T

where AHvap is the enthalpy of vaporisation and T now refers to the boiling point. Entropy

Figure 3.1 Melting of a solid involves a change from an ordered arrangement of molecules, represented by (a), to a more chaotic liquid, represented by (b). As a result, the melting process is accompanied by an increase in entropy.

Figure 3.1 Melting of a solid involves a change from an ordered arrangement of molecules, represented by (a), to a more chaotic liquid, represented by (b). As a result, the melting process is accompanied by an increase in entropy.

changes accompanying other phase changes, such as change of the polymorphic form of crystals (see section 1.2), may be calculated in a similar manner.

At absolute zero all the thermal motions of the atoms of the lattice of a crystal will have ceased and the solid will have no disorder and hence a zero entropy. This conclusion forms the basis of the third law of thermodynamics, which states that the entropy of a perfectly crystalline material is zero when T = 0.

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