L5

Question 1

In a valence orbital diagram why is the Cl valence orbital much lower in energy than the Na orbital?

Answer 1

Because it has a larger Zeff due to a lack of shielding

Question 2

What is the meaning of d0 (d nought) in the Born Mayer equations?

Answer 2

d nought is the equilibrium distance at which attraction and repulsion are balanced

Question 3

What is Vo?

Answer 3

Minimum potential energy

Question 4

Why doesn’t NaCl form a molecule under normal circumstances?

Answer 4

• There is no directional bonding between ions
• Opposite ions attract each other so the ions surround themselves with as many ions of the opposite charge as possible
• The ionic lattice reflects the balance between maximising attractive forces and minimising repelling forces

Question 5

What is the Madelung constant?

Answer 5

A constant for each structure type that takes into account all the interactions in an ionic solid.

Question 6

Which lattice would have a higher lattice potential energy out of MgO and CaF2 and why?

Answer 6

MgO because its ions have a higher charge and lattice potential energy, Vl is proportional to the product of the charges in the ionic solid

Question 7

Which would have a higher lattice potential energy out of NaCl and KBr and why? d0 (NaCl = 283pm) d0 (KBr = 334pm)

Answer 7

NaCl would have a higher lattice potential energy because it has a smaller ionic radius that KBr and Vl is inversely proportional to d0, i.e. a larger ionic radius means a smaller lattice potential energy.

Question 8

Define lattice potential energy Vl

Answer 8

Vl is the energy released when ions in the gas phase combine to form an ionic lattice

Question 9

Define lattice dissociation enthalpy

Answer 9

Lattice dissociation enthalpy is the enthalpy change when an ionic solid dissociates into gaseous ions

Question 10

What is -deltaLH

Answer 10

Lattice formation enthalpy, the enthalpy change when gaseous ions combine to form an ionic solid. This corresponds to the lattice potential energy Vl

Question 11

MgF3 has a lattice dissociation enthalpy of 6032kJ/mol and an enthalpy change of formation of +2975kJ/mol what does this tell us about the solid?

Answer 11

The lattice dissociation enthalpy indicates thats the lattice is very stable, however it has a +ve formation enthalpy which tells us that the ionisation enthalpies of Mg must outweigh the lattice dissociation enthalpy to make the solid unstable.