Question

(a) Provide a brief explanation for why strontium chloride as a higher melting point (874 ºC) than lithium chloride (605 ºC). Provide a brief explanation for why strontium chloride as a lower melting point than boron nitride (2,973 ºC). (3)

(b) H2O (MW = 18.0 g/mol) boils at 100 ºC, HF (MW = 20.0 g/mol) boils at 19.5 ºC, while H2Se (MW = 81.0 g/mol) boils at –41.3 ºC. Provide an explanation for these observations based on the type and magnitude of intermolecular forces. (3)

Answer 1

(a) An ionic compound is composed of positively charged cation(s) and negatively charged anion (s). These are held together in an ionic lattice by electrostatic forces of attraction.

Melting point of ionic compounds depends upon their lattice energy. Higher the charge on the constituent ions and smaller the size of these ions, higher is the lattice energy hence higher is the energy needed to separate the ions by overcoming the electrostatic forces of attraction and melt the compound.

Strontium chloride (SrCl2) has Sr2+ and Cl- ions whereas Lithium chloride (LiCl) has Li+ and Cl​​​​​- ions. So there is more charge on Sr​​​​​2+ ions than on Li+. This means the electrostatic forces of attraction are stronger in SrCl​​​2 than in LiCl which results in Strontium chloride having a higher lattice energy and hence a higher melting point than lithium chloride.

Boron nitride (BN) has B​​​​​​3+ and N​​​​​​3- ions whereas Strontium chloride (SrCl​​​2) has Sr​​​​​2+ and Cl​​​​- ions. Thus, the higher charge on B​​​​​​3+ and N​​​​​3- ions means these are held together by stronger electrostatic forces of attraction in boron nitride than Sr​​​​​2+ and Cl​​​​​- in Strontium chloride. So boron nitride has a higher lattice energy and hence a higher melting point than strontium chloride.

(b) Boiling point of a substance indicates the strength of intermolecular forces of attraction in the substance.

Molecules in which there is a hydrogen directly attached to a highly electronegative atom (F, O, Cl) are capable of forming intermolecular hydrogen bonds. Hydrogen bonds lead to very high boiling point because of the high energy required to break these.

In linear shaped H-F there can be two hydrogen bonds per molecule. In bent shaped H2O there can be four hydrogen bonds per molecule. This means the hydrogen bonding is more extensive in water than in HF resulting in H2O having a higher boiling point than HF.

Now Se is lower in periodic table than O (O is in period 2, Se is in period 4). So it has a much lower electronegativity than O and hence H2Se is not capable of forming hydrogen bonds. So H2Se has a lower boiling point than H2O.