[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.

Question 13

Draw the Born-Haber cycle of sodium chloride and calculate the enthalpy of lattice formation with the values given below.

Enthalpy of atomisation of Na = 107
Enthalpy of 1st ionisation energy of Na = 496
Enthalpy of atomisation of Cl = 122
Enthalpy of 1st electron affinity of Cl = -349
Enthalpy of formation of NaCl = -411

Answer 13

Question 14

Draw the Born-Haber cycle of magnesium chloride if you were to calculate the enthalpy of lattice formation.

Answer 14

(Note in this example the first and second ionisation energies of Mg are needed as Mg is a +2 ion)

Question 15

Draw the Born-Haber cycle of calcium oxide if you were to calculate the enthalpy of lattice dissociation.

Describe the equation you would use to reach your answer.

Answer 15

Question 16

Describe the trends in lattice enthalpies.

Answer 16

1. The sizes of ions.
   • The larger the ions, the less negative the enthalpies of lattice formation (i.e. a weaker lattice). As the ions are larger, the charges become further apart and so have a weaker attractive force between them.
2. The charges on the ion.
   • The bigger the charge of the ion, the greater the attraction between the ions so the stronger the lattice enthalpy (more negative values).
     
     • The lattice enthalpies become less negative down any group.

Question 17

What is the perfect ionic model?

Answer 17

-Theoretical lattice enthalpies assumes a perfect ionic model where the ions are 100% ionic and spherical and the attractions are purely electrostatic.

Question 18

What are the differences between theoretical and Born-Haber (experimental) lattice enthalpies?

Answer 18

• The Born-Haber lattice enthalpy is the real experimental value.
• When a compound shows covalent character, the theoretical and the Born-Haber lattice enthalpies differ.
• The more covalent character, the bigger the difference between the values.
  
  • e.g. When a compound has some covalent character, it tends towards giant covalent so the lattice is stronger than if it was 100% ionic. Therefore, the Born-Haber value would be larger than the theoretical value.

Question 19

When is there a tendency towards covalent character in ionic substances?

Answer 19

When:
- The positive ion is small.
- The positive ion has multiple charges.
- The negative ion is large.
- The negative ion has multiple negative charges.