Physical 2 Section 1 - Unit 17: Thermodynamics

Question 1

Enthalpy change of formation

Answer 1

The energy transferred when 1 mole of a compound is formed from its elements under standard conditions with all reactants and products in their standard states

Question 2

Enthalpy of atomisation

Answer 2

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

Question 3

Bond dissociation enthalpy

Answer 3

The standard molar enthalpy change when one mole of a covalent bond is broken into two gaseous atoms (or free radicals)

Question 4

First ionisation enthalpy

Answer 4

The enthalpy change required to remove 1 mole of electrons from 1 mole of gaseous atoms to form 1 mole of gaseous ions with a +1 charge

Question 5

Enthalpy of lattice formation

Answer 5

The standard enthalpy change when 1 mole of an ionic crystal lattice is formed from its constituent ions in gaseous form

Question 6

Enthalpy of lattice dissociation

Answer 6

The enthalpy of lattice dissociation is the standard enthalpy change when 1 mole of an ionic crystal lattice form is separated into its constituent ions in gaseous form

Question 7

Enthalpy of hydration

Answer 7

Enthalpy change when one mole of gaseous ions become aqueous ions

Question 8

Enthalpy of solution

Answer 8

The standard enthalpy change when one mole of an ionic solid dissolves in a large enough amount of water to ensure that the dissolved ions are well separated and do not interact with one another

Question 9

In terms of electrostatic forces, suggest why the first electron affinity of an element has a negative value (2 marks)

Answer 9

• Attraction between the nucleus and the added electron

- So energy is released when the electron is gained

Question 10

Explain why the theoretical enthalpy of lattice dissociation for a compound may be different from the experimental value that can be calculated using a Born–Haber cycle (2 marks)

Answer 10

• The experimental lattice enthalpy value takes into account the covalent interaction
• The theoretical lattice enthalpy value assumes only ionic interaction/perfectly ionic

Question 11

Suggest why hydration of the chloride ion is an exothermic process (2 marks)

Answer 11

• Water is polar / water has Hδ+

- Chloride ion attracts (the H in) water molecules

Question 12

Equation for entropy

Answer 12

∆S˚ = ΣS˚products - ΣS˚reactants

Question 13

Equation for Gibbs free cycle

Answer 13

• ∆G = ∆H - T∆S

Question 14

Equation for enthalpy of solution

Answer 14

Enthalpy of solution = Enthalpy of lattice dissociation + enthalpy of hydration
OR
Enthalpy of solution = - Enthalpy of lattice formation + enthalpy of hydration

Question 15

Explain, in terms of molecules, why the entropy is zero when the temperature is zero Kelvin (2 marks)

Answer 15

• Particles are stationary / not vibrating

- So there is perfect order / no disorder

Question 16

Explain why the evaporation of water is spontaneous even though this change is endothermic (4 marks)

Answer 16

• The molecules become more disordered when water changes from a liquid to a gas / evaporates
• Therefore the entropy change is positive
• TΔS>ΔH
• ΔG<0

Question 17

The freezing of water is an exothermic process. Give one reason why the temperature of a sample of water can stay at a constant value of 0 °C when it freezes (1 mark)

Answer 17

• The heat given out escapes

Question 18

Using a Born–Haber cycle, a value of –905 kJ mol–1 was determined for the lattice enthalpy of silver chloride. A value for the lattice enthalpy of silver chloride using the ionic model was –833 kJ mol–1.
Explain what you would be able to deduce from a comparison of these values (3 marks)

Answer 18

• The model used assumes the ions are spherical and in a lattice
• The calculated value is smaller than the cycle value or
  stronger attraction
• Indicating some covalent character or ions are polarised