Topic 19 lattice energy (enthalpy)

Question 1

Lattice Energy

Answer 1

The energy change when one mole of ionic solid is formed from its gaseous ions under standard conditions

Question 2

Enthalpy Change of Formation

Answer 2

The enthalpy change when one mole of a substance is formed from its elements in their standard states under standard condition

Question 3

1st Ionisation Energy

Answer 3

The enthalpy change when one mole of gaseous atoms lose one mole of electrons to form one mole of singly charged gaseous ions under standard conditions

Question 4

1st Electron Affiinity

Answer 4

The enthalpy change when one mole of gaseous atoms gains one mole of electrons to form one mole of singly charged gaseous ions under standard conditions

Question 5

Trends in electron affinities

Answer 5

-greater the nuclear charge, the greater the attractive force between the nucleus and the outer electrons
-the further away the outer shell electrons are from the positive nuclear charge the less attractive the force between the nucleus and the outer shell electrons is
-The greater the number of electron, shells, the greater the ability of inner shell electrons to shield the outer shell electrons from the nuclear charge

Question 6

As the size of the ion increases, the energy becomes?

Answer 6

less exothermic-this applies to both anions and cations

Question 7

Explain: ions with the same ionic charge have a lower charge density if their radius is larger -why?

Answer 7

This is because the same charge is spread out over a larger volume

Question 8

What does lower charge density result in which respect to ion attractions?

Answer 8

A lower charge density results in weaker electrostatic forces of attraction in the ionic lattice

Question 9

Ion polarisation(covalent character)

Answer 9

Perfect ionic model: model of an ionic lattice- we think of the ions as being spherical (charges)

Question 10

Distortion

Answer 10

+ve charge on the cation can attract electrons in the anion (toward it); this results in a distortion of the electron cloud of the anion and hence the anion is no longer spherical (this is distortion)

Question 11

Define ion polarisation

Answer 11

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

Question 12

Polarising power

Answer 12

The ability of a cation to attract the electron cloud of an anion and distort it

Question 13

Factors affecting ion polarisation (increase in covalent character)

Answer 13

The degree of polarisation of an anion depends on
- the charge density of the cation
-the ease with which the anion can be polarised (its polarisability)

Question 14

An anion is more likely to be polarised if

Answer 14

-the cation is small
-the cation has a higher charge (2+ or 3+)
-The anion is large
-The anion has a higher charge (2- or 3-)

Question 15

Enthalpy change of solution

Answer 15

The energy absorbed or released when one mole of an ionic solid dissolved in sufficient water to form a very dilute solution under standard conditions

Question 16

A compound is likely to be soluble in water only if…

Answer 16

• value for enthalpy of solution is -ve or very small +ve value
  -substances with large positive values of enthalpy of solution are relatively insoluble

Question 17

Enthalpy change of hydration

Answer 17

The enthalpy change when one mole of a specified gaseous ion dissolved in sufficient water to form a very dilute solution under standard conditions