Hello UltimaOnline, I have a
qn on energetics :) :
Liquid hydrazine (N2H4) reacts with oxygen to give nitrogen gas and
steam.
(i) By using appropriate bond energy data from the Data Booklet
calculate a value for the enthalpy change of reaction. [Ok, no
problem.]
The actual enthalpy change of the reaction in (i) is more
exothermic than the calculated value in (b). One reason for this
discrepancy in values is that the bond energy values from the Data
Booklet are average bond energies. Suggest another possible reason
for this discrepancy.
_______
Remarks: I can think of 2 reasons which would suggest a difference
in the actual enthalpy change of reaction being different from what
would have been calculated in the Data Booklet. (i) Presence of
hydrogen bonds in liquid hydrazine and energy is needed to overcome
the H bonds before covalent bonds are broken/formed; (ii) Hydrazine
is in a liquid state and thus energy is needed to vapourise
hydrazine first before the covalent bonds are
broken/formed.
However, I have some disquiet about this: Wouldn’t these
possibilities result to the actual enthalpy change of the reaction
being less exothermic? Since in reality energy would be required to
vapourise liquid hydrazine/overcome H bonds and such energies are
not taken into account if one only use use the values in the Data
Booklet and thus the initial energy level of hydrazine would have
been lower, leading to a less exothermic enthalpy change of
reaction.
Thank you, UltimaOnline. :)