kp questions

Question 1

Why are lattice enthalpies from born haber cycles often higher than the value calcukated from perfect ionic model ?

Answer 1

many ionic compounds show some covalent character, they are not purely ionic

Question 2

For Mgcl2 why is the experimental lattice enthalpy of formation more exothermic than theoretical value ( calculated from perfect ionic model)

Answer 2

• MgCl2 has some covalent character
• this is because Mg ions are small and have a highly positive charge so are highly polarising
• Cl ions are large and negatively charged, so easily polarised
• this charge polarisation allows a degree of electron sharing, representing some covalent bonding between the Mg and Cl

Question 3

How does ionic radius affect enthalpy ?

Answer 3

• ionic radius, if it is larger, then the charge is spread out more ( less concentrated)
• also more distance between the positively charged centres of the ions
• this results in weaker electrostatic forces of attraction between the ions
• this makes the enthalpy LESS exothermic

Question 4

How does ionic charge affect enthalpy ?

Answer 4

• increasing ionic charge of an ion means that the charge density will be greater
• so stronger forces of electrostatic attraction between oppositely charged ions
• increases the exothermic energy needed to form the lattice from its gaseous ions