# Lattice Enthalpies and Born Haber Cycle (Worksheet)


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## Q1.1

Define the terms

• lattice dissociation enthalpy
• lattice formation enthalpy.

## Q1.2

$$\ce{NaCl}$$, $$\ce{NaBr}$$ and $$\ce{MgO}$$ all have the same crystal structure.

• Explain why the lattice dissociation enthalpy of $$\ce{NaBr}$$ is a bit less than that of $$\ce{NaCl}$$.
• Explain why the lattice dissociation enthalpy of $$\ce{MgO}$$ is about 5 times greater than that of $$\ce{NaCl}$$.

## Q1.3

• Define the term standard atomization enthalpy.
• The standard atomization enthalpy of bromine is +112 kJ mol-1, and for sodium is +107 kJ mol-1. Write equations for the reactions that these figures relate to.

## Q1.4

The Born-Haber cycle for the formation of sodium chloride is:

• The +107 transition in the figure is the atomization enthalpy of sodium. Explain what all the other transitions represent.
• Use the figures on the diagram to calculate the lattice formation enthalpy of $$\ce{NaCl}$$.
• Draw the equivalent diagram which would enable you to calculate the lattice formation enthalpy of magnesium chloride, $$\ce{MgCl2}$$. Write the names of the enthalpy changes against each arrow rather than using actual values.

## Q1.5

Lattice enthalpies can be calculated from figures obtained experimentally using Born-Haber cycles. They can also be calculated theoretically. The Table 1 gives experimental and theoretical values for the silver halides. (The values are listed as lattice dissociation energies. Don't worry about the difference between lattice energy and lattice enthalpy. All I am asking you to do is to compare the values without worrying about the exact difference between the two terms.)

 experimental (kJ mol-1) theoretical (kJ mol-1) AgF +967 +953 AgCl +915 +864 AgBr +904 +830 AgI +889 +808
• For AgF, the experimental and theoretical values are very close. What does that show?
• For AgI, there is a much greater difference between the two values. What does that suggest?
• Why do you think the difference between the two values increases as you go from AgF to AgI?

## Q1.6

Two of the biggest enthalpy changes during a Born-Haber cycle are the energy needed to ionize the metal, and the energy released when the positive and negative ions come together to make the lattice - the lattice formation enthalpy. In terms of these energy changes, explain why magnesium forms a chloride with the formula $$\ce{MgCl2}$$ and not $$\ce{MgCl}$$ or $$\ce{MgCl3}$$. (You do not have to quote any numbers here - you can discuss it perfectly well in general terms.)

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