Question

Which of the following reaction involves the liberation of energy?

• A. $N{a}_{\left(s\right)}\to N{a}_{\left(g\right)}^{+}$
• B. $C{l}_{2\left(g\right)}\to 2C{l}_{\left(g\right)}$
• C. $N{a}_{\left(g\right)}^{+}+C{l}_{\left(g\right)}^{-}\to NaC{l}_{\left(s\right)}$
• D. $NaC{l}_{\left(s\right)}\to N{a}_{\left(g\right)}^{+}+C{l}_{\left(g\right)}^{-}$

Answer 1

The correct option is C $N{a}_{\left(g\right)}^{+}+C{l}_{\left(g\right)}^{-}\to NaC{l}_{\left(s\right)}$

(A) $NaCO\to NaC{O}^{+}$

This is equivalent to

$Na\left(s\right)\underset{{\Delta H}_1}{\overset{}{\to }}Na\left(g\right)\underset{{\Delta H}_2}{\overset{}{\to }}N{a}^{+}\left(8\right)$

$\Delta H_1=$ Heat of cultivation

$\Delta H_2=$ Ionisation energy

Thus in this case net energy is absorbed

(B) $C{l}_2\left(q\right)\to 2Cl\left(g\right)$

This reaction involves dissociation of $C{l}_2$ molecules in chlorine atoms

Thus Net energy is required (for bond breaking)

(C) $N{a}^{+}\left(g\right)+Cl\left(g\right)\to NaCl\left(s\right)$

This represent the formation of an ionic lattice from the gaseous isolated sodium and chloride ions

Since lattice energy of formation is negative

Net energy is liberated

(D) $NaCl\left(s\right)\to N{a}^{+}\left(g\right)+C{l}^{\oplus }\left(g\right)$

Thus involves breaking the lattice crystal which simply requires energy and also lattice energy of acisociation is positive

Thus Net energy is absorbed.

$\therefore$ Correct Ans $\left(C\right)$