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SOLUTIONSMANUAL TO ACCOMPANY ATKINS' PHYSICAL CHEMISTRY 71
E3B.7(b) Because entropy is a state function, ∆S between the initial and �nal states is
the same irrespective of the path taken.�us the overall process can be broken
down into steps that are easier to evaluate. First consider heating the ice at
constant pressure from the initial temperature to the melting point, Tm. �e
variation of entropy with temperature at constant pressure is given by [3B.7–
90], S(Tf) = S(Ti) + Cp ln (Tf/Ti).�us the change in entropy, ∆S = S(Tf) −
S(Ti), for this step is
∆S1 = Cp ln(
Tm
Ti
) = nCp ,m(H2O(s)) ln(Tm
Ti
)
Next consider the phase transition from solid to liquid at the melting temper-
ature. �e entropy change of a phase transition is given by [3B.4–89], ∆trsS =
∆trsH/Ttrs, thus
∆S2 = n
∆fusH−○
m
Tm
�en the liquid is heated to the boiling temperature, Tb. In analogy to the �rst
step
∆S3 = nCp ,m(H2O(l)) ln( Tb
Tm
)
�e next phase transition is from liquid to gas
∆S4 = n
∆vapH−○
m
Tb
Finally, the vapour is heated from Tb to Tf
∆S5 = nCp ,m(H2O(g)) ln( Tf
Tb
)