption that we have or can estimate the properties of the individual pure substances that make up the mixtures. The next few sections describe where pure substance tables like the steam tables come from, and how we would make up such a table (or the part we need of it) for some new substance. 

The third law of thermodynamics (see Appendix E) shows us that at 0.00 K all perfectly regular crystaUine substances have the same value of the entropy, which we assign a datum value of s = 0.00. Entropies based on this value are called absolute entropies. They are used only in the study of chemical equUibrium (Chapter 12). The datum chosen for the steam tables is m = s = 0.00 for the saturated liquid at the triple point (solid, liquid, and gas in equihbrium at 32.018°E and 0.08866 psia). Eor refrigerants the common choice is h=s=0.00 for the saturated liquid at —40°F = —40°C. The light hydrocarbon tables choose h = s = 0.00 for the elements at 0.00 K, which is one of the common reference states for chemical reaction calculations. There are a variety of choices of datums, all of which seem like the right choice for some class of substances or problems. These datum or reference state values can be used in Eqs. 2.9 and 2.14 to calculate those tables, by methods shown below. There is no logical or thermodynamic reason why the values in the datum state must be chosen as 0.00. They could just as well be chosen as 

7 or y/n or the first three digits of your social security nmnber. There is no thermodynamic reason why we cannot choose the datum for h at one condition and that for at 

In later chapters we will see other datum or reference states that are chosen for physical or chemical equilibrium calculations. 

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