Question

Why are the enthalpies of fusion for all reactions positive?

Answer 1

Hint: We need to know that the enthalpy of fusion is also called the heat of fusion. The enthalpy of a fusion of a substance indicates the change in the enthalpy or heat developed from providing the energy. The particular amount of energy of a substance to change the phase of a material from solid state to liquid phase at a constant pressure. The enthalpy of the reaction can be found by adding the internal energy with the product of pressure and the volume and the enthalpy is represented as $'H'$.

Complete answer:
As we know that the change of the phase from the solid phase to liquid phase is known as fusion. The enthalpies of fusion for all reactions are positive. Because, if an organized crystalline solid is transform in to a disorganized structure of a liquid, the degree of disorder is increases in that process. And the melting of a solid is an endothermic reaction. In that reaction, the heat is supplied in to it. Therefore, the sum of enthalpies of the reactant will be less than the sum of enthalpies of the product. Hence, the enthalpies of fusion for all reactions are positive.

Note:
We need to remember that the quantity of heat energy for the conversion of the definite mass of a solid at its melting point to the liquid is known as enthalpy of fusion. In that process, there will not increase the temperature. The melting is the endothermic process. Therefore, the enthalpy of fusion is positive for all the reactions. But the freezing of water is an exothermic process. Hence, the enthalpy of freezing is negative.