Thermodynamics

Question 1

What is enthalpy of formation?

Answer 1

• when one mole of a substance is formed from its constituent elements with all substances in their standard states

Question 2

What is enthalpy of combustion

Answer 2

• enthalpy change when one mole of a substance undergoes complete combustion in oxygen with all substances in standard states

Question 3

What is first ionisation enthalpy

Answer 3

The enthalpy change when one mole of gaseous atoms loses one electron per atom to produce gaseous 1+ ions

Question 4

What is second ionisation enthalpy

Answer 4

When one mole of gaseous 2+ ions is produced from one mole of 1+ ions

Question 5

What is second ionisation enthalpy

Answer 5

The enthalpy change when one mole of gaseous 2+ ions is produced from one mole of 1+ ions

Question 6

What is electron affinity

Answer 6

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

Question 7

What is the second electron affinity?

Answer 7

• the enthalpy change when one mole of gaseous 1- ions gains one electron per ion to produce gaseous 2- ions

Question 8

What is enthalpy of atomisation

Answer 8

When one mole of gaseous atoms is produced from an element in its normal state

Question 9

What is hydration enthalpy?

Answer 9

• when one mole of gaseous ions becomes hydrated

Question 10

What is enthalpy of solution?

Answer 10

• 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

Question 11

What is bond dissociation enthalpy?

Answer 11

• enthalpy change when one mole of covalent bonds is broken in the gaseous state

I2 (g) -> 2I (g)

Question 12

What is the lattice enthalpy of formation/disassociation?

Answer 12

Enthalpy change when one mole of a solid ionic compound is formed/broken up into its constituent ions in the gas phase

Question 13

What factors affect lattice enthalpy?

Answer 13

• ionic radius
• ionic charge
• covalent character

Question 14

What is lattice enthalpy an indicator of?

Answer 14

• ionic radius
• ionic charge
• covalent character

Question 15

When a reaction takes place, the main compound that will form will be…

Answer 15

The most energetically stable (at lower energy)

Question 16

The more exothermic the enthalpy of formation..

Answer 16

The more stable the compound is relative to its elements

Question 17

Why is CaF2 more likely to form than CaF and CaF3

Answer 17

• CaF3 is at a higher energy than its substituent elements
• CaF has a less exothermic enthalpy of formation

Question 18

What would happen if CaF is formed?

Answer 18

CaF will decompose into Ca and CaF2

Question 19

What assumptions does the perfect ionic model make?

Answer 19

• perfectly spherical ions
• no electron density between the ions

Question 20

What is covalent character?

Answer 20

• when electron density is found between the two nuclei within a covalent bond

Question 21

Describe a polarising cation

Answer 21

• small ionic radius
• highly charged

Question 22

Describe a polarisable anion

Answer 22

• diffuse electron cloud

Question 23

The greater the different between the experimental and theoretical values of lattice enthalpy…

Answer 23

The further the bonding is away from the perfect ionic model and the more covalent character present in the bond

Question 24

What is enthalpy change of solution equal to?

Answer 24

Lattice enthalpy + enthalpy of hydration
Ionic solid -> gaseous ions -> dissolved ions

Question 25

What affects lattice enthalpy?

Answer 25

The strength of the interactions between cations and anions - ionic radius - ionic charge

Question 26

What affects the size of hydration enthalpies

Answer 26

Charge density

Question 27

Explain why larger charge density leads to a more exothermic enthalpy of hydration

Answer 27

- More charge dense ions attract water molecules more strongly - more energy is released when ion-dipole attractions formed

Question 28

How can we tell if a solid would dissolve by enthalpy changes?

Answer 28

- a solid will always dissolve if hydration of the anions and cations is sufficient to provide enough energy to dissociate the lattice. - some may dissolve if Hyd(cat+ani) provides almost enough energy - enthalpy of solution must be very slightly endothermic

Question 29

What is entropy?

Answer 29

- a measure of disorder in a chemical system - measured in JK mol

Question 30

Phase changes always cause…

Answer 30

A sudden change in entropy

Question 31

Do more electrons mean higher or lower entropy

Answer 31

higher

Question 32

Does diamond or graphite have higher entropy

Answer 32

Graphite - has delocalised electrons q

Question 33

What four processes have positive entropy change?

Answer 33

- change in state - dissolving (ions in a solution are delocalised) - solid -> gas - increase in number of moles

Question 34

How do you calculate entropy change?

Answer 34

- entropy of products - entropy of reactant

Question 35

What does feasibility refer to

Answer 35

- when a reaction is able to take place

Question 36

What does the word spontaneous refer to?

Answer 36

- reactions that happen of their own accord

Question 38

Are exothermic reactions spontaneous?

Answer 38

Yes

Question 39

Why are some endothermic reaction spontaneous?

Answer 39

- increase in entropy

Question 40

What does it mean when a reaction has entropy change

Answer 40

Reactions that bring about an increase in disorder also tend to be feasible

Question 41

For a reaction to be feasible we ideally want…

Answer 41

A exothermic enthalpy change and a positive entropy change

Question 42

What is Gibbs’s free energy change

Answer 42

ΔG = ΔH – TΔS

Question 43

Why might a reaction still not happen even if gibbs is smaller than 0

Answer 43

Kinetics - may have a very high activation energy