19. LATTICE ENERGY

Question 1

Why is lattice energy always negative

Answer 1

• bonds are formed only

- bond formation releases energy

Question 2

What does the large exothermic value of lattice energy show

Answer 2

the ionic compound formed is very stable with respect to its gaseous ions
the stronger the ionic bonding in the lattice

Question 3

What is lattice energy

Answer 3

the internal energy change when one mole of an ionic compound is formed from its gaseous ions at 0K

Question 4

Why are values of the standard enthalpy change of atomisation always positive

Answer 4

because energy must be supplied to break the bonds holding the atoms in the element together

Question 5

Why are the second and third electron affinities positive

Answer 5

energy is required to overcome the repulsion between the negatively charged ions and the negatively charged electrons

Question 6

What factors affect electron affinity

Answer 6

• ionic radius
• nuclear charge
• shielding effect

Question 7

What is electron affinity

Answer 7

• a measure of the attraction between the incoming electron and the nucleus
• the stronger the attraction, the greater the energy released

Question 8

Why do successive ionisation energies increase

Answer 8

removing an electron from a more positively charged ion requires a greater amount of energy

Question 9

Where does lattice energy arise from

Answer 9

from the electrostatic forces of attraction between oppositely charged ions when the crystalline lattice is formed

Question 10

What factors affect lattice energy

Answer 10

• ion size

- ion charge

Question 11

Why do ions with the same charge but a larger radius have a lower charge density

Answer 11

the charge is spread out over a larger volume

Question 12

What is the effect of ion size on ions with the same charge

Answer 12

• ions with the same charge but a larger radius have a lower charge density
• lower charge density results in weaker electrostatic forces of attraction in the ionic lattice
• the lattice energy is less exothermic

Question 13

What is the effect of ion charge on ions with the same radius

Answer 13

• the greater the charge for the same ionic radius, the greater the charge density
• a higher charge density results in stronger electrostatic forces of attraction in the ionic lattice
• the lattice energy is more exothermic

Question 14

Why does MgO have a more exothermic lattice energy than LiF

Answer 14

• Mg2+ and Li+ have almost the same ionic radius while Mg2+ has a greater charge density
• O2- and F- have almost the same ionic radius while O2- has a greater charge density
• the doubly charged ions in MgO attract each other more strongly than the singly charged ions in LiF

Question 15

Why does the thermal stability of Group II carbonates increase down the group

Answer 15

• the Group II ions increase in ionic radius down the group so the charge density decreases
• decreasing the distortion (polarisation) of the CO32- ion
• the greater the polarisation, the easier it is to weaken the C-O bond and form carbon dioxide and oxygen on heating

Question 16

At what values of the enthalpy change of solution is a compound likely to be soluble

Answer 16

• negative values of enthalpy change of solution

- small positive values of enthalpy change of solution

Question 17

What are ion-dipole bonds

Answer 17

• when an ionic solid dissolves in water, bonds are formed between water molecules and the ions
• these bonds are called ion-dipole bonds

Question 18

How is the lattice energy supplied by the water to break down the ionic compound

Answer 18

the energy released in forming ion-dipole bonds is sufficient to compensate for the energy that must be put in to separate the anions and cations that are bonded together in the ionic lattice

Question 19

Why is the enthalpy change of hydration always exothermic

Answer 19

because ion-dipole bonds are formed and bond formation is always exothermic

Question 20

What factors affect enthalpy change of hydration

Answer 20

• ionic radius (smaller)

- ionic charge (greater)

Question 21

What happens to the enthalpy change of hydration down Group II

Answer 21

• enthalpy change of hydration becomes less exothermic
• ions with a smaller ionic radius and same charge have greater enthalpy changes of hydration
• ionic radius decreases down Group II

Question 22

What happens to the lattice energy of Group II sulfates down the group

Answer 22

• lattice energy decreases
• ionic radius decreases down the group
• the smaller the ionic radius, the greater the lattice energy
• sulfate ion is much larger than the Group II cations
• so the sulfate ion contributes a relatively large part to the change in lattice energy down the group
• so the decrease in lattice energy is relatively small down the group and is determined more by the size of the large sulfate ion than the size of the cations

Question 23

Why does solubility of Group II sulfates decrease down the group

Answer 23

• ionic radius increases down the group
• hence both Hlatt and Hhyd decrease down the group but Hhyd decreases by a greater extent
• Hsol = Hhyd - Hlatt so Hsol becomes less exothermic and more endothermic down the group