[docteurdeno]

From the NIST database, the enthalpy of formation of alcohols, aldehydes and carboxylic acids with increasing carbon chain lengths are as follows: https://imgur.com/a/kRXrYYJ

Why is this the case? It probably has something do to with intermolecular forces, but I don't really understand why.

Another thing I don't understand is the overall trend of the graph, 1-propanol (hydroxyl/3) and 1-pentanal (carbonyl/5) deviate from the overall trend of the graph.

Also, what can be deduced from the enthalpy of formation? Does it affect volatility, flammability or something else? I would really appreciate it if you could help me.

[Corribus]

First, always put units on your plots. A plot axis should never be without a label and a unit.

Second, consider how the heats of formation are calculated/determined. Are these values that are actually measured experimentally, or are they derived from something else that is determined experimentally? If the latter, it may be helpful to look at those values instead.

Third, are you comparing apples to apples in terms of the substitution point (is the functional group on carbon position 1?) In the case of carboxy-terminated, is the C of the carboxy group part of the chain or in addition to the chain?

Finally, I don't think your numbers are right. Just as a check, I looked up the gas phase values of methanol, ethanol, 1-propanol, 1-butanol, and 1-pentanol at the webbook and found, respectively: -205, -234, -256, -277, -298 kJ/mol, which seems like a consistently decreasing trend. Are you sure you recorded the right values? As a side note, 2-propanol (isopropanol) has a heat of formation of -273 kJ/mol, compared to -256 kJ/mol for 1-propanol.

[docteurdeno]

Thank you for replying. I forgot to mention some important details: all functional groups are at position 1 (i am working with alcohols, aldehydes and carboxylic acids), and the compounds are all linear. I have figured out why the trend occurs, but you are probably right, my data is off. Why have you taken the gas phase data? I thought I was supposed to take the liquid phase data since these compounds are liquids at standard conditions. Also, the enthalpy of combustion data for the gas phase is missing for a lot of compounds on nist.

[Corribus]

You can determine formation enthalpies for substances that exist in various phases and states. Actually this is kind of the point. This is why you can find formation enthalpies for gas and liquid phases of many substances. What must be in standard state are the elements doing the formation, as this provides a common reference point for all formation enthalpies. 

I had no specific reason for choosing gas phase other than habit - you can do liquid phase if that's your preference. Gas phase thermodynamic quantities are sometimes easier to explain (in terms of trends) because generally they aren't influenced by intermolecular forces.