3.1.8 Thermodynamics

Question 1

Define lattice formation enthalpy

Answer 1

Enthalpy change when one mole of solid ionic compound is formers from its gaseous ions

Question 2

Define lattice dissociation enthalpy

Answer 2

Enthalpy change when one mole of solid ionic compound is completely dissociated into its gaseous ions

Question 3

Define bond dissociation enthalpy

Answer 3

Enthalpy change when all of the bonds of the same type in one mole of gaseous molecules are broken

Question 4

Define enthalpy of atomisation

Answer 4

Enthalpy change when one mole of gaseous atoms is formed from an element in its standard state

Question 5

Define first electron affinity

Answer 5

The enthalpy change when one mole of gaseous 1- ions is formed from one mole of gaseous atoms

Question 6

Define enthalpy of hydration

Answer 6

The enthalpy change when one mole of aqueous ions is formed from gaseous ions

Question 7

Define enthalpy of solution

Answer 7

Enthalpy change when one mole of an ionic substance dissolves in enough solvent to form an infinitely dilute solution

Question 8

Why is second electron affinity endothermic?

Answer 8

The 1- ions repel the second electron so energy must be put in to form 2- ions

Question 9

How is lattice enthalpy found experimentally?

Answer 9

Born-Hager cycles

Question 10

How is lattice enthalpy found theoretically?

Answer 10

Coulomb’s Law

Question 11

What assumptions does Coulomb’s Law rely on? (2)

Answer 11

100% ionic bonding

Perfectly spherical ions

Question 12

Why do theoretical and experimental lattice enthalpy values differ?

Answer 12

The ionic substance in question shows some covalent character

Question 13

How does lattice enthalpy indicate stability?

Answer 13

The more exothermic the enthalpy of formation is, the more stable the compound is

Question 14

What factors affect stability of ionic lattices? (4)

Answer 14

Charge density
Radius of negative ions (polarisability)
Polarising power of positive ions
Magnitude of electrostatic attraction

Question 15

How does the polarising power of positive ions change throughout the periodic table?

Answer 15

Polarising power increases up the groups and across the periods

Question 16

How does the polarisability of negative ions change?

Answer 16

Negative ions with a larger radius are more polarisable

Question 17

Why can enthalpy of solution calculations be inaccurate? (3)

Answer 17

Errors in mass measurements
Heat loss to surroundings/apparatus
Non-standard conditions

Question 18

Define entropy

Answer 18

A measure of the disorder of a system

Question 19

Define standard molar entropy

Answer 19

Entropy per mole of a substance under standard conditions

Question 20

What does entropy determine?

Answer 20

The feasibility of a reaction - change occurs in the direction of increasing entropy

Question 21

What is the equation for Gibbs free energy?

Answer 21

ΔG = ΔH - TΔS

Question 22

For a reaction to be feasible what must be true about ΔG?

Answer 22

Less than or equal to 0

Question 23

How is the minimum feasible temperature for a reaction calculated?

Answer 23

T = ΔH/ΔS