Why is enthalpy of vaporization nonzero in an isothermal phase change?

[RUSI16]

is enthalpy change during isothermal process zero? if it is zero then what is the significance of enthalpies like enthalpy of vapourization or fussion since during phase transformation temperature remains constant.

 

[Andrew Mason]

is enthalpy change during isothermal process zero? if it is zero then what is the significance of enthalpies like enthalpy of vapourization or fussion since during phase transformation temperature remains constant.

H = U +PV so dH =dU + PdV + VdP. For an ideal gas, dU =0 for an isothermal process, so dH = PdV +VdP.

For melting ice which occurs at constant T, there may be negligible change in P and V, but is ΔU =0?

AM

 

[Chestermiller]

is enthalpy change during isothermal process zero? if it is zero then what is the significance of enthalpies like enthalpy of vapourization or fussion since during phase transformation temperature remains constant.

You answered your own question. Enthalpy is not just a function of temperature. It is also a function of pressure, state of aggregation, and chemical composition.