Lattice Energy: Experimental vs. Calculated values
Lattice energy is the amount of energy required to break an ionic solid into its ions in their gas phase. It is difficult to determine lattice energy directly through experimentation. Therefore, several methods have been developed in order to estimate values for lattice energy.
Lattice energy is usually estimated by using the Born-Fajans-Haber cycle, an application of Hess' Law. It is important to be able to calculate lattice energy because it can be used as a way to predict the melting points and solubilities of ionic compounds.
Lattice energy gives an idea of how strongly ions in an ionic solid are interacting. This is why these values can be used to predict properties such as the melting point of the substance. Lattice Energy is equal to the change in enthalpy that converts one mole of solid crystal into gaseous ions.
Application of the Born-Mayer equation represents the experimental value because the data is derived from experiment. It gives:
ΔfH˚(MX,s)=ΔaH˚(M,s)+nΔaH˚(X,g)+ΣIE(M,g)+nΔEA H(X,g)+Δlattice H˚(MX,s)
ΔfH˚(MX,s)=standard enthalpy of formation
ΔaH˚(M,s)=enthalpy of atomization of metal M
nΔaH˚(X,g)=enthalpy of atomization of X
ΣIE(M,g)=sum of the ioninzation energies for the processes (M(g) --> M+(g) + e- --> M2+(g) + 2e-...)
ΔEA H(X,g)=enthalpy change associated with the attachment of an electron
Δlattice H˚(MX,s)=Lattice enthalpy change
The calculated value is an approximation determined by using the Born-Haber cycle. Rearrangement of the equation gives: ΔU(0K)≈ΔfH˚(MX,s)-ΔaH˚(M,s)-nΔaH˚(X,g)-ΣIE(M,g)-nΔEA H˚(X,g)
NaCl: ΔU(0K)≈ -411-108-496-(244/2)-(-349)≈ -788 kJ/mol
1. Which one of the following has the greatest lattice energy?
Answer: A)MgO. It has ions with the largest charge.
2. Which one of the following has the greatest Lattice Energy?
Answer: C)AlCl3. According to the periodic trends, as the radius of the ion increases, lattice energy decreases.
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