Brief Notes on Enthalpy of Fusion

Enthalpy of phase transition: Introduction

Energy is required to convert a solid into a liquid, which is the process of melting. Energy is also required to convert a liquid into gas, which is the process of evaporation. It is due to the fact that intermolecular forces of attraction between different phases of solids, liquids, and gases are different. Hence, for phase transitions, enthalpy change occurs, which is termed as enthalpy of phase transition. 

Enthalpy of fusion

When 1 mole of a solid substance is converted into its liquid state at its melting point, the enthalpy change which takes place is termed enthalpy of fusion. It is also known as molar enthalpy of fusion because it involves the melting of 1 mole of solid. 

Enthalpy of fusion can also be expressed as:

                △fusHo       =    Hliquido      –     Hsolido

Unit of enthalpy of fusion: The SI unit of enthalpy of fusion is joules per mole.

Special case: When the process of melting is carried out at a constant temperature and under standard pressure, i.e., 1 bar, it is called standard enthalpy of fusion, △fusHo. 

For example,

Enthalpy of fusion of ice per mole,

 When one mole of ice melts at 273 K, the heat absorbed is 6.0 KJ mol–

H2O(s)   →  H2O(l)      , △fusHo  =  6.0 KJ mol– 

       (ice)             (water)

If water freezes, then the process is reversed and an equal amount of heat is evolved.

Enthalpy of fusion of water, △freezeHo  =  -6.0 KJ mol– 

                                     △freezeHo    =    -(△fusHo)

The melting of a solid is endothermic, so enthalpy of fusion is always positive.

Application of enthalpy of fusion

It may be noted that enthalpies of the fusion of substances give an idea of the nature of solid and the magnitude of forces acting between the particles constituting the solids. The solids such as O2, H2S, etc. which are molecular solids (consisting of molecules) have low enthalpies of fusion because of the weak forces of attraction between the molecules. 

                             △fusHo  (O2) = +0.45 KJ mol– 

                             △fusHo  (H2S) = +2.0 KJ mol– 

On the other hand, ionic solids such as NaCl, MgCl2, etc. have high enthalpies of fusion because they have strong electrostatic forces of attraction between the ions constituting them.  

                             △fusHo  (NaCl) = +29.0 KJ mol– 

                             △fusHo  (MgCl2) = +43.0 KJ mol–

The general formula for the heat/enthalpy of fusion

                                                 q = m ΔfusH  

                       Where q is the energy, m is the mass of the substance and  △fusH is the enthalpy of fusion.

Numericals

1. When 1g of liquid naphthalene (C10H8 ) solidifies, 149J of heat is evolved. Calculate the heat of fusion of naphthalene. 

Solution: Molecular mass of naphthalene = 128g

               Heat evolved when 1g of naphthalene solidifies = -149 J

               Heat evolved when 128g of naphthalene solidifies = -149 x 128 J

                                                                                               = -19072 J 

               In the question, the liquid is changing into solid but in the fusion solid is changed to liquid. So, △fusHo = -(-19072) J

                             = 19072J or 19.072 KJ 

1. When 25.23g of methanol, CH3OH, froze 4.01 KJ of heat was released. What is the enthalpy of fusion of methanol? 

Solution: As we know, q = m ΔfreezeH  

                          -4.01 KJ = 25.23 * ΔfreezeH     ( q is negative because energy is released)

                           Solving, ΔfreezeH  = -0.1589 

We need to find the solution for 1 mole of methanol, so we will multiply it by the molecular mass of methanol which is 32 g.

                                                  ΔfreezeH  = -0.1589 * 32 

                                                  ΔfreezeH = -5.09 KJ/mole

And we know,  △freezeHo    =    -(△fusHo)

So, △fusHo = 5.09 KJ/mole

Conclusion

Different phases of solids, liquids, and gases have different intermolecular forces of attraction. Hence, conversion from one phase to another requires a change in energy. It can be studied using enthalpy of phase transitions. And when 1 mole of a solid substance is converted into its liquid state at its melting point, the enthalpy change which takes place is termed enthalpy of fusion. Enthalpy of fusion is very important as it is related to enthalpy of freezing also. With the help of the enthalpy of fusion, we can get an idea of the nature of solids and the magnitude of forces acting between particles of solids. So, it plays a major role in understanding solids and is hence very important.Energy is also required to convert a liquid into gas, which is the process of evaporation. It is due to the fact that intermolecular forces of attraction between different phases of solids, liquids, and gases are different. Hence, for phase transitions, enthalpy change occurs, which is termed as enthalpy of phase transition.