Does the sign of the enthalpy of fusion change depending on the direction of the phase change for example the enthalpy of fusion of water is 6025 JK per mol and going from liquid to ice is -6025?

Answer 1

The sign only depends on whether it's an exothermic or endothermic reaction. So no, it doesn't depend on phase change, and in some reactions, there is no phase change. But you will find correlation between the sign and the direction of the phase change because they will either be endothermic or exothermic. The enthalpy of fusion is positive because melting is an endothermic reaction (think - you put in heat to melt an ice cube). Endothermic reactions are represented by positive enthalpy. Conversely, freezing is an exothermic process (heat is taken out of the molecules in order to slow them down). We represent exothermic reactions with negative enthalpy.

Always be conscious of the sign when working with thermochemistry calculations. Unlike the usual math, the signs of these numbers are somewhat arbitrary. We say that the system is losing heat in an exothermic reaction; thus, the energy must be represented by a negative. However, there are a few areas (like electrical engineering) where an exothermic reaction is represented by a positive number (because usually the goal is to produce heat).