Question

PartI: Rank the following compounds in order of decreasing boiling point:

potassium fluoride (KF),

carbon tetrafluoride (CF4),

and formaldehyde (CH2O)

PartII: The melting point of a solid is related to the way in which that substance is held together. Arrange the solids in order of decreasing melting point.

Rank from highest to lowest melting point.

a) graphite

b) solid ammonia

c) sodium chloride

Answer 1

compounds in order of decreasing boiling point:

potassium fluoride (KF) > formaldehyde (CH2O) > carbon tetrafluoride (CF4)

Boiling point depents on the intermolecular forces. KF is an ionic compound and it involve strong electrostatic bonds and hence it has highest boiling point, since higher energy is required to break these bonds. Formaldehyde has weak london dispersion forces and dipole dipole (hydrogen bonding ) interaction between H and O-atoms.

O = C - H --------- O = C -H    | | H H

Though each C-F bond is polar but the net dipole moment is zero and hence CF₄ is non polar and involves weak london forces among its molecules and hence less energy is required to break these bond. Therefore it has low boiling point.

PartII:  Rank from highest to lowest melting point: Graphite > NaCl > Solid NH₃

Graphite is a network covalent solid. It has carbon atoms joined together by covalent bonds to form a giant lattice.Since very large number of covalent bonds are there , hence it has higher melting point than the ionic solids (ex. NaCl).

NaCl is an ionic solid and it involves strong electrostatic force and hence it has high melting point han solid NH₃.

Solid NH₃ is a molecular solid.Nitrogen and hydrogen atoms are joined by strong covalent bonds but their molecules are held together by weak london forces(i.e. intermolecular forces are weak), so it has low melting.