F325: Module 2: Born-Haber cycles, Lattice and hydration enthalpy

Question 1

I) Define Lattice Enthalpy.

Answer 1

The enthalpy change when 1 mole of an ionic compound is formed from its gaseous ions under standard conditions.

Question 2

I) Is lattice enthalpy exothermic or endothermic.

Answer 2

ALWAYS exothermic

Question 3

I) What does a large exothermic value for lattice enthalpy indicate about the bonding and structure?

Answer 3

It means a large electrostatic attraction between the oppositely charged ions in the lattice.

The more energy released (more exothermic), the more stable the new compound is and therefore the stronger the ionic lattice.

Question 4

I) What happens to the lattice enthalpy if the ionic charge increases?

Answer 4

It becomes more exothermic.

Question 5

I) What happens to the lattice enthalpy if the ionic radius decreases?

Answer 5

It becomes more exothermic.

Question 6

I) In a born-haber cycle, what are the two possible routes?

Answer 6

The enthalpy change of formation

Atomisation of the metal and non-metal, followed by the ionisation of the metal and the first electron affinity of the non-metal and finally lattice enthalpy.

Question 7

I) In a born-haber cycle, how can you calculate the enthalpy change of formation?

Answer 7

Enthalpy change of formation = Atomisation Enthalpies + Ionisation enthalpies + Electron affinities + Lattice Enthalpy

Question 8

I) In a born-haber cycle, how can you calculate the lattice enthalpy?

Answer 8

Lattice Enthalpy = Enthalpy change of formation - Atomisation Enthalpies - Ionisation enthalpies - Electron affinities

Question 9

I) Sort the stages in a born-haber cycle into Exothermic and Endothermic.

Answer 9

Exothermic:
Lattice Enthalpy
Enthalpy Change of Formation
First Electron Affinity (non-metals)

Endothermic:
Enthalpy Change of Atomisation
Ionisation Energy (metals)
Second Electron Affinity (non-metals)

Question 10

I) What is the largest exothermic enthalpy change affecting the enthalpy change of formation?

Answer 10

Lattice Enthalpy

Question 11

I) What is the largest endothermic enthalpy change affecting the enthalpy change of formation?

Answer 11

Ionisation Energy

Question 12

I) How would you calculate the total ionisation energy for Mg given that 1st IE = 738kJ/mol and 2nd IE = 1451kJ/mol?

Answer 12

To go from Mg to Mg2+ you need to add the 1st and 2nd ionisation enthalpies.

Therefore the total would be 738 + 1451 = 2189 kJ/mol

Question 13

I) Which of the first and second electron affinities are exothermic or endothermic?

Answer 13

1st is Exothermic since energy is released.

2nd is Endothermic since it requires energy to add the electron to the negative ion.

Question 14

I) Define the Enthalpy Change of Solution.

Answer 14

The enthalpy change when 1 mole of a compound is completely dissolved in water under standard conditions.

Question 15

I) Define the Enthalpy Change of Hydration.

Answer 15

The enthalpy change when 1 mole of isolated gaseous ions is dissolved in water to form 1 mole of aqueous ions under standard conditions.

Question 16

I) Why are hydration enthalpies always exothermic?

Answer 16

The are always exothermic because they involve the electrostatic attraction between a charged ion and the dipoles on water molecules.

Question 17

I) What happens to the magnitude of the enthalpy of hydration and the charge density of the ion increases?

Answer 17

The magnitude also increases.

Question 18

I) Can lattice enthalpy be determined experimentally?

Answer 18

No It has to be done using a born-haber cycle.

Question 19

I) Can The enthalpy of hydration be determined experimentally?

Answer 19

No It has to be done using a born-haber cycle.

Question 20

I) How would you calculate the enthalpy of solution using lattice enthalpy and hydration enthalpy?

Answer 20

Hsol = Hhyd - Hle

Question 21

I) Define the enthalpy change of formation.

Answer 21

The enthalpy change when 1 mole of gaseous atoms is formed from its elements in their standard stated under standard conditions.

Question 22

I) Define the enthalpy change of atomisation.

Answer 22

The enthalpy change when 1 mole of gaseous atoms is formed from an element in its standard state.

Question 23

(2 marks)
Highest melting point
NaBr
RbCl
KI
Lowest melting point 
Explain this trend in boiling points

Answer 23

Attraction and ionic size linked:
-Greater attraction from smaller ions/closer ions/larger
charge density (COMPARISON NEEDED)
Energy and attraction/ breaking bonds linked:
-More energy/heat to overcome attraction (between ions)
OR
More energy/heat to break (ionic) bonds

Question 24

(3 marks)
Explain why it would be difficult to predict whether the lattice enthalpy of magnesium sulphide would be more or less exothermic than the lattice enthalpy of sodium oxide/.

Answer 24

Comparison of size AND charge of cations
Mg2+ is smaller AND Mg2+ has a greater charge
OR
Mg2+ has a greater charge density

Comparison of size of anions
S2– is larger
OR
S2– has a smaller charge density

Comparison of attraction of a cation and an anion
Mg2+ has stronger attraction OR Na+ has weaker attraction
AND
S2– has weaker attraction OR O2– has stronger attraction


Question 25

The student wanted to determine the lattice enthalpy of Na2CO3. Unfortunately it is very difficult to do using a standard born haber cycle. SUggest why it is very difficult.

Answer 25

-Cycle needs formation of CO3^2- ions i.e. NOT breaking up of CO32– ion