Lattice Enthalpies and Born Haber Cycle (Worksheet)
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- 96819
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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.)