Change of Entropy and the Diffusion Process

Let us suppose that two parts of substances (Part 1 and 2 of Fig.E.l) of different concentration are in contact with each other. We can prove that diffusion occurs from the higher concentration part (Part 1) to the lower concentration part (Part 2) because of the First Law of Thermodynamics. Here the total system is closed, the temperature is the same, T, the chemical potentials are fi and 1x2 (ffi O ), and the amounts of substance are ni and n2, respectively. We set the stoichiometric coefficients of Part 1 and Part 2 as —1 and 1, respectively thus the extent of reaction d is 

That is, transport of the substance is autonomously caused, which implies diffusion from the higher concentration to the lower concentration. It should be noted that this is a result of the First Law of Thermodynamics. 

Thus if Cv, Cp are known as functions of temperature, the entropies that are exchangeable with the surroundings are given by 

Here we assume that, by virtue of the Third Law, the entropy is zero at 0 K. 

When a substance undergoes a phase change, heat is adsorbed or discharged under constant temperature. In general, this change occurs under constant pressure, therefore AQ =. Table E.3 shows the temperature Tfus and change in enthalpy 

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