[ajax0604]

I'm studying calorimetry at the moment and this type of question often comes up in practice questions - "Explain why the calculated value of ΔH for this reaction might not be the same as the published value of ΔH".
I know the main reason is heat loss to the surrounding due to inadequate insulation but I was wondering if it would also be correct to say that the heat transferred to the calorimeter itself and its components (e.g. stirrer, thermometer etc) has been ignored.

I'm not sure what kind of calorimeters have been used to find the published ΔH values but would the heat transferred to the calorimeter itself have been taken into account to ensure the results were as accurate as possible?

[Enthalpy]

Hi Ajax0604,

What do you call "calculated" value: deduced from your measurements? Or (yuk!) estimated by software?

I'd trust (maybe I'm horribly naive) some experimenters like Dorofeeva to have made the corrections they found meaningful. Though, published data is brutally inconsistent, even for banal components.

One reason, in the case of the enthalpy of formation, is that it's deduced from the enthalpy of combustion, which is big and whose small relative errors have big consequences on the heat of formation. Hydrogenolysis would be better than combustion for that purpose, but it must be impractical.

One other reason is that the reactants' state of aggregation isn't always indicated.

Finally, existing literature shows so much dispersion that, if your measures happen to fit some published data, they will disagree with other. Have a look at Nist, they list the sources, and the discrepancy is discouraging.

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In case you have some choice about the compounds, I'd be interested in measures made by one person with one setup for:

• 2,4-dimethylpentane and its 2-aza and 2,4-diaza variants
  http://www.chemicalforums.com/index.php?topic=46407.msg302977#msg302977
• 1,3,5-trimethylcyclohexane and aza variants up to trimethyltriazinane
  http://www.chemicalforums.com/index.php?topic=46407.msg273296#msg273296
• Aminoguanidine and diaminoguanidine, directly as rocket fuels. Their enthalpy of recomposition should characterize the heat of formation better than the enthalpy of combustion. Found no data, probably paywalled.

[Babcock_Hall]

Is it possible that by "calculated," what is meant is enthalpies estimated using bond strengths?

[ajax0604]

"Calculated" means using the numbers that the question supplies. For example, one of the questions I'm looking at says "In one experiment, 1.50g of sucrose reacted with excess oxygen in a bomb calorimeter. The temperature rose by 14.0 deg Celsius etc". Using the data, you have to calculate the calibration factor of the calorimeter then the ΔH of sucrose. The third part of the question asks why this calculated ΔH might not be the same as the published value in a chemistry database for example. So, when the published ΔH was established, would the heat transferred to the calorimeter and its components have been taken account?