3.1.8 Thermodynamics (A-level only)

Question 1

Enthalpy of formation

Answer 1

Enthalpy change when 1 mole of a substance is formed from its constituent elements with all substances in their standard states

( exothermic)

Question 2

Enthalpy of combustion

Answer 2

enthalpy change when 1 mole of substance undergoes complete combustion in oxygen with all substances in their standard states

(exothermic)

Question 3

Enthalpy of neutralisation

Answer 3

Enthalpy change when 1 mole of water is formed in a reaction between an acid and alkali under standard conditions

(Exothermic)

Question 4

Ionisation Enthalpys

Answer 4

(same definition as normal but apart from )
2nd IE enthalpy - AH when each 1 mole of gaseous 1+ ions loses 1 electron to form 1 mole of gaseous 2+ ions

(endothermic)

Question 5

Enthalpy of Electron Affinity -

Answer 5

the enthalpy changes when one mole of electrons is added to one mole of gaseous atoms/ions

exothermic for 1st electron affinity
endothermic for 2nd electron affinity

Question 6

Enthalpy of Atomisation

Answer 6

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

(endothermic)

Question 7

hydration enthalpy

Answer 7

Enthalpy change when 1mole of gaseous ions become hydrated (dissolved in water )

(Exothermic)

Question 8

Enthalpy of solution

Answer 8

Enthalpy change when 1 mole of an ionic solid dissolves in an amount of water large enough to make sure the dissolved ions are well separated and dont interact with eachother

(Either exo or endo)

Question 9

Bond dissociation Enthalpy

Answer 9

Enthalpy change when 1 mole of covalent bonds are broken in the gaseous state

(endo)

Question 10

Lattice enthalpy of formation

Answer 10

enthalpy change when 1 mole of a solid ionic compound is formed from its constituent ions in the gas phase

(endothermic)

Question 11

Lattice enthalpy of dissociation -

Answer 11

enthalpy change when 1 mole of a solid ionic compound is broken up into its constituent ions in the gas phase

the bigger the MAGNITUDE of lattice enthalpy of dissociation (more negative) , the stronger the ionic bonding
(endothermic)

Question 12

Lattice enthalpy of vaporisation

Answer 12

Enthalpy change when 1 mole of liquid into gas
( endothermic)

Question 13

Lattice enthalpy of fusion

Answer 13

Enthalpy change when 1 mole of solid turned into a liquid
(endothermic)

Question 14

what proves covalent character in a compound

Answer 14

a differrence between the theoretical and actual value for the lattice enthalpy, the greater the differrence, (% ) the more covalent character a compound shows

Question 15

how is the experimental value calculated

Answer 15

using the charge + size of ions assuming the structure is perfectly ionic

Question 16

why might compounds show covalent character

Answer 16

the differrence between experimental and true value may be due to some distortion of the ions, also the spheres may not be perfectly spherical

larger sized ions tend to show more covalent character since theyre easier to distort - they are more polarisable

the less soluable a compound is, the more covalent character it shows

Question 17

going from 4 moles of gas to 2 moles of Gas means -

Answer 17

the entropy decreases

Question 18

what does the G ( gibbs free energy value) need to be for the reaction to be feasible

Answer 18

the G value needs to be negative for the reaction to be feasible

Question 19

make sure that

Answer 19

for diatomic molecules like CL2, their hydration, and atomisation will be 2x

Question 20

why would a solid get dissolved in a solvent such as water