Topic 13: Energetics II 1️⃣

Question 1

Define standard lattice enthalpy change

Answer 1

Energy change when 1 mole of the ionic solid is formed from its gaseous ions
@standard conditions

Question 2

Which 2 factors affect the magnitude of lattice energy ?

Answer 2

• magnitudes of the charges on the ions
• the sum of the ionic radii

Question 3

As the ionic radius of the ions increases, what is the effect on the lattice energy ?

Answer 3

Lattice energy becomes less exothermic

Question 4

As the ionic charge of the ions increases, what is the effect on the lattice energy ?

Answer 4

Lattice energy gets more exothermic

Question 5

Define the standard enthalpy change of atomisation

Answer 5

Energy change measured at a standard conditions when 1 mole of gaseous atoms is formed from an element in its standard state

Question 6

Define 1st electron affinity

Answer 6

Energy change when each atom in 1 mole of gaseous atoms gains an electron to form one mole of gaseous 1- ions

Question 7

What are the directions of the arrows representing the following in Born Harber Cycles?
- enthalpy change of formation
- enthalpy change of atomisation
- enthalpy change of ionisation
- electron affinity
- lattice enthalpy

Answer 7

Enthalpy change of formation = downwards
Enthalpy change of atomisation = upwards
Enthalpy change of ionisation = upwards
Electron affinity = downwards/ upwards
Lattice enthalpy = downwards

Question 8

What does the experimental lattice energy refer to

Answer 8

The value of the lattice energy calculated using the born haber cycle

Question 9

What 3 assumptions must be made before calculating theoretical lattice energy

Answer 9

• the ions are all in contact with one another
• the ions are perfectly spherical
• ion charge is evenly distributed

Question 10

The greater the percentage difference between the experimental and theoretical values…

Answer 10

The father away from the ‘ionic’ side of the scale the lattice is.
ie. the lattice has more covalent character

Question 11

Describe & explain what we mean by covalent character of a lattice

Answer 11

A region where electrons are existing in an area of orbital overlap.
why?
In an ionic lattice the cation will attract the electrons of the anion.
If the electrons are pulled towards the cation, the anion is said to be polarised because the even distribution of its electron density has been distorted

Question 12

What are the 2 factors that change the extent to which an anion is polarised ? (Fajan’s rules)

Answer 12

• high charge and small size of the cation
• high charge and large size of anion

Question 13

If the experimental and theoretical lattice energy values are similar what do we say about the lattice?

Answer 13

The lattice is nearly 100% ionic

Question 14

Define the enthalpy change of solution

Answer 14

Energy change when 1 mole of ionic solid completely dissolves in water (to form an infinitely dilute solution)
Eg. Eg. NaCl(s) —-> Na+ (aq) + Cl- (aq)

Question 15

Define the enthalpy change of hydration

Answer 15

Energy change when 1 mole of gaseous ions are completely hydrated by water (to form an infinitely dilute solution)
Eg. Na+ (g) —-> Na+ (aq) & Cl- (g) —-> Cl- (aq)

Question 16

What factors affect the magnitude of hydration enthalpy ?

Answer 16

Charge of ions: greater charge = stronger interaction with water molecules = more negative hydration value
Size of ions: larger ion = weaker attraction with water molecule = less negative hydration value