Question

Which compound in each of the following pairs of ionic substances has the most exothermic lattice energy? Justify your answers.

$$\begin{gathered} \mathbf{a}\mathbf{.}\mathbf{}\mathbf{NaCl}\mathbf{,}\mathbf{}\mathbf{Kcl} \\ \mathbf{b}\mathbf{.}\mathbf{}\mathbf{LiF}\mathbf{,}\mathbf{}\mathbf{LiCl} \\ \mathbf{c}\mathbf{.}\mathbf{}\mathbf{Mg}{\mathbf{\left(}\mathbf{OH}\mathbf{\right)}}_{\mathbf{2}}\mathbf{,}\mathbf{}\mathbf{MgO} \\ \mathbf{d}\mathbf{.}\mathbf{}\mathbf{Fe}{\mathbf{\left(}\mathbf{OH}\mathbf{\right)}}_{\mathbf{2}}\mathbf{,}\mathbf{}\mathbf{Fe}{\mathbf{\left(}\mathbf{OH}\mathbf{\right)}}_{\mathbf{2}}\mathbf{}\mathbf{e}\mathbf{.}\mathbf{}\mathbf{NaCl}\mathbf{,}\mathbf{}{\mathbf{Na}}_{\mathbf{2}}\mathbf{O} \\ \mathbf{f}\mathbf{.}\mathbf{}\mathbf{MgO}\mathbf{,}\mathbf{}\mathbf{BaS} \end{gathered}$$

Short answer

a) NaCl

b) LiF

c) MgO

d)$\mathrm{Fe}{\left(\mathrm{OH}\right)}_3$

e)${\mathrm{Na}}_2\mathrm{O}$

f) MgO

Step-by-step solution

Describe Lattice Energy:

The energy needed to convert a mole of an ionic solid into gaseous ionic constituents is called lattice energy.

Ex: $786$ kJ energy is required to separate NaCl into gaseous ${\mathrm{Na}}^{+}\mathrm{and}{\mathrm{C}}^{-}$ ions.

Elaborate the factors efficiently lattice energy:

(i)Charge of ions– greater the charge means greater force of attraction, hence stronger lattice.

(ii) Distance between ions – The larger the distance between ions, the weaker will be the electronic forces between them hence lower lattice energy.

Use these concepts to compare the ionic substances:

a) NaCl and KCl

${\mathrm{Na}}^{+}\mathrm{and}{\mathrm{K}}^{+}$ have same charge but sign of ${\mathrm{Na}}^{+}$is smaller.

So, NaCl will have most exothermic lattice energy.

b) ${\mathrm{F}}^{-}\mathrm{and}{\mathrm{Cl}}^{-}$ have the same charge but since ${\mathrm{F}}^{-}$ is smaller.

So, LiF will have most exothermic lattice energy.

c) Mg has $+2$ charge but ${\mathrm{O}}^{2-}$ has greater charge than ${\mathrm{OH}}^{-}$. So, MgO will have most exothermic lattice energy.

d) ${\mathrm{OH}}^{-}$is common in both however ${\mathrm{Fe}}^{3+}$ has greater charge than ${\mathrm{Fe}}^{2+}$. So, will have$\mathrm{Fe}{\left(\mathrm{OH}\right)}_3$ most exothermic lattice energy.

e) ${\mathrm{Na}}^{+}$is common however ${\mathrm{O}}^{2-}$ has greater charge than ${\mathrm{Cl}}^{-}$. So ${\mathrm{Na}}_2\mathrm{O}$ will have most exothermic lattice energy.

f) ${\mathrm{Mg}}^{2+}$and ${\mathrm{O}}^{2-}$ are both smaller in size than ${\mathrm{Ba}}^{2+}$ and ${\mathrm{S}}^{2-}$. So MgO will have the most exothermic lattice energy.