Rank the following in order of decreasing vapor pressure, and briefly explain the observed trend in vapor pressure:
Acetic Acid
Isobutane
Acetamide
Propanal
I think the order of decreasing vapor pressure is Acetamide, Acetic Acid, Propanal, and Isobutane. The trend is the higher the boiling point, the lower the vapor pressure.
Is this right? If not, please explain.
To rank the substances in order of decreasing vapor pressure, we need to consider their molecular structures and intermolecular forces.
The trend in vapor pressure can be explained by the strength of intermolecular forces of each substance. The weaker the intermolecular forces, the higher the vapor pressure.
1. Isobutane:
Isobutane is an alkane with the molecular formula C4H10. It has only weak London dispersion forces between its molecules. Because it is a smaller molecule than the others on the list, it has a larger surface area-to-volume ratio, resulting in stronger dispersion forces. Isobutane has the highest vapor pressure among the compounds listed.
2. Propanal:
Propanal is an aldehyde with the molecular formula C3H6O. It has a polar carbonyl group (C=O) that can participate in dipole-dipole interactions. The presence of the polar group increases the intermolecular forces compared to isobutane, resulting in a lower vapor pressure than isobutane.
3. Acetamide:
Acetamide is an amide with the molecular formula C2H5NO. It has both a polar carbonyl group (C=O) and a polar nitrogen atom. The presence of both polar groups increases the intermolecular forces further, causing a lower vapor pressure compared to propanal.
4. Acetic Acid:
Acetic acid is a carboxylic acid with the molecular formula CH3COOH. It has a polar carbonyl group (C=O) and a highly polar OH group, which can form strong hydrogen bonds. Hydrogen bonds are stronger intermolecular forces than dipole-dipole interactions. As a result, acetic acid has the highest intermolecular forces among the compounds listed, leading to the lowest vapor pressure.
So, the rank of the substances in decreasing vapor pressure is:
Isobutane > Propanal > Acetamide > Acetic Acid.