Question

What will be the sign of entropy for the change of liquid into it's vapour?
b) Define standard enthalpy of fusion, solution, formation.

Answer 1

Dear Student,

When a liquid change into vapour the process which is involved in it is called Endothermic process which means vaporisation. In such case entropy change is positive, reflecting high degree of dissociation in the phase that is formed during the transition.

At constant pressure, q = ΔH_trs , which is defined as the enthalpy change accompanying the transition. The molar entropy change of the transition is thus given by:
$$\begin{gathered} ∆S_{\mathrm{trs}}=\frac{∆{\mathrm{H}}_{\mathrm{trs}}}{{\mathrm{T}}_{\mathrm{trs}}} \\ \mathrm{therefore}, \\ \mathrm{In}\mathrm{endothermic}∆{\mathrm{H}}_{\mathrm{trs}}>0 \end{gathered}$$

The enthalpy of a substance, also known as (latent) heat of fusion, is the change in its enthalpy resulting from providing energy, typically heat, to a specific quantity of the substance to change its state from a solid to a liquid at constant pressure.

The enthalpy of solution, or heat of solution is the enthalpy change associated with the dissolution of a substance in a solvent at constant pressure resulting in infinite dilution. The enthalpy of solution is most often expressed in kJ/mol at constant temperature.

​The standard enthalpy of formation is defined as the change in enthalpy when one mole of a substance in the standard state (1 atm of pressure and 298.15 K) is formed from its pure elements under the same conditions.