5.2.1 Lattice enthalpy

Question 1

Define enthalpy of formation

Answer 1

The enthalpy change when 1 mole of a compound is formed from it’s constituent elements (in their standard states under standard conditions)

Question 2

Is the enthalpy of formation exo or endothermic?

Answer 2

Exo/endothermic

Question 3

What is the enthalpy of formation formula of NaCl (s)?

Answer 3

Na (s) + 1/2 Cl (g) –> NaCl (s)

Question 4

Define first ionisation enthalpy

Answer 4

The energy needed to remove 1 electron from each atom in a mole of gaseous atoms to produce one mole of gaseous 1+ ions

Question 5

Define second ionisation enthalpy

Answer 5

The energy needed to remove a second electron from each ion in a mole of gaseous 1+ ions to produce 1 mole of gaseous 2 + ions

Question 6

Is ionisation enthalpy exo or endothermic?

Answer 6

Endothermic

Question 7

What is the first ionisation enthalpy of Na (g)?

Answer 7

Na (g) –> Na+ (g) + e-

Question 8

What is the second ionisation enthalpy of Na + (g)?

Answer 8

Na + (g) –> Na 2+ (g) + e-

Question 9

Define first electron affinity

Answer 9

The enthalpy change when 1 mole of gaseous atoms gains 1 electron per atom to produce gaseous 1 - ions

Question 10

Define second electron affinity

Answer 10

The enthalpy change when 1 mole of gaseous 1 - ions gain 1 electron per ion to produce gaseous 2 - ions

Question 11

Is first electron affinity exo or endothermic?

Answer 11

Exothermic

Question 12

Is second electron affinity exo or endothermic?

Answer 12

Endothermic

Question 13

Why is the second electron affinity positive?

Answer 13

An electron is added to an already negative ion so energy is needed to overcome the repulsion

Question 14

What is the first electron affinity of Cl (g)?

Answer 14

Cl (g) + e - –> Cl - (g)

Question 15

What is the second electron affinity of O - (g)?

Answer 15

O - (g) + e - –> O 2- (g)

Question 16

Define enthalpy of atomisation

Answer 16

The enthalpy change when 1 mole of gaseous atoms is produced from an element in its standard state

Question 17

Is the enthalpy of atomisation exo or endothermic?

Answer 17

Exothermic

Question 18

What is the enthalpy go atomisation of Na (s)?

Answer 18

Na (s) –> Na (g)

Question 19

What is the enthalpy go atomisation of 1/2 Cl 2 (g)?

Answer 19

1/2 Cl 2 (g) –> Cl (g)

Question 20

Define lattice enthalpy

Answer 20

The enthalpy change of formation of 1 mole of a solid ionic compound from its gaseous ions under standard conditions

Question 21

Is lattice enthalpy exo or endothermic?

Answer 21

Exothermic

Question 22

What is the lattice enthalpy of K+ (g) and Cl- (g)?

Answer 22

K+ (g) and Cl- (g) –> KCl (s)f

Question 23

Define enthalpy change of solution

Answer 23

The enthalpy change when 1 mole of an ionic solid compound is completely dissolved in water

Question 24

Is the enthalpy change of solution exo or endothermic?

Answer 24

Exo/endothermic

Question 25

What is the enthalpy change of solution for MgCl2 (s)?

Answer 25

MgCl2 (s) + aq –> Mg2+ (aq) + 2Cl- (aq)

Question 26

What is the enthalpy change of solution for Na+Cl- (s)?

Answer 26

Na+Cl- (s) + aq –> Na+ (aq) + Cl- (aq)

Question 27

How do you calculate the enthalpy change of solution?

Answer 27

Enthalpy change of solution = hydration enthalpies - lattice enthalpy

Question 28

How do we determine if a substance is soluble or not?

Answer 28

By comparing the lattice enthalpy with the sum of the hydration energies of the ions
- If the LE is greater, solid doesn’t dissolve
- If the sum of hydration energies is grater, solid will dissolve

Question 29

What two factors affect charge density?

Answer 29

Ionic radius
Ionic charge

Question 30

How does charge density increase?

Answer 30

Ionic radius decreases
Ionic charge increases

Question 31

Define enthalpy change of hydration

Answer 31

The enthalpy change when 1 mole of isolated gaseous ions are dissolved in water to form 1 mole of aqueous ions

Question 32

Is hydration enthalpy exo or endothermic?

Answer 32

Exothermic

Question 33

What is the hydration enthalpy of Na + (g)?

Answer 33

Na+ (g) + aq –> Na+ (aq)

Question 34

What is the hydration enthalpy of Cl - (g)?

Answer 34

Cl- (g) + aq –> Cl- (aq)

Question 35

What makes the lattice enthalpy more exothermic?

Answer 35

Greater the attraction between ions, the stronger the ionic bonding and greater the charge density

Question 36

Why is the hydration enthalpy of Na+ lower than that of Mg2+?

Answer 36

Mg2+ has a higher ionic charge and a smaller ionic radius. Mg2+ has a higher charge density than Na+ and therefore, a stronger electrostatic force of attraction on the water molecules. More energy is released so hydration enthalpy of Mg2+ is more exothermic

Question 37

Describe and explain the factors that affect lattice enthalpy values

Answer 37

Higher ionic charge and smaller ionic radius = a higher charge density. Therefore, there is stronger electrostatic forces of attraction on oppositely charged ion so more energy released. More exothermic LE

Question 38

How do you work out the lattice enthalpy from a born harber cycle?

Answer 38

Lattice enthalpy = enthalpy of formation - other enthalpies

Question 39

In a born harber cycle, an upwards arrow shows what energy change?

Answer 39

Endothermic - positive

Question 40

In a born harber cycle, a downwards arrow shows what energy change?

Answer 40

Exothermic - negative