Now, according to Hess’s Law all we need to do is to add all the enthalpies associated with each step of the Born-Haber Cycle to calculate the lattice energy. So to write the enthalpy reactions required in the formation of MgF 2(s), we need to write the reactions in the direction required and we need to adjust for the number of moles of each reactant used: Remember if we reverse the direction of the reaction, the sign of the change in enthalpy will change. To calculate the total enthalpy change using Hess’s law we will need to find the enthalpy change for each component of the Born-Haber cycle and add them together. This definition is represented by the chemical equation: I am now going to ask you to accept that this rather strange enthalpy change was a fundamental jigsaw. The enthalpy of solution of an ionic compound can be calculated from its lattice enthalpy and the enthalpies of hydration of the individual ions. Mg 2 +(g) + 2F -(g) –> MgF 2(s) ΔH = Lattice Energy b) Lattice energy of the ionic compound X + Y c) Enthalpy of hydration of the gaseous ion Z + (g) 2. Remember the lattice energy is energy released when ions in the gaseous form combine to form a solid. Hess’s law states that the total enthalpy change during the complete course of a reaction is same whether the reaction is made in one step or in several steps. Now, writing the component reactions of the Born-Haber Cycle:
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