First slide

Enthalpy of Atomisation and bond dissociation

Question

$N - N$ single bond enthalpy is “X”. $N = N$ double bond enthalpy is “Y”. Correct relation is

Difficult

A

X > 2Y
B

2X > Y
C

2X < Y
D

2X = Y

Solution

Generally
A) a single bond is a sigma bond
B) a double bond has one sigma and one pi bond
C) a pi bond is weaker than a sigma bond
Applying to the given question
$Expected$ --
N–N single bond enthalpy = X
$∴ “ 2 X ” denotes two sigma bond enthalpy values$
‘Y’ denotes double bond enthalpy (one sigma bond and one pi bond)
$Since pi bond is weaker than sigma bond$
2X > Y
$But in the case of nitrogen$
$\overset{. .}{N} - \overset{. .}{N}$ single bond is too weak due to the repulsions between lone pairs of adjacent
Strength of two sigma bonds, 2 N–N < strength of double bond (N = N )
$∴ 2 X < Y$

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