Why are absolute entropies given the units J mol−1 K−1?

I've just started looking at the concept of entropy and can't understand why absolute (or standard molar entropies) are given the units $\pu{J mol^−1 K^−1}$. Could someone explain the rationale for these units?




The best explanation I can give is that in order to measure entropy for a process we can exploit the fact that it's a state function. Entropy doesn't depend on the pathway that we take. So if you take for example ice melting at 273 K, this process is thermodynamically reversible. At 273 K ice and liquid water are in a state of equilibrium, but if we apply heat we can cause ice to melt. So this allows us to measure $ \Delta S$ directly by looking at how much heat we apply to cause this process to proceed. So we look at the amount of heat in joules and compare that to the temperature where we applied the heat. 


If you want to think conceptually, think what adding heat will do to the system. We associate adding heat with an increase in entropy. But the magnitude of the change is related to the amount of energy the system currently has (which is directly related to its temperature in kelvin).


Standard entropies of formation are given in molar quantities because they assume the process is taking place to create 1 mole of the substance. Entropy itself is traditionally described with the units of J/K.


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