H
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?
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.
Diagram:
_Clamp stand
_
_ _I__Thermometer
_ I I Beaker
_ I-----I
_ I___I Water
_ I Wick
_ -----
_ I I
_ ----- Alcohol
_ I __I
______________
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?
The structural diagram you provided represents a molecule with a central carbon atom bonded to three hydrogen atoms (H-C-H) and also bonded to an oxygen atom (C-O). It seems that you have a concern about the C-O bond and its inclusion or exclusion in certain calculations. Let me explain further.
In chemical reactions, understanding the energies associated with the breaking and forming of bonds is important. Both the bonds broken and the bonds formed contribute to the overall enthalpy change (ΔH) of a reaction. Bond enthalpy (also known as bond energy) is a measure of the energy required to break a specific bond in a gaseous molecule, usually averaged over a large number of different compounds.
Now, in the case of the molecule you presented, the C-O bond exists and can be broken or formed during a chemical reaction. However, in some cases, the C-O bond is not explicitly mentioned because it's part of an average bond enthalpy value. When using average bond enthalpies, the C-O bond is implicitly accounted for in the calculation.
Regarding the use of O-H average bond enthalpy in the products side of the equation, it's because water (H2O) is one of the products. The O-H bond in water is explicitly mentioned because it is not part of an average bond enthalpy value. The average bond enthalpy for the O-H bond is used as a good approximation for the actual O-H bond energy in water.
Now, moving on to the question about the diagram and its relation to part b, I'm assuming "part b" refers to a specific question or problem linked to the diagram. Unfortunately, without additional context or details, I cannot provide a specific answer. Could you please provide more information about part b, so I can assist you further?
Lastly, regarding the statement you mentioned about "good data" and "tabulated values," it seems to suggest a preference for using experimental data obtained from a practical (experimental) procedure. The practical data, which involves directly measuring the energy changes during a reaction, is considered more accurate and reliable compared to using average bond enthalpy values. The practical data provides actual values specific to the reaction under investigation, whereas average bond enthalpies are more general approximations.
To summarize, the "good data" and "tabulated values" usually refer to experimental data obtained from a practical procedure, which is considered more accurate than relying on average bond enthalpy values for specific reactions.