Lattice Energy

Question 1

What is Lattice Energy?

Answer 1

The enthalpy change when one mole of an ionic compound is formed from it’s gaseous ions under standard conditions.

Question 2

Why is lattice energy always negative?

Answer 2

Because it has to do with bond forming (of ions) which is exothermic.

Question 3

What does a large exothermic value of lattice energy show?

Answer 3

That the ionic lattice is very stable with respect to it’s gaseous ions.

OR

The more exothermic the lattice energy the stronger the ionic bonding in the lattice.

Question 4

What is enthalpy change of atomization?

Answer 4

This is the enthalpy change when 1 mole of gaseous atoms is formed from its element under standard conditions.(always positive,as energy must be applied to break bonds)

Question 5

What is first electron affinity?

Answer 5

This is the enthalpy change when 1 mole of electrons is added to 1 mole of gaseous atoms to form 1 mole of gaseous 1-ions under standard conditions. (For 2nd you say- to form gaseous 2-ions)

Question 6

Relationship between lattice energy and ion size.

Answer 6

As ion size increases, the ion size becomes less exothermic.

Question 7

Relationships between lattice energy and charge on ions.

Answer 7

The lattice energy becomes more exothermic as the ionic charge increases.

Question 8

What is ion polarisation.

Answer 8

This is the distortion of the electron cloud (shape) of the anion by the cation when it attracts the electrons in the anion towards it such that the anion is no longer spherical.

Question 9

What is polarising power?

Answer 9

The ability of a cation to attract electrons and distort an anion.

Question 10

Polarisation is more likely to occur when…

Answer 10

• The cation is small
• The anion is large
• The ions have a charge of (-)2 or (-)3

Question 12

Define enthalpy change of solution.

Answer 12

The energy absorbed or released when 1 mole of an ionic solid dissolves in sufficient water to form a very dilute solution.

Question 13

Factors affecting solubility of ionic compounds in water.

Answer 13

A compound is likely to be soluble in water only if enthalpy change of solution is negative or has a small positive value. Substances with high positive values are relatively insoluble.

Question 14

Enthalpy change of hydration.

Answer 14

This is the enthalpy change when 1 mole of a specified gaseous ion dissolves in sufficient water to form a very dilute solution.

Question 15

The value of enthalpy change of hydration depends on…

Answer 15

• The size of ion (is more exothermic the smaller the ion)

* The charge on the ion ( is more exothermic the greater the charge)

Question 16

What is the trend of solubility of group 2 sulfates?

Answer 16

It decreases down the group.

Question 17

Applying Hess’s law in this topic shows that….

Answer 17

E.C of solution = E.C of hydration - lattice energy.

Question 18

Why does the thermal stability of group 2 nitrates and carbonates increase down the group?

Answer 18

Because as you go down, there is less polarisation of the anion, so it is harder to weaken the carbon-oxygen bond in the carbonate and form CO₂ and O₂.