13: Energetics II

Question 1

Lattice energy definition

Answer 1

The energy change when one mole of an ionic solid is formed from its gaseous ions

Question 2

Enthalpy change of atomisation definition

Answer 2

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

Question 3

Electron affinity enthalpy

Answer 3

The enthalpy change when one mole of gaseous atoms gain one mole of electrons to form one mole of gaseous ions with a -1 charge

Question 4

Is electron affinity endothermic or exothermic?

Answer 4

1st is exothermic, subsequent are endothermic

Question 5

Process for constructing Born Haber cycles

Answer 5

• Atomise the metal
• Ionise the metal however many time needed
• Atomise the non-metal
• Add the electrons to the non-metal (e.a.)
• The remaining difference to the formation is lattice enthalpy

Question 6

Effect of bond strength of lattice enthalpy

Answer 6

Lattice enthalpy is higher for compounds with stronger bonds

Question 7

Assumptions for lattice enthalpies

Answer 7

Perfect ionic model

Question 8

How to tell degree of covalency from lattice enthalpy

Answer 8

The experimental model will be different from the theoretical - the greater the difference the greater the covalency

Question 9

How do cations polarise anions

Answer 9

They pull the negative charge cloud towards the cation

Question 10

Factors affecting the polarisation of anions?

Answer 10

• Cation size (smaller polarises more as closer to electrons)
• Anion size (bigger allows for easier distortion)
• Charge difference (greater attraction between cation and negative charge field)

Question 11

Enthalpy change of solution

Answer 11

The enthalpy change when one mole of ionic substance dissolves in water

Question 12

Enthalpy change of hydration

Answer 12

The enthalpy change when one mole of gaseous ions dissolve in water

Question 13

Is enthalpy of hydration endothermic or exothermic?

Answer 13

Exothermic

Question 14

Factors affecting lattice enthalpy?

Answer 14

• Size of ions (as radius increases, the attractive force decreases so lattice enthalpy values increase as the bonds are weaker)
• Charge difference (greater the difference in charges , the greater the attraction so lattice enthalpy decreases)

Question 15

Factors affecting hydration enthalpy?

Answer 15

-Charge density (as charge density increases, the hydration enthalpy decreases as the ions attract water molecules more strongly)

Question 16

Is hydration enthalpy endothermic/exothermic?

Answer 16

Exothermic

Question 17

What are spontaneous reactions?

Answer 17

Reactions that occur without external influence

Question 18

What is entropy?

Answer 18

A measure of disorder

Question 19

What is the overall trend of entropy?

Answer 19

Entropy always increases

Question 20

Entropy of different states

Answer 20

Solid (ordered structure) < Liquid < Gas

Question 21

What happens to entropy when a solute dissolves?

Answer 21

Entropy increases as the regular structure of the solid is broken down

Question 22

What happens to entropy when the number of moles increase?

Answer 22

Entropy increases

Question 23

Equation for change in enthalpy

Answer 23

ΔS(total) = ΔS(system) + ΔS(surrounding)

Question 24

Units for entropy

Answer 24

J/(K+mol)

Question 25

Equation for change in enthalpy surroundings

Answer 25

ΔS(surroundings) = -ΔH(reaction) / T

Question 26

Equation for Gibbs Free Energy

Answer 26

ΔG = ΔH - TΔS(system)

Question 27

Requirement for a reaction to take place

Answer 27

ΔG needs to be negative

Question 28

Equation for Gibbs Free Energy (in terms of equilibria)

Answer 28

ΔG = -RTln(K)

Question 29

Reason why a reaction might not occur even if ΔG is negative

Answer 29

A reaction has a high activation energy (limited by kinetic factors)