Originally posted by Zarathustra
What is the correct formula for delta enthalpy?
Depends on what you want to define it in terms of - for example, defining deltaH as H(products) - H(reactants) would be non-controversial, but also essentially useless. There are formal definitions from Physical Chemsitry, but many of them are not terribly useful either. For the purposes of calculating deltaH from HSC-type experimental data, I would use:
deltaH = - deltaq<sub>surr</sub> * coeff(X) / n(X), where X is the limiting reagant
but you won't find this in any HSC textbook - I favour it because it's true and it works, but it also ignores the data sheet, so some may consider it risky...
Why would the chemistry syllabus have dodgy info in it?
(With apologies to Yes, Prime Minister): It is not for a humble mortal such as myself to speculate on the complex and elevated deliberations of the mighty.
In other words, I have no idea. (Sorry.
) If I had to guess, I'd suggest it is a lame attempt to 'simplify' a concept, suggested by some stakeholder group that doesn't understand the concept properly.
Also, bear in mind that the BoS seems to have no problem with ignoring convention in making its declarations - After all, it has chosen to redefine the internationally accepted meaning of 'standard state', and it did so based on a misunderstanding of scientific convention.
In my half yearly I remembered that molar heat of combustion was the absolute value of delta H (or so I thought) but I think that the answer the school wanted was negative.
There was a question to find out the molar heat of combustion of ethanol I wrote 1330kJ/mol I think they wanted negative because the next question was - the person doing experiment got -570kj/mol why did they get such a low value . I wrote it was about 2000kj/mol out - I also wrote about the invalidity of the experiment - heat escapes etc.
As I pointed out above, the question of whether heat of combustion is deltaH<sub>c</sub> or -deltaH<sub>c</sub> depends as much on the teacher's opinion as anything else. As such, you need to look for clues in questions as to which you should use. (I know this is silly, but it is the way it is).
IMO, the follow on question is a clear clue that the examiner takes heat of combustion and enthalpy of combustion as the same, so I would definitely have given a negative answer.
The difference is then due to factors like heat loss to the environment / equipment. Whether I would accept your giving this answer in the situation that you describe would depend on exactly what you wrote. By highlighting a 2000 kJ/mol difference, you have emphasised the difference in sign. This raises the question of whether your answer implies that the student measured an endothermic reaction as exothermic. In this sort of situation, you need to make it clear that you know combustion is exothermic, or you leave yourself wide open to being penalised.
btw is the correct formula to do with the bonds broken and formed during combustion?
See above, but in short, no. Bonds breaking (endothermic) and bond forming (exothermic) are certainly the origin of the energy change associated with a chemical reaction, but it isn't a good place to use as the basis for a definition, as it raises lots of questions - like how much energy does it take to break a bond, do all bonds of the same type have the same strength, and what do we do about states - bond breaking / forming is much easier to discuss in gas phase.
