Energetics Flashcards
What is lattice energy?
Lattice energy is the energy change when one mole of an ionic solid forms its gaseous ions.
Why is lattice energy always negative?
Because forming ionic bonds is always exothermic.
Why is CaCl2 lattice energy more negative than KCl?
Ca2+ has higher charge than K+
Ca2+ is slightly smaller than K+
Cl- has the same charge and size in both
Attraction between ions is stronger in CaCl2
so more energy is released when CaCl2 forms
What properties should be compared when explaining differences in lattice energy?
Ion charges
Ionic radii
Attraction between ions
Energy released
What are the two sources that lattice energies come from and what are their advantages and disadvantages?
Theoretical LEs that are calculated using maths and physics.
Advantage: all you need is a calculator
Disadvantage: assumes perfect ionic bonding
Experimental LEs that are measured using chemistry.
Advantage: reflects reality
Disadvantage: needs lots of other experimental data
Why do the theoretical LEs differ from the experimental LEs?
The theoretical LE assumes perfect ionic bonding whereas in reality there is ion polarisation which causes covalent characteristics.
Explain the trend in difference between theoretical LEs and experimental LEs in the series:
AgCl, AgBr, AgI
Halide ions get larger which makes them easier to polarise. Ag+ polarises halide ion more which increases the covalent character in the bonding therefore the difference between the theoretical and experimental LEs increase.
13.2 ppt
