Trends and predicting lattic enthalpies

Question 1

What is the trend in ion size to lattic enthalpies?

Answer 1

• larger the ion smaller the lattice enthalpies
• because the opposite charges do not approach each other as closely when the ions are larger

Question 2

How does charge affect the lattice enthalpy?

Answer 2

• ions of approximitely of a similar size in the same group
  
  • the lattice enthalpy increases with the size of the charge
  • give out more energy when joined together

Question 3

How can a theoretical value for the lattice fromation enthalpy be calculated?

Answer 3

• if you know the charge, distance apart, and geometry of its structure

Question 4

Why is the experimental value for the lattice formation of enthalpy of ZnSe (Zn²⁺ + Se^2-) more negative than the theoretical value? 10% differnece

Answer 4

• suggests other bonding present
• Zn is relatively small and has high positive charge
• Se is large and has a high negative charge
• Zn can approach closely and distort the electron clouds of Se and distort them by attracting them towards it

Question 5

Why is Se easy to distort:?

Answer 5

• large size means the electrons are far from the ncleus and its double charge means there is plenty of negative charge to distort

Question 6

How is Se polarised?

Answer 6

• distortion means there are more electrons than expected concentrated between the Zn and Se nuclei
• and represents a degree of electron sharing or covalency
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Question 7

What factors increase polarisation?

Answer 7

• cation
  
  • small size high charge
• negative ion
  
  • large size
  • high charge