Answers for Group II

Qn 1. i only

Qn 2. i. A low melting point suggests that beryllium nitrate is simple molecular with covalent

bonds. Conversely, a high melting point suggests that calcium nitrate is giant ionic with

ionic bonds.

ii. 2Mg(NO3)2 (s) → 2MgO (s) + 4NO2 (g) + O2 (g)

Ca is below Mg in the Group II Hence it will have a bigger ionic radius. As a result, the

Ca2+

has a lower charge density, and hence a lower polarizing ability towards nitrate ion.

Thus calcium nitrate is thermally more stable than magnesium nitrate.

Qn 3. i.

ii. Mg3N2 + 6 H2O → 3 Mg(OH)2 + 2 NH3

Amount of Mg3N2 = 2.0 / (24.3 x 3 + 14 x 2) = = 0.198 mol

Mass of solid formed = 0.0261 x 3 x (24.3 + 17 x 2) = 3.47g

Qn 4.

i. lattice Energy ∝ q+.

q-/ (r+

+ r-)

where q+ and q

- = charge on ions ; (r+

+ r-) = inter-ionic distance

ii. Down the group, the lattice energy will be numerically smaller (i.e. less exothermic)

iii. The magnitude of the lattice energy of Mg3N2 will be greater than MgO because the

anion involved in the ionic bonding of Mg3N2 has a higher charge (i.e. N3-

) compared to

that of MgO (i.e. O2-

). The anionic sizes of N3-

and O2-

, however, are similar.

2 N

3–

3 Mg 2+