3.1.8 Thermodynamics

Question 1

Hydration enthalpy

Answer 1

Enthalpy change when one mole of gaseous ions become hydrated i.e. dissolved in water

Question 2

Enthalpy change of a solution

Answer 2

The enthalpy change when one mole of an ionic solid is dissolved in a volume of water large enough so that the dissolved ions are well separated and do not interact with each other

Question 3

Lattice enthalpy of formation

Answer 3

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

Question 4

Lattice enthalpy of dissociation

Answer 4

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

Question 5

Entropy (JK^-1mol^-1)

Answer 5

The degree of disorder in a system is called entropy (S) (from the Greek word for change)
More disordered=higher entropy

Question 6

Suggest a reason for incomplete combustion or heat transfer to surroundings in enthalpy of combustion experiment

Answer 6

Evaporation of alcohol
Reaction not under standard conditions

Question 7

3rd Law of thermodynamics

Answer 7

entropy is zero (or close to zero) at absolute zero (K) and increases with temperature

Question 8

Entropy change

Answer 8

Sum of product entropies-
Sum of reactant entropies

Question 9

Gibb’s Free Energy Change

Answer 9

∆Gθ = ∆Hθ - T∆Sθ
Standard free energy change= enthalpy change - total entropy increase

Question 10

Why is entropy of water 0 at 0K

Answer 10

Particles in max state of order

Question 11

Why is entropy change larger when water vaporises than when it melts

Answer 11

Larger increase in disorder

Question 12

Molecular formula for standard used in 13C spectroscopy + reasons why used

Answer 12

C4H12Si
Single peak
Inert
Low boiling point/volatility

Question 13

Why might the theoretical value of lattice dissociation for NaF be less endothermic than the experimental value calculated using a Born-Haber cycle

Answer 13

Theoretical assumes perfect ionic spheres
Some covalent character in NaF

Question 14

Why might enthalpy change differ between experimental and data book value apart from heat loss to surroundings?

Answer 14

Energy absorbed in equipment
Error in temp readings
Volumes calculated inaccurately
Concentrations not accurate

Question 15

Suggest one reason why the enthalpy change for the hydration of magnesium chloride can’t be determined by calorimetry

Answer 15

Not possible to prevent some dissolving

Question 16

Enthalpy of atomisation

Answer 16

Enthalpy change when one mole of gaseous atoms is formed from the element in its standard state, under standard conditions

Question 17

Electron affinity

Answer 17

Enthalpy changes when one electron per atom is added to one mole of gaseous atoms to form 1- ions

Question 18

Ionisation energy

Answer 18

Enthalpy change when one electron is removed per atom from one mole of gaseous atoms to form 1+ ions

Question 19

Why does increase in entropy make reactions favourable

Answer 19

Free-energy change negative