Metrological aspect of enthalpy

H=U+p*V

U is the 'inner energy'

p is the pressure

V is the volume

A table tells us that enthalpy, ΔH, for combustion of fat is 3700000 J/kg. Rather it is the conjugate: minus 3700000 J/kg, since the energy is given away from the system. But how do we make this intelligible to the common man? What relevance have pressure and volume? None. What we actually measure is ΔH. There is no method of measuring U or ΔU. We burn fat and read the termometer. From this reading we calculate the change in enthalpy. You might believe scientists have a special machine that goes 'ping' and reveals everything on the computer screen. It is often much simpler. Medieval instruments like balance scales are still in use in our laboratories. In these latter days there is still not a reproducible definition of 'kilogramme'.

When you read

• ΔG=ΔH-T*ΔS

it may confuse you. The difference in free energy equals difference in ethalpy minus the actual temperature times the difference in entropy. How do we measure this? Do we have an entropymeter in the laboratory? No ΔS is calculated from thermometer readings. What is the purpose of this mumbo-jumbo? It is to tell if a process is spontaneous. If ΔG is less than zero, the process is spontaneous. This means that a process could be spontaneous if it demands heat, if the entropy is increasing. I am not sure this is the simplest way of viewing it, but it is the conventional way.

Alas, if our calculations will only show the process beeing spontaneous, there is no way of telling whether it will happen. Try and try again!

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Warning! The Shannon entropy is often denoted 'H'. However, 'S' is more appropriate.