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.
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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. :)
It's -618kJ/mol
Originally posted by UltimaOnline:Your thought process and reasoning is correct.
From most exothermic to least exothermic :
[most exothermic] hydrazine (gas) --> water (liquid)
[moderately exothermic] - hydrazine (gas) --> water (gas)
[moderately exothermic] - hydrazine (liquid) --> water (liquid)
[least exothermic] - hydrazine (liquid) --> water (gas)
Don't let the question confuse you. Notice the question setter didn't ask "why the actual enthalpy more exothermic?" because it may not actually be so (and probably isn't so). The question setter just used some values plucked from the internet, and an internet search will reveal conflicting values from different sources involving different states of reactants and products.
Hence the question setter simply asked "suggest reasons for the discrepancy" instead of specifying "why more exothermic" for which the question setter him/herself has no answer to (because it shouldn't be more exothermic).
Hi UltimaOnline,
Although the question did ask for the reason for discrepancy, they asked for the discrepancy in the context of previous sentence - “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…” Since you agreed that the actual enthalpy change should not be more exothermic, I am taken aback by this mistake in the question because it was obtained from the promos of a top-tier school! :S