Lattice Energy

Question 1

The more exothermic the lattice energy

Answer 1

the stronger the ionic bonding in the lattice

Question 2

What is lattice energy

Answer 2

Lattice energy is the internal energy change when 1 mole of an ionic compound is formed from its gaseous ions at 298K

Question 3

Enthalpy of Atomization is

Answer 3

always positive

Question 4

EA1, EA2 and EA3 are

Answer 4

always negative, positive and positive

Question 5

Trend in electron affinity across period

Answer 5

The pattern is not always clear but generally non-metal atoms get more negative (more exothermic) across the period.

Question 6

Trend in electron affinity down the group (Group 6 and 7 more precisely)

Answer 6

Electron affinity becomes less negative (less exothermic) except the first element of the group.

Question 7

Why does first element not follow the trend of decreasing electron affinity down the group

Answer 7

The value of first electron affinity depends on electron and the positively charged nucleus. The stronger the attraction, the greater the amount of energy released.

Question 8

Factors affecting electron affinity down the group

Answer 8

1. The greater the nuclear charge, the greater the attractive force between nucleus and the outer electrons.
2. The greater the ionic size, the lesser the electron affinity.
3. Greater the shielding effect, lesser the electron affinity.

Question 9

Reason for fluorine not fitting in the trend for electron affinity

Answer 9

The high electron density causes a greater repulsion b/w the electrons within the atom. Thus, reducing the attractive effect b/w incoming electrons and the nucleus.

Question 10

In Born-Haber energy level diagram upward and downward arrow represent

Answer 10

Going upward represents an increase in energy ΔH° positive and going downwards represents a decrease in energy ΔH° negative.

Question 11

Lattice energy arises from

Answer 11

the electrostatic force of attraction of oppositely charged ions when the crystalline lattice is formed.

Question 12

For any given anion i.e. F −

Answer 12

the lattice energy gets less exothermic as the size of the cation increases from Li+ to Cs+.

Question 13

For any given cation i.e. Li+

Answer 13

the lattice energy gets less exothermic as the size of the anion increases from F− to I−.

Question 14

Lattice energy due to ions with same opposite charge

Answer 14

Ions with same ionic charge have a lower charge density if their radius is larger. A lower charge density results in weaker electrostatic forces of attraction in the ionic lattice. Sodium fluoride has a less exothermic lattice energy than lithium fluoride.

Question 15

Lattice energy due to ions of similar size

Answer 15

For ions of similar size, the greater the ionic charge, the higher the charge density. This results in stronger ionic bonds being formed.

Question 16

Polarising power

Answer 16

The ability of a cation to attract electrons and distorts an anion is called the polarising power of the cation.

Question 17

Ion polarisation

Answer 17

The distortion of the electron cloud of an anion by a neighboring cation. The distortion is greatest when the cation is a small and highly charged.

Question 18

Factors affecting ion polarisation

Answer 18

o the cation is small
o the cation has a charge of 2+ or 3+
o the anion is large
o the anion has a charge of 2- or 3-

Question 19

Trend on thermal stabilities of Group 2 carbonates

Answer 19

Thermal stabilities of Group 2 carbonates increases down the group. The more positive the enthalpy change, the more stable the carbonate relative to its oxide and carbon dioxide. (The same pattern is observed down the group 2 with nitrates as well)

Question 20

Why does thermal stability of carbonates increases down group 2

Answer 20

the smaller the ionic radius of the cation, the better it is at polarising the carbonate ion. (Mg2+ > Ca2+ > Sr2+ > Ba2+) The greater the polarisation of the carbonate ion, the easier it is to weaken a carbon-oxygen bond in the carbonate and form carbon dioxide and the oxide on heating.

Question 21

Enthalpy of sol vs Enthalpy of hyd

Answer 21

Sol: when 1 mole of an ionic solid dissolves in sufficient water to form a dilute solution.
(ΔH sol, can be positive or negative)
Hyd: when 1 mole of specified gaseous ions dissolve in water to form a dilute solution.
(ΔH hyd is always negative)

Question 22

What does ΔHsoln value indicate

Answer 22

A compound is soluble if ΔHsol is negative or has a small positive value; and it is insoluble if it has a large positive value.

Question 23

Factors affecting ΔH hyd

Answer 23

The value of ΔH hyd is more exothermic for ions with the same charge but smaller ionic radii.
The value of ΔH hyd is more exothermic for ions with same radii but a larger charge.

Question 24

Solubility of Group 2 sulfates

Answer 24

o the lattice energy of the sulfates decreases (gets less exothermic) by relatively smaller values down the group
o the enthalpy change of hydration decreases (gets less exothermic) by relatively larger values down the group
o the value of ΔHsol gets more endothermic down the group so the solubility of group 2 sulfates decreases down the group.

Question 25

Trend in bond energies of the bonds in Cl2, Br2 and I2

Answer 25

Larger atomic radii and more shielding down the the group hence weaker bonds and lower bond energy. (FLUORINE does not follow this trend due to lone pairs repulsion/ high charge density which weakens the bond therefore lower bond energy) :.F.:− :.f.: both have 3 lone pairs so greater repulsion b/w them.

Question 26

Enthalpy change formula

Answer 26

always make sure regardless of sign with product/reactant ‘product is supposed to be negative’ and ‘reactant is supposed to be positive’ in the eqn. you’re using.
(because their is variation in types of eqns. with R − P and P − R, so use any)

Question 27

Past paper repeated Q: Explain whether MgCO3 would require a higher or a lower temperature than CaCO3 for its decomposition.

Answer 27

Mg2+ has greater charge density and more polarising power which polarises CO3 2- ion leading to easier decomposition.

Question 28

Past paper repeated Q: Describe and explain how the solubilities of the sulfates of Group II elements vary down the group.

Answer 28

Solubility of sulfates decreases down the group. cationic radii increases down the group, both ΔH (L.E) and ΔH (hyd) decreases down the group but ΔH (hyd) decreases more than ΔH (L.E) . Hence ΔH (sol) becomes more endothermic down the group.

Question 29

For enthalpy change of solution exp. formulas

Answer 29

q = −mcΔT
and then later for 1 mole;
ΔH (sol) = − Mr/m x q(without negative sign as already been put in front)