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Enthalpy Changes During Phase Transformations

It is a matter of common observation that energy is required to convert a solid into liquid or a liquid into gas. The conversion of solid into liquid is called melting or fusion and the process of conversion of liquid into gas is termed as vaporisation. These processes are collectively known as phase transformations or phase changes. The magnitude of enthalpy change accompanying the phase transition depends largely on the strength of intermolecular forces in the substance undergoing phase transformations. Let us study the enthalpy changes accompanying the phase changes.

(a) Standard or Molar Enthalpy by of Fusion (ΔfusH0)

It is the enthalpy change accompanying the melting of I mole of a solid substance in standard state at its melting point. For example, when one mole of ice changes into water at its melting point 273 K, 6.0 kJ of heat is absorbed. It is represented as:

H2O(s) à H2O(l) ; Δ fus HO  = + 6.0 kJ mol-1

 

The values of enthalpy of fusion give an idea about the magnitude of interparticle forces in solids. For example, ionic solids have very strong interparticle forces. Consequently, they have high values of enthalpy of fusion. On the other hand, molecular solids have weak interparticle forces which reflect that their values of enthalpy of fusion are low. The enthalpies of fusion of some substances are given in Table 16.1.

Freezing of ice is exothermic. When water freezes, attractive interactions between water molecules increase and . energy is released. The freezing of ice, is thus. Exothermic process. The enthalpy of freezing of ice is same in magnitude as enthalpy of fusion but opposite in sign.

H2O(l) à H2O(s); ΔH = – 6.0 KJ mol-1

 (b) Standard or Molar Enthalpy of Vaporisation (Δ vap H0) It is the enthalpy change accompanying the conversion of one mole of a liquid in standard state into its vapours at

the boiling point and standard pressure (100 kPa). For example, when one mole of water is converted into steam at 273 K, the enthalpy change accompanying the process is 40.6 kJ.

It can be expressed as:

H2O(l) à H2O(g); Δ vap H0= + 40.6 kJ mol-1

The values of enthalpy of vaporisation give some idea about the magnitude of interparticle forces in liquids. The values of enthalpy of vaporisation of some substances are given in Table 16.1.

Table 16.1. Standard Enthalpy of Fusion and Vaporisation

(c) Enthalpy o Sublimation (Δ sub HO)

 It is the enthalpy change accompanying the sublimation of one mole of a solid substance into its gaseous state at a constant temperature below its melting point at the standard pressure.

Sublimation is direct conversion of a solid into vapour. For example, solid CO2 (dry ice) sublimes at 195 K with Δ sub Ho = 25.2 kJ moi-1 Similarly 6 sublimes’ of naphthalene and iodine are 73.0 kJ moi-1 and 62.4 kJ moi-1 respectively. The thermochemical equations are as under

CO2(s) à  CO2(g); Δ sub HO = 25.2 kJ mol-1

C10H8(s) à  C10H8(g); Δ sub HO = + 73 kJ mol-1

I2 (s)  à I2(g); Δ sub HO = + 62.4 kJ

 

The process of sublimation consists of change of solid state into vapour state in one step. The same change may be brought about in two steps:

 

(i) The melting of solid into its liquid and the enthalpy change is Δ fus H

(ii) The vaporisation of the liquid into its gaseous state and the enthalpy change is Δ vap H.

Δ sub H = Δ fus H + Δ vap H