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Question

Which of the following reaction involves the liberation of energy?

A
Na(s)Na+(g)
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B
Cl2(g)2Cl(g)
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C
Na+(g)+Cl(g)NaCl(s)
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D
NaCl(s)Na+(g)+Cl(g)
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Solution

The correct option is C Na+(g)+Cl(g)NaCl(s)
(A) NaCONaCO+
This is equivalent to
Na(s)−−ΔH1Na(g)−−ΔH2Na+(8)
ΔH1= Heat of cultivation
ΔH2= Ionisation energy
Thus in this case net energy is absorbed

(B) Cl2(q)2Cl(g)
This reaction involves dissociation of Cl2 molecules in chlorine atoms
Thus Net energy is required (for bond breaking)

(C) Na+(g)+Cl(g)NaCl(s)
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(s)Na+(g)+Cl(g)
Thus involves breaking the lattice crystal which simply requires energy and also lattice energy of acisociation is positive
Thus Net energy is absorbed.

Correct Ans (C)

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