Question

Question: Rationalize the following lattice energy values.

Compound Lattice energy (kj/mol)

CaSe -2862

${\mathbf{Na}}_{\mathbf{2}}\mathbf{Se}$ -2130

CaTe -2721

${\mathbf{Na}}_{\mathbf{2}}\mathbf{Te}$ -2095

Short answer

The lattice energy is the energy change upon the formation of one mole of a crystalline ionic compound from its constituents ions which are assumed to be in the gaseous state.

$\mathrm{L}.\mathrm{E}=\frac{\mathrm{K}.{\mathrm{Q}}_1.{\mathrm{Q}}_2}{\mathrm{r}}$

where K = Constant

${\mathrm{Q}}_1,{\mathrm{Q}}_2$ = Charge on cation and anion

r = distance between ions

Step-by-step solution

Arrange the following compound in order of their size.

${\mathrm{Se}}^{2-}<{\mathrm{Te}}^{2-}$

${\mathrm{Se}}^{2-}$ion is smaller than ${\mathrm{Te}}^{2-}$ion.

${\mathrm{Na}}^{+}>{\mathrm{Ca}}^{2+}$

${\mathrm{Na}}^{+}$has a greater size than${\mathrm{Ca}}^{2+}$

Explain how a double bond character is possible.

Charges: ${\mathrm{Na}}^{+}$has +1 charge while${\mathrm{Ca}}^{2+}$ has +2 charge and we know that the effect of charge on the lattice energy is greater than the effect of size.

Therefore, we may expect the trend by using the lattice energy formula from the most exothermic lattice to the lowest exothermic.

Order: - CaSe (2852 kj/mol) > CaTe (-2721 kj/mol) > ${\mathrm{Na}}_2\mathrm{Se}$(2130 kj/mol) >${\mathrm{Na}}_2\mathrm{Te}$ (-2095 kj/mol)

Hence all the lattice energy for compounds are correctly rationalized.