Define boiling point - NSC Physical Sciences - Question 3 - 2018 - Paper 2

The functional group common to these compounds is the carboxyl group (-COOH).

The IUPAC name of compound C is propanoic acid.

The structural formula of the functional isomer of compound B (which is butanoic acid) can be represented as follows:

  H   O
  |  ||
H—C—C—C—C—OH
  |  |  |  
  H  H  H

This represents the four-carbon backbone with a carboxyl group at one end.

Compound A, which is formic acid (HCOOH), has the highest vapor pressure among the three compounds due to its lowest boiling point. A lower boiling point typically translates to a higher vapor pressure as it indicates that the substance more readily transitions into the gaseous state.

This is a fair comparison because both compounds B and X have similar molecular weights and structural characteristics, allowing for a reliable assessment of their boiling points under similar conditions.

Compound X ($CH_3CH_2CH_2OH$) is a primary alcohol because the -OH (hydroxyl) group is attached to a carbon atom that is bonded to only one other carbon atom.

Both compounds B (propanoic acid) and X (1-propanol) exhibit hydrogen bonding due to the presence of -OH and -COOH groups. However, propanoic acid can form two hydrogen bonds due to its carboxyl group, making its intermolecular forces stronger compared to 1-propanol, which has fewer hydrogen bond sites. Thus, propanoic acid has a higher boiling point than 1-propanol. Additionally, the stronger van der Waals forces in propanoic acid due to its higher molecular weight contribute to its higher boiling point. Therefore, the intermolecular forces in compound B (propanoic acid) are indeed stronger than those in compound X (1-propanol).