Hess's Law state that when a reactants are converted to products, the change in enthalpy is the same whether the reaction take place in one step or series of step.We can apply Hess's Law to find enthalpy change of any reaction for which we can write an equation,even if it is impossible to carry out.An example of calculation method that applied the Hess's Law is Born-Haber Cycle.
Born-Haber cycle is a series of chosen steps from elements to ionic compound for which all the enthalphies are known except the lattice energy.
Born-Haber cycle is a series of chosen steps from elements to ionic compound for which all the enthalphies are known except the lattice energy.
To construct a Born-Haber Cycle, students must first know the definition of lattice energy.Lattice energy is the energy change when 1 mole of solid ionic compound is formed from its gaseous ion.For example:
1. Draw x-axis and y-axis and labelled the y-axis as enthalpy
2. Draw the formation of LiF compund where:
3. Atomization of Lithium where:
4. Atomization of Fluorine where:
5. Ionization of Lithium atom where:
6. Electron affinity of Fluorine atom where:
BUT WHAT IS THE LATTICE ENERGY?
Back to the definition:
Thus:
