Key explanations!

Question 1

Compare the lattice enthalpies of NaCl and KCl

Answer 1

Na+ and K+ have the same charge
Na+ is a smaller ion than K+
Na+ has a higher charge density than K+
Greater attraction between Na+ and Cl-
NaCl has a more exothermic lattice enthalpy

Question 2

Compare the hydration enthalpy of Cl- and Br-

Answer 2

Both Cl- and Br- has the same charge
Cl- is a smaller ion than Br-
Cl- has a greater charge density
Cl- attracts water molecules more strongly-stronger ion-dipole interactions
Cl- has a more exothermic hydration enthalpy

Question 2

Explain why different substances conduct electricity

Answer 2

Ionic- ions can move
Metallic- delocalised electrons can move
covalent- no mobile charge carriers (except graphite-delocalised electrons can move)

Question 2

Explain the trend of boiling points in straight chain alkanes

Answer 2

boiling point increases with chain length
More electrons so stronger Id-Id bonds form
Require more energy to break

Question 3

Explain the increase in successive ionisation energies

Answer 3

Same number of protons attract fewer electrons
Radius decreases
Nuclear attraction increases

Question 4

Why is sodium pentanoate more soluble than pentanoic acid

Answer 4

Forms ion-dipole bonds between ions and polar water molecules
Pentanoic acid only forms hydrogen bonds with water molecules
Ion dipole bonds are stronger than hydrogen bonds so sodium pentanoate is more soluble