3.1.8 Thermodynamics - BHC

Question 1

Define ‘enthalpy change’

Answer 1

heat energy change at a constant pressure

Question 2

Define ‘standard enthalpy change’

Answer 2

enthalpy change measured under standard conditions

Question 3

Define ‘enthalpy of formation’.

Answer 3

Enthalpy change when 1 mole of compound is formed from its elements (under standard condtions - 298k and 100 kpa) with all reactants and products in their standard states

Question 4

Define ‘enthalpy of atomisation’.

Answer 4

Enthalpy change when 1 mole of gaseous atoms is formed from an element in its standard state.

Question 5

Define ‘1st ionisation enthalpy’.

Answer 5

The enthalpy change when 1 mole of gaseous 1+ ions is formed from its gaseous atoms.

Question 6

Define ‘2nd ionisation enthalpy’.

Answer 6

The enthalpy change when 1 mole of gaseous 2+ ions are formed from gaseous 1+ ions.

Question 7

Define ‘1st electron affinity’

Answer 7

The enthalpy change when 1 mole of gaseous 1- ions are formed from gaseous atoms.

Question 8

Define ‘2nd electron affinity’.

Answer 8

The enthalpy change when 1 mole of gaseous 2- ions is formed from gaseous 1- ions.

Question 9

Define ‘bond dissociation enthalpy’.

Answer 9

The enthalpy change when 1 mole of covalent bonds is completely separated to form gaseous atoms.

Question 10

Define ‘lattice enthalpy of formation’.

Answer 10

The enthalpy change when 1 mole of solid ionic compound is formed from gaseous ions.

Question 11

Define ‘lattice enthalpy of dissociation’.

Answer 11

The enthalpy change when 1 mole of solid ionic compound dissociates into gaseous ions.

Question 12

How can we use the Born-Haber Cycle to work out lattice enthalpies?

Answer 12

Elements converted to gaseous atoms
Electrons lost/gained to form ions
Ions attract to form a lattice
Write start/finish then go around to find enthalpy change

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Question 13

What is the perfect ionic model?

Answer 13

Ions are perfect spheres
with evenly distributed charge

Question 14

State why some ionic compounds display covalent character.

Answer 14

+ve ions attract the outer electrons of the -ve ions
Electron clouds are polarised / charge cloud distorted

Question 15

In what scenario is covalent character more likely for an ionic compound?

Answer 15

Cation has high charge density
Anion has low charge density

Question 16

How does covalent character change the lattice enthalpy?

Answer 16

larger than expected
as it requires more energy to break / form

Question 17

What two factors does the strength of lattice enthalpy depend on and how?

Answer 17

Size of ions (bigger size, charges are further apart, weaker attraction)
Charge on the ion (bigger charge, stronger attraction)

Question 18

What happens when NaCl dissolves?

Answer 18

Water molecules are polar
Na+ attracted to δ- O atoms
Cl- ions are attracted to δ+ H atoms

Question 19

Define ‘enthalpy of solution’ - ΔHsolnӨ:

Answer 19

The enthalpy change when 1 mole of ionic compound dissolves in water.

Question 20

Define ‘enthalpy of hydration’ ΔHhydӨ:

Answer 20

The enthalpy change when one mole of aqueous ions is formed from gaseous ions.

Question 21

Using enthalpy cycles, qualitiavely describe how we can work out enthalpy of solution.

Answer 21

Ionic lattice broken down into gaseous ions
Gaseous ions hydrated by polar water molecules

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Question 22

What are the two factors affecting solubility of an ionic compound?

Answer 22

Higher charge - greater attraction:
ΔHLӨ increases
ΔHhydӨ increases

Larger ion - less charge density
ΔHLӨ decreases
ΔHhydӨ decreases

Question 23

Recall the equations for enthalpy of solution when given enthalpy of hydration and either lattice enthalpy. Use enthalpy cycles! (think of LiCl as an example)

Answer 23

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