[3.1.8] Thermodynamics

Question 1

What is the definition of the enthalpy of formation?

Answer 1

• The standard enthalpy change of formation of a compound is the energy transferred when 1 mole of the compound is formed from its elements under standard conditions (298K & 100kPa), all reactants and products being in their standard states.
  
  • e.g. Na (s) + 1/2 Cl₂ (g) -> NaCl (s) [ΔfH = -411.2 KJ mol⁻¹)

Question 2

What is the definition of enthalpy of atomisation?

Answer 2

• The enthalpy of atomisation of an element is the enthalpy change when 1 mole of gaseous atoms is formed from the element in its standard state.
  
  • e.g. Na (s) -> Na (g) [ΔatH = +148 KJ mol⁻¹]
  • e.g. 1/2 O₂ (g) -> O (g) [ΔatH = +249 KJ mol⁻¹]

Question 3

What is the definition of bond dissociation enthalpy (bond energy)?

Answer 3

• The bond dissociation enthalpy is the standard molar enthalpy change when 1 mole of a covalent bond is broken into two gaseous atoms (or free radicals).
  
  • e.g. Cl₂ (g) -> 2Cl (g) [ΔdissH = +242 KJ mol⁻¹]
  • e.g. CH₄ (g) -> CH₃ (g) + H (g) [ΔdissH = +435 KJ mol⁻¹]

(For diatomic molecules the ΔdissH of the molecule is the same as 2xΔatH of the element

Cl₂ (g) -> 2Cl (g) [ΔdissH = +242 KJ mol⁻¹]
1/2 Cl₂ (g) -> Cl (g) [ΔdissH = +121 KJ mol⁻¹])

Question 4

What is the definition of first ionisation enthalpy?

Answer 4

• The first ionisation enthalpy is 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.
  
  • e.g. Mg (g) -> Mg⁺ (g) + e⁻ [ΔIE1H]

Question 5

What is the definition of second ionisation enthalpy?

Answer 5

• The second ionisation enthalpy is the enthalpy change required to remove 1 mole of electrons from 1 mole of gaseous 1⁺ ions to form 1 mole of gaseous ions with a 2⁺ charge.
  
  • e.g. Mg (g) -> Mg²⁺ (g) + e⁻ [ΔIE2H]

Question 6

What is the definition of first electron affinity?

Answer 6

• The first electron affinity is the enthalpy change that occurs when 1 mole of gaseous atoms gain 1 mole of electrons to form 1 mole of gaseous ions with a 1⁻ charge.
• The first electron affinity is exothermic for atoms that normally form negative ions. This is because the ion is more stable than the atom and there is an attraction between the nucleus and the electron.
  
  • e.g. O (g) + e⁻ -> O⁻ (g) [Δea1H = -141.1 KJ mol⁻¹]

Question 7

What is the definition of second electron affinity?

Answer 7

• The second electron affinity is the enthalpy change that occurs when 1 mole of gaseous 1- ions gains 1 electron per ion to produce gaseous 2⁻ions.
• The second electron affinity is endothermic because it takes energy to overcome the repulsive force between the negative ion and the electron.
  
  • e.g. O⁻ (g) + e⁻ -> O²⁻ (g) [Δea2H = +798 KJ mol⁻¹]

Question 8

What is the definition of enthalpy of lattice formation?

Answer 8

• The enthalpy of lattice formation is the standard enthalpy change when 1 mole of an ionic crystal lattice is formed from its constituent ions in gaseous form.
  
  • e.g. Na⁺ (g) + Cl⁻ (g) -> NaCl (s) [ΔlattH = -787 KJ mol⁻¹]

Question 9

What is the definition of enthalpy of lattice dissociation?

Answer 9

• 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.
  
  • e.g. NaCl (s) -> Na⁺ (g) + Cl⁻ (g) -> [ΔlattH = +787 KJ mol⁻¹]

Question 10

What is the definition of enthalpy of hydration?

Answer 10

• Enthalpy change when 1 mole of gaseous ions become aquous ions.
  
  • e.g. X⁺ (g) + aq -> X⁺ (aq) For Li⁺ [ΔhydH = -519 KJ mol⁻¹]
  • e.g. X⁻ (g) + aq -> X⁻ aq) For F⁻ [ΔhydH = -506 KJ mol⁻¹]

Question 11

What is the definition of enthalpy of solution?

Answer 11

• The enthalpy of solution is the standard enthalpy change when 1 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.
  
  • e.g. NaCl (s) + aq -> Na⁺ (aq) + Cl⁻ (aq)

Question 12

How do we calculate the lattice enthalpy?

Answer 12

• Lattice enthalpy cannot be determined directly.
• We calculate it indirectly by making use of changes for which data are available and link them together in an enthalpy cycle, the Born-Haber cycle.