Vapour pressures of Propanal and propanoic acid

[Lua-Anne]

Hi, I am looking for more information on vapour pressures of propanal and propanoic acid at room tempreature and need to explain in in terms of molecular structure. Couldn't find any thing on it though.

 

[pzona]

I'm not an expert in organic chemistry, but I like to think I have a pretty solid understanding of basic molecular theory. First of all, which stereoisomers are you looking at? Their molecular structure will affect the vapor pressure, so this is important. Look at the polarity of propanol and propanoic acid. Are they polar? Nonpolar? This tells you quite a bit about the intermolecular forces, and from this you can predict the energy needed to break the hydrogen bonds and whatnot. I'm not quite sure how to figure out the vapor pressure (Clausius-Clapeyron equation maybe? I think this requires a little more information), but hopefully this should give you a good start.

 

[quicknote]

The vapor pressure of a substance is the pressure exerted by its vapor when it is in equilibrium with its liquid or solid phase at a given temperature. In the case of propanal and propanoic acid, the vapor pressures at room temperature (25°C) are 20.4 mmHg and 5.9 mmHg, respectively.

The difference in vapor pressures between propanal and propanoic acid can be explained by their molecular structures. Propanal, also known as propionaldehyde, has a linear structure with three carbon atoms and an aldehyde functional group (-CHO). This structure allows for strong intermolecular interactions, resulting in a higher vapor pressure compared to propanoic acid.

On the other hand, propanoic acid, also known as propionic acid, has a carboxylic acid functional group (-COOH) which can form hydrogen bonds with other molecules. This leads to stronger intermolecular forces and a lower vapor pressure compared to propanal.

In addition, propanoic acid has a larger molecular size compared to propanal, which also contributes to its lower vapor pressure. The larger size of the molecule results in a lower vapor pressure due to decreased mobility and weaker intermolecular interactions.

In summary, the difference in vapor pressures between propanal and propanoic acid can be attributed to their molecular structures and sizes. Propanal has a linear structure and smaller size, leading to stronger intermolecular interactions and a higher vapor pressure. Propanoic acid, on the other hand, has a larger size and can form hydrogen bonds, resulting in weaker intermolecular interactions and a lower vapor pressure.