The enthalpies of fusion for all reactions are always positive because melting of a solid is an endothermic process in which heat is supplied to the reaction Hence the sum of enthalpies of the product is greater than the sum of enthalpies of the reactant.
- The amount of heat energy required to convert a unit mass of a solid at its melting point into a liquid without an increase in temperature is called enthalpy of fusion.
- Melting is an endothermic process, so all enthalpies of fusion are positive because ice requires heat for melting.
- While in freezing of water equal amount of heat is given to the surrounding i.e. freezing is an exothermic process. Thus, all enthalpies of freezing are negative.