3.1.8 Thermodynamics

Question 1

What is enthalpy of atomisation?
Is it exothermic or endothermic?

Answer 1

The enthalpy change when one mole of gaseous atoms is formed from its element under standard conditions.
Always endothermic

Question 2

What is electron affinty?

Answer 2

The enthalpy change when one mole of electrons are gained by one mole of gaseous atoms of an element to form 1 mole of gaseous 1- ions under standard conditions.

Question 3

Why is the first electron affinty exothermic and the second electron affinty endothermic?

Answer 3

2nd electron affinty = electron repulsions make it more difficult to add a second electron to the negative ion.

Question 4

What is bond dissociation enthalpy?
Is it exothermic or endothermic?

Answer 4

• The energy required to break 1 mole of a specific covalent bond (gas) under standard conditions.
• Always endothermic

Question 5

What is the lattice energy of formation?
Is it exothermic or endothermic?

Answer 5

• Enthalpy change when one mole of an ionic compound is formed from its gaseous ions under standard conditions.
• Always exothermic

Question 6

What is the lattice enthalpy of dissociation?
Is it exothermic or endothermic?

Answer 6

• Enthalpy change when one mole of an ionic compound is broken up into its gaseous ions under standard conditions
• Always endothermic

Question 7

What is the order of Born-Haber cycle in reference to enthalpy of formation?

Answer 7

1. Enthalpy of atomisation
2. Ionisation energy
3. Bond dissociation enthalpy (not always, mainly with halogens)
4. Electron affinity
5. Enthalpy of lattice formation

Question 8

What is required to calculate the theoretical lattice enthalpy?

Answer 8

Shape of ionic compounds, ion charges, distacne between ions

Question 9

Is the latice enthalp of formation calculated from a Born-Haber cylce theoretical or experimental?

Answer 9

Experimental

Question 10

What is the meaning of the term perfect ionic model?

Answer 10

No covalent character

Question 11

Why is the experimental lattice enthalpy of dissociation greater than the theortical valie obtained from the perfect ionic model?

Answer 11

• The ionic compound in experimental displays some covalent character.
• Ionic model displays none.

Question 12

How do you calculate hydration enthalpy?

Answer 12

enthalpy of solution + enthalpy of formation

Question 13

What is the standard enthalpy of solution?
Is it exothermic or endothermic?

Answer 13

• The enthalpy change when one mole of an ionic compound dissolves in sufficient water to form an infinitely dilute solution. Under standard conditions.
• Either exothermic or endothermic

Question 14

What is hydration enthalpy?
Is it exothermic or endothermic?

Answer 14

• The enthalpy change when 1 mole of a gaseous ion dissolves in sufficient water to form an infintitely dilute solution. Under standard conditions.
• Always exothermic

Question 15

What factors impact the hydration enthalpy of different ions?

Answer 15

• Nuclear charge
• Ionic radius
• Ion charge
• Shielding

Question 16

What is entropy?
What are the units of entropy?

Answer 16

• The total disorder of a system
• J mol-1 K-1

Question 17

In which state does a substance have the highest entropy and why?

Answer 17

• Gaseous state
• More disorder

Question 18

How do you calculate entropy change of a reaction?

Answer 18

ΔS = ΔS(products) - ΔS(reactents)

Question 19

How do you calculate Gibss Free Energy?

Answer 19

ΔG = ΔH - TΔS