Paper 3

Question 1

Explain why the F-F standard bond enthalpy is twice the value of the enthalpy of atomisation of fluorine (5 marks)

Answer 1

• the enthalpy change of atomisation is the enthalpy required to form 1 mole of gaseous atoms from an element in its standard state under standard conditions (1)
• Fluorine is already gaseous in its standard state (1)
• therefore the only enthalpy change required to achieve atomisation is the breaking of F-F bonds (1)
• bond enthalpy is the enthalpy required to break 1 mole of a bond into its gaseous atoms (1)
• but atomisation of fluorine only requires the breakage of half a mole of bonds (1 mark) 1/2 f2 (g) -> F (g)

Question 2

Describe how the extent of ionic bond polarisation influences lattice enthalpy values (4)

Answer 2

• in an ionic lattice, cations attract and distort electron clouds of adjacent anions (1)
• lattices with more polarising cations and more polarisable anions have ionic bonds with more covalent character (1 mark)
• lattices with large anions and small cations show more covalency (1)
• the more covalent character there is, the greater the lattice enthalpy is (1 mark)