The Born-Haber Cycle is essentially Hess's Law applied to an ionic solid. Hess's Law states that the overall change in energy of a process can be determined by breaking the process down into steps, then adding the changes in energy of each step.This may be positive or negative, depending on the atoms involved and how they interact. The heat of formation is the change in energy when forming a compound from its elements.It may also be referred to as the energy of atomization. This is an input of energy, and thus has a positive value. Sublimation energy is the energy required to cause a change of phase from solid to gas, bypassing the liquid phase.The magnitude of the dissociation energy depends on the electronegativity of the atoms involved. Therefore, the change in energy is always positive. The dissociation of a compound is always an endothermic process, meaning it will always require an input of energy. Dissociation energy is the energy required to break apart a compound. In general, electron affinity increases from left to right across the periodic table and decreases from top to bottom. Therefore, when used in calculating the lattice energy, we must remember to subtract the electron affinity, not add it. Usually, energy released would have a negative value, but due to the definition of electron affinity, it is written as a positive value in most tables.
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