Question:
Bond Dissociation Energy
Suppose there is an element X which occurs naturally as X2(g).
X2(g) + 2O2(g) → X2O4(g)
X2O4 has a structure
ΔHof of O(g) is 249 kJ/mol
ΔHof of X(g) is 478.5 kJ/mol
ΔHof of X2O4(g) is 9 kJ/mol
The X-X single bond energy is 120. kJ/mol
Use the above data to estimate the average bond energy in X2O4.
| Give your answer to the nearest 1 kJ/mol. |
Answer:
Given data :
Reaction
X2(g) + 2O2(g) → X2O4(g)
Heat of dissociation.
ΔHof of O(g) is 249 kJ/mol
There 4 mole of oxygen atom as in the form of 2 mole of O 2 gas. Hence heat generated for formation of 4 mole of O atom
= 4 mole × 249 KJ / mole = 996 KJ
ΔHof of X(g) is 478.5 kJ/mol
There 2 mole of X atom as in the form of 1 mole of X 2 gas. Hence heat generated for formation of 2 mole of X atom
= 2 mole × 478.5 KJ / mole = 957 KJ
Hof of X2O4(g) is 9 kJ/mol
The X-X single bond energy is 120. kJ/mol
In the given product molecule there 1 X - X bond and
4 X = O bond
Hof of X= O = ??
From the reaction we can say
Hof of X2O4 = { ( 2 × ΔHof of X(g) ) + ( 4 × ΔHof of O(g)) } - { ( ΔHof of X - X) + ( 4 × ΔHof of X= O ) }
Hof of X2O4 + { ( ΔHof of X - X) + ( 4 × ΔHof of X= O ) } = { ( 2 × ΔHof of X(g) ) + ( 4 × ΔHof of O(g)) }
Putting the values in equation.
9 KJ / mol + 120 KJ / mol + ( 4 × ΔHof of X= O ) = 957 KJ + 996 KJ
129 KJ / mol + ( 4 × ΔHof of X= O ) = 1953 KJ
(4 × ΔHof of X= O )= 1953 KJ - 129 KJ / mole
( ΔHof of X= O ) =( 1824 KJ / mole) / 4 = 456 KJ / mole
Average bond energy for the formation of X= O in X2O4
= 456 KJ / mole
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