`Delta S = Delta H//T` holds good for

Options

A. a process under any condition
B. an isothermal reversible phase change
C. an adiabatic process
D. a process at constant pressure

Answer

B

Explanation

Whenever heat is added to the system, it increases molecular motions causing increased randomness ikn the system. Thus,k heat `(q)` has randomizing influence on the system. A system at higher temperature has greater randomness in it than the one at lower temperature. Thus, temperature is the meausre of average chaotic motion of particles in the system. This suggests that the entropy change is directly proportional to the temperature.
The entropy change `(Delta S)` is given by the equation
`Delta S = (q_(rev))/(T)`
when `q_(rev)` is the heat exchanged when the process is carrid out reversibly and isothermally (i.e., at constant temperature).Phase change occurs constant temperature at a given pressure and the process is reversible. For example, when a solid melts `[H_(2) O (s) rarr H_(2) O (1)]`, there is an equilibrium between the solid and the liquid at the freezing or melting point. The process involes latent heat which is equal to the standard enthalpy of fusion, `Delta_(fus) H^(@)`, at constant temperature and pressure. Thus,
`Delta_(fus) S^(@) = (q_(rev))/(T) = (Delta_(fus) H^(@))/(T)` (T is melting point)
Similarly, when a liquid is transformed into vapor at constant temperature and pressure,
`Delta_(vap) S^(@) = (Delta_(vap) H^(@))/(T)` (T is boiling point)