H

Question

I
H- C - O - H
I
H
I thought there is a C-O bond? Is that not used?
If it is ignored because we used the full equation average bond enthalpy then why in the products side when the water equation average bond enthalpy is used do we also use the O-H avergae bond enthalpy information?

How does the diagram come into the question? Is there anything I can say about it for part b (why the value is different to the one obtained in the practical)?

"If I wanted a dHrxn I would want to do it with the good data. It amazes me that so many instructures use this Bond Energy thing when it is only good for a good guess. And frankly, at least in my opinion, I think using the tabulated values are easier to calculate AND you end up with values that are more likely to be correct."

Is the good data and the tabulated values that are easier to calculate the ones from the practical?

Answer 1

I'm so sorry but I made an error. There certainly is a C-O bond there which I completely ignored. I thought,erroneously, that breaking the OH bond was breaking the C-O-H bond. You're right and that 358, if I remember the number correctly, does belong on the reactants side.

As for your other question, I don't know what the "practical" is but I assume it is exam related.So I'm guessing the answer would be something like this. If the questioin lists bond energies you're expected to use the dHrxn = BEreactants - BEproducts. But if the problem gives dH^o_f (that's standard heat formation) then dHrxn = (n*dHo f products) - (n*dHo f reactants). BE are listed in kJ/mol and dHf are given in kJ/mol but they are called standard heats of formation. And for the record I may have been a little harsh on using BE data when I say it's a good guess. Some calculations give answers to within 1% of the formation data (the gold standaard) but not all of them turn out that way. Also, I don't know anything about the diagram. I didn't see one.

Answer 2

Diagram:

_Clamp stand
_
_ _I__Thermometer
_ I I Beaker
_ I-----I
_ I___I Water
_ I Wick
_ -----
_ I I
_ ----- Alcohol
_ I __I
______________

Answer 3

When you said:
"good data"

Is the good data the one for the experiment?

When you said:
"tabulated values are easier to calculate AND you end up with values that are more likely to be correct."

Are you talking about the "tabulated values" for the experiment or average bond enthalpy data?