som week 11

Question 1

define lattice energy

Answer 1

the difference in coulombic potential energy between having ions in a solid state to a gas state

Question 2

what is enthalpy of the lattice energy reaction?
give the equation that represents this
give the equation for working out this enthalpy

Answer 2

A𝑥B𝑦(s) → 𝑥A⁺(g) + 𝑦B⁻(g)
energy required to go from solid to gas
V = Q₁Q₂/4πℇ₀r → interaction potential between just two ions

lattice enthalpy = Σ -Q₁Q₂/4πℇ₀r summation of all the ions interactions across the lattice

Question 3

looking at unit cell, describe how you would calculate lattice enthalpy of a structure

Answer 3

• first, take the central atom and look at the oppositely charged ions directly bonded to it, these are nearest neighbour atoms, count how many there are and do the V= calculation to get this interaction
• from the central atom, look at the similarly charged atoms around it, count how many there are, work out the distance in terms of r and use the V= equation to get this interaction
• then from the central atom look at the second nearest neighbour oppositely charged atoms, count how many there are, wrk out the distance in terms of r and use the V = equation to get this interaction

to work out total lattice enthalpy, add all these interactions

Question 4

what does a negative lattice energy show

Answer 4

negative = favourable interactions

Question 5

what does a positive lattice energy show

Answer 5

positive = unfavourable interactions

interaction between like charged ions is greater