TOPIC 13: ENERGETICS II

Question 1

Enthalpy of formation (fHº):

Answer 1

1. Enthalpy change when one mole of a substance is formed from its constituent elements with all substances in their standard states.
2. Exothermic (-ve) for most substances

Question 2

Enthalpy of combustion (cHº)

Answer 2

1. Enthalpy change when one mole of a substance undergoes complete
   combustion in oxygen with all substances in standard states.
2. Exothermic (-ve)

Question 3

Enthalpy of neutralisation (neutHº)

Answer 3

1. Enthalpy change when 1 mole of water is formed in a reaction between an
   acid and alkali under standard conditions.
2. Exothermic (-ve)

Question 4

Ionisation enthalpy (ieHº)

Answer 4

1. First ionisation energy = enthalpy change when each atom in one mole of
   gaseous atoms loses one electron to form one mole of gaseous 1+ ions.
2. Second ionisation energy = enthalpy change when each ion in one mole of
   gaseous 1+ ions loses one electron to form one mole of gaseous 2+ ions.
3. Endothermic (+ve)

Question 5

Electron affinity (eaHº)

Answer 5

1. First electron affinity = enthalpy change when each atom in one mole of
   gaseous atoms gains one electron to form one mole of gaseous 1– ions.
   -(Exothermic (-ve) for many non-metals)-
2. Second electron affinity = enthalpy change when each ion in one mole of
   gaseous 1– ions gains one electron to form one mole of gaseous 2– ions.
3. Endothermic (+ve) as we are adding -ve electron to -v ion- energy is required to overcome this repulsion

Question 6

Enthalpy of atomisation (atHº)

Answer 6

1. Enthalpy change when one mole of gaseous atoms is produced from an
   element in its standard state.
2. Endothermic (+ve - as you need to supply energy to break an element into its separate gaseous atoms.

Note: If the element is a gas the enthalpy change of atomisation is related to bond enthalpy.

Question 7

Hydration enthalpy (hydHº)

Answer 7

1. Enthalpy change when one mole of gaseous ions become hydrated (dissolved in water) to form one mole of aqueous ions.
2. Exothermic (-ve)

Question 8

Enthalpy of solution (solHº)

Answer 8

1. Enthalpy change when one mole of an ionic solid dissolves in an amount of water large enough so that the dissolved ions are well separated and do
   not interact with each other.
2. Varies

Question 9

Bond dissociation enthalpy (disHº)

Answer 9

1. Enthalpy change when one mole of covalent bonds is broken in the
   gaseous state.
2. Endothermic (+ve)

Question 10

Lattice enthalpy of formation (LEFHº)

Answer 10

1. Enthalpy change when one mole of a solid ionic compound is formed from
   into its constituent ions in the gas phase.
2. Exothermic (-ve)

Question 11

Lattice enthalpy of dissociation (LEDHº)

Answer 11

1. Enthalpy change when one mole of a solid ionic compound is broken up
   into its constituent ions in the gas phase.
2. Endothermic (+ve)

Question 12

Enthalpy of vaporisation (vapHº):

Answer 12

1. Enthalpy change when one mole of a liquid is turned into a gas.
2. Endothermic (+ve)

Question 13

Enthalpy of fusion (fusHº)

Answer 13

1. Enthalpy change when one mole of a solid is turned into a liquid.
2. Endothermic (+ve)

Question 14

Process of forming a Born-Haber Cycle:

Answer 14

1. Enthalpy of formation
2. Atomisation enthalpies of the atoms
3. First ionisation energies of the element
4. Electron affinities

Question 15

Process of forming a Born-Haber Cycle for Group 2 Elements:

Answer 15

1. Group 2 elements form 2+ ions so you have to include the second ionisation energy.
2. If there are two moles in a compound double the atomisation energy.
3. Double the first electron affinity if not given the second.

Question 16

What are the two ways to work out a lattice energy:

Answer 16

1. The experimental way- Using experimental enthalpy values in a Born-Haber cycle.
2. The theoretical way- Doing some calculations based on the purely ionic model of a lattice.

Question 17

What is the Born-Lande equation

Answer 17

The product of the charges/the sum of the radii

Question 18

What is the electrostatic theory:

Answer 18

1. All bonds are ionic

2. All ions are perfect spheres/ point charges

Question 19

State the relationship between the Experimental and theoretic lattice enthalpy value on ionic character

Answer 19

1. The closer the experimental and theoretical value the more perfectly ionic the compounds.
2. If the experimental value is greater than the theoretical value this tells us that the bonding is stronger and that the compounds have a ‘covalent character’

Question 20

Equation for charge density

Answer 20

Charge/Ionic radius

Question 21

Enthalpy change for formation and combustion

Answer 21

Formation= products-reactants
Combustion= reactants - products

Question 22

What is polarisation?

Answer 22

1. When a small, high charge Cation is bonded next to a large high charge anion, it will attract some of the negative charge away.
2. This distorts the electron cloud around the anion and there is a degree of electron sharing (covalency).

Question 23

Factors which increase polarisation?

Answer 23

1. Small high charge Cation

2. Large high charge anion

Question 24

What is happening in hydration?

Answer 24

Electrostatic attractions are formed between the water molecules and the ion in question.

Question 25

Factors affecting the size of the hydration enthalpy

Answer 25

Higher charge density means a stronger attraction

The hydrat