1.8.1 Thermodynamics

Question 1

What is the standard enthalpy of formation (∆Hɵf)?

Answer 1

It is the enthalpy change when one mole of a compound is formed from its elements under standard conditions, with all reactants and products in their standard states.

Question 2

What is meant by the first ionisation enthalpy (1st∆Hɵi)?

Answer 2

It is the enthalpy change when one mole of electrons is removed from one mole of gaseous atoms to form one mole of gaseous ions with a single positive charge.

Question 3

Define the standard enthalpy of atomisation (∆Hɵat).

Answer 3

It is the enthalpy change when one mole of gaseous atoms is formed from an element in its standard state.

Question 4

What is the mean bond enthalpy (ΔHBEΘ)?

Answer 4

It is the enthalpy change when one mole of gaseous molecules each break a covalent bond to form two free radicals, averaged over a range of compounds.

Question 5

Define the first electron affinity (1st∆Hɵea).

Answer 5

It is the enthalpy change when one mole of gaseous atoms is converted into a mole of gaseous ions, each with a single negative charge under standard conditions.

Question 6

What is lattice formation enthalpy (ΔHLFΘ)?

Answer 6

It is the enthalpy change when one mole of a solid ionic compound is formed from its gaseous ions.

Question 7

Write the equation representing the standard enthalpy of formation of NaCl.

Answer 7

Na(s) + ½Cl₂(g) → NaCl(s)

Question 8

Write the equation for the first ionisation enthalpy of calcium.

Answer 8

Ca(g) → Ca⁺(g) + e⁻

Question 9

Write the equation representing the enthalpy of atomisation for bromine.

Answer 9

½Br₂(l) → Br(g)

Question 10

Provide the equation representing the mean bond enthalpy for chlorine gas.

Answer 10

Cl₂(g) → 2Cl(g)

Question 11

Write the equation for the first electron affinity of oxygen.

Answer 11

O(g) + e⁻ → O⁻(g)

Question 12

Give the equation for the lattice formation enthalpy of magnesium bromide.

Answer 12

Mg²⁺(g) + 2Br⁻(g) → MgBr₂(s)

Question 13

Which has a stronger ionic bond: NaCl or MgO?

Answer 13

Charge: MgO has Mg²⁺ and O²⁻ ions, while NaCl has Na⁺ and Cl⁻. Radius: Mg²⁺ and O²⁻ are smaller than Na⁺ and Cl⁻. Strength of attraction: The higher charges and smaller radii in MgO create a stronger attraction. Conclusion: MgO has the stronger ionic bond.

Question 14

Compare the ionic bond strength of KBr and NaF.

Answer 14

Charge: Both have +1 and -1 charges. Radius: Na⁺ and F⁻ are smaller than K⁺ and Br⁻. Strength of attraction: The smaller radii in NaF result in a stronger attraction. Conclusion: NaF has the stronger ionic bond.

Question 15

Which compound has stronger ionic bonds: CaO or BaS?

Answer 15

Charge: Both have +2 and -2 charges. Radius: Ca²⁺ and O²⁻ are smaller than Ba²⁺ and S²⁻. Strength of attraction: Smaller ions in CaO create a stronger attraction. Conclusion: CaO has the stronger ionic bond.

Question 16

Compare the ionic bond strength of LiCl and CsCl.

Answer 16

Charge: Both have +1 and -1 charges. Radius: Li⁺ is much smaller than Cs⁺. Strength of attraction: The smaller Li⁺ ion leads to a stronger attraction. Conclusion: LiCl has the stronger ionic bond.

Question 17

Which has a stronger ionic bond: Al₂O₃ or Na₂O?

Answer 17

Charge: Al³⁺ and O²⁻ have higher charges compared to Na⁺ and O²⁻. Radius: Al³⁺ is smaller than Na⁺. Strength of attraction: The higher charge and smaller size in Al₂O₃ create a stronger attraction. Conclusion: Al₂O₃ has the stronger ionic bond.

Question 18

Compare the ionic bond strength of SrCl₂ and MgCl₂.

Answer 18

Charge: Both have +2 charges. Radius: Mg²⁺ is smaller than Sr²⁺. Strength of attraction: The smaller Mg²⁺ leads to a stronger attraction. Conclusion: MgCl₂ has the stronger ionic bond.

Question 19

Given that the purely ionic lattice enthalpy of NaCl is +770 kJ/mol and the Born-Haber value is +780 kJ/mol, why might there be a difference?

Answer 19

The slight difference suggests that NaCl has some covalent character not accounted for in the purely ionic model.

Question 20

The purely ionic lattice enthalpy for MgO is +3850 kJ/mol, while the Born-Haber value is +3950 kJ/mol. Why is this?

Answer 20

The larger difference indicates significant covalent character in the bonding of MgO, which is not captured by the purely ionic model.

Question 21

The purely ionic lattice enthalpy for CaCl₂ is +2200 kJ/mol, whereas the Born-Haber value is +2250 kJ/mol. What does this suggest?

Answer 21

The small difference suggests that CaCl₂ is mostly ionic, with only a minor degree of covalent character.

Question 22

What assumption does the perfect ionic model make about ions?

Answer 22

It assumes ions are perfect spheres or point charges.

Question 23

How does the Born-Haber cycle model differ from the perfect ionic model?

Answer 23

The Born-Haber model accounts for polarised ions, while the perfect ionic model does not.

Question 24

What is the nature of bonding in the perfect ionic model?

Answer 24

Purely ionic.

Question 25

What additional character is present in bonding when using the Born-Haber model?

Answer 25

The bonding is ionic with some covalent character.

Question 26

Why is the enthalpy of hydration important in Born-Haber cycles?

Answer 26

It helps calculate the overall enthalpy change when an ionic compound dissolves in water.

Question 27

Which process would have a more exothermic enthalpy change: the formation of NaCl(s) or the formation of CaO(s)?

Answer 27

The formation of CaO(s) because Ca²⁺ and O²⁻ have higher charges, resulting in a stronger attraction and a more exothermic enthalpy change.

Question 28

Why is the first electron affinity often exothermic?

Answer 28

Because energy is released when an electron is added to a neutral atom due to the attraction between the atom and the added electron.

Question 29

Why might lattice enthalpies differ between experimental and theoretical calculations?

Answer 29

Theoretical calculations assume perfect ionic models, whereas experimental data account for polarisation and covalent character.

Question 30

Why is the second electron affinity always endothermic?

Answer 30

Because adding an electron to a negatively charged ion requires energy to overcome the repulsion between the electron and the ion.