Lattice enthalpy

Question 1

Trend in lattice enthalpy across a period

Answer 1

• Ionic charge increases
• Ionic radius decreases
• Electrostatic attraction between ions increases so more exothermic lattice enthalpy value

Question 2

Trend in lattice enthalpy down a group

Answer 2

• Ionic stays the same
• Ionic radius increases
• Electrostatic attraction between ions decreases so less exothermic lattice enthalpy value

Question 3

Which has the most exothermic lattice enthalpy; CaCl2 or KCl?

Answer 3

• Ca2+ has a higher ionic charge than K+
• Ca2+ has a smaller ionic radius than K+
• Stronger electrostatic attraction between oppositely charged ions in CaCl2 so more exothermic lattice enthalpy.

Question 4

Which has the most exothermic hydration value; Al3+ or Mg2+?

Answer 4

• Al3+ has a higher ionic charge than Mg2+
• Al3+ has a smaller ionic radius than Mg2+
• Stronger electrostatic attraction between Al3+ and polar water molecule so more exothermic hydration value.

Question 5

Why is the first electron affinity exothermic?

Answer 5

• There is an attraction between positive nucleus and negative electron which releases energy

Question 6

Why is the second electron affinity endothermic?

Answer 6

• There is a repulsion between the negative ion and the negative electron.
• Energy required to overcome the repulsion and add electron to ion.

Question 7

Why is the second ionisation energy more endothermic than the first ionisation energy?

Answer 7

• After first electron is removed there is a higher proton to electron ratio
• Ionic radius decreases
• Nuclear attraction on outer electron increases
• and so more energy is required to remove outer electron.