Values of lattice energy

1. Values of lattice energy Lattice energy is “The enthalpy change when one mole of an ionic lattice is formed from the gaseous ions under standard conditions.” X+(g) + Y-(g) → XY (s) ΔH =ΔHΘlat XY The value of lattice energy will depend on the degree of attraction between ions. Anything that increases the attraction between ions will make the lattice energy more negative.

2. Charge density The degree of attraction between ions depends on their charge density. Charge density depends on; 1) The size of an ion. 2) The charge on an ion.

3. 1) Ionic charge Multiply charged ions will attract more than the singly charged. Na+ and Cl- will attract each other less. Mg 2+ and O2- Na+ Cl- Mg 2+ O 2- Hence have a less negative lattice energy than…

4. 2) Ionic radius Smaller ions have larger charge densities and will attract each other more strongly to give more negative lattice energies. Large ions also cannot approach each other so closely. Eg Na+ and Cl- are small ions. Cs+ and I- I- Na+ Cs+ Cl- So attract each other more strongly and have a more negative lattice energy than…

5. Explain the trend in lattice energies of the sodium halides.

6. Radius increases down the group. Ions do not attract each other so strongly The cation is constant, so differences must be due to the anion. The charge on the anion is constant; -1. Charge density decreases. So lattice energies decrease.

7. 3) Degree of covalent character. Ionic compounds are assumed to be made up of spherical ions with complete charge separation. If the bond is polarised it will lose ionic character. + - The attraction between ions is reduced and lattice energy will be less negative.

8. Ionic bonds tend to be polarised if; 1) The cation is small. Eg; Li+ is smaller than Na+ and so its compounds are more likely to be polarised. Li+ Na+ 2) The cation has a multiple charge. Eg; Be2+ is twice the charge as Li+ and so is more likely to be polarised. Li+ Be2+ Generally the greater charge density the more likely is polarisation.

9. Similarly for polarisation an anion should be; 1) Large. Eg; Iodide ions are more likely to be polarised than chloride ions. I- Cl- 2) Multiply charged. Eg; S2- is more likely to be polarised than Cl-. S2- Cl-

10. Which of these compounds are likely to have classic ionic bonding?

11. Ans;Those with the smallest differences between theoretical and practical lattice energies;

12. Using electronegativities to predict polarisation. Generally the larger the differences in electronegativities between a metal and a non-metal the more likely their bonding is to be classic ionic. Eg; Use following electronegativities to decide whether sodium halides or silver halides are more likely to have classic ionic bonding.

13. Sodium halides have the greatest differences in electronegativities and hence are more likely to have classic ionic bonds.

14. For Group 1 halides the theoretical and practical lattice energies are similar. So their bonding is typically ionic.

15. Do silver halides have stronger or weaker bonds than predicted? Are they more or less stable? What type of bonding is present?

16. Practical > Theoretical So bonding is stronger. Compounds are more stable. A significant degree of strong, covalent bonding must be present.