Let's say we have three spoons. In the first one, we put 5 drops of water; in the second, we put 5 drops of alcohol and in the third, 5 drops of acetone. After waiting a while, we will see that acetone will quickly change to a gaseous state, followed by alcohol, and only after a long time will the water evaporate.
This example shows us that substances do not go into a gaseous state or a vapor state at the same time and, consequently, their boiling points are also different.
To understand why this happens, we first need to understand when this transition from liquid to gas (or to vapor in the case of water) occurs. Molecules of liquids in a container are constantly under agitation, as they have some freedom to move. Atmospheric pressure exerts a force on these molecules that prevents them from passing into the gaseous state. Furthermore, the molecules make intermolecular bonds with each other, which also make it difficult to change their physical state.
However, when these molecules acquire a determined kinetic energy, they manage to break their intermolecular bonds and inertia, changing to the gaseous or vapor state.
When we increase the temperature of this liquid, we are supplying energy to the system, which causes these molecules more quickly acquire the energy needed to change state, which happens when they reach the your boiling point.
In the case of the example given, the boiling points of acetone, alcohol and water are, respectively, 56.2 °C, 78.5 °C and 100 °C at sea level. This explains the order of evaporation mentioned for these liquids.
But why this difference?
There are two basic factors that justify the differences in the boiling points of substances, which are: intermolecular interactions and molar masses.
Let's look at the following list to see how these factors influence the boiling point of substances:
- Intermolecular Interactions:
If the intermolecular interaction is intense, it will be necessary to supply even more energy to the system so that it breaks down and the molecule is able to pass to the gaseous state.
The intensity of these interactions between molecules follows the following descending order:
Hydrogen bonds > permanent dipole > induced dipole
For example, in the table, we see that the boiling points of butan-1-ol and ethanoic acid are higher than those of other substances. This is because these two substances have hydrogen bonds, which are more intense interactions than the others.
Also, the boiling point of propanone is higher than that of pentane because the interaction of propanone is permanent dipole, which is more intense than the induced dipole, which is the interaction performed by the pentane.
But why is the boiling point of propanone not higher than that of hexane, since it also performs the induced dipole interaction?
This is where the second factor that interferes with the boiling point of a substance comes in: the molar mass.
- Molar Masses:
If the molecule's mass is large, it will be necessary to supply more energy to the system so that the molecule can overcome the inertia and move to the gaseous state.
For example, pentane and hexane perform the same interaction, which is that of an induced dipole, but the molar mass of hexane is greater. Therefore, the boiling point of hexane is higher than that of pentane.
In the case of butan-1ol and ethanoic acid, both make hydrogen bonds and butan-1-ol has a higher molar mass. However, the boiling point of ethanoic acid is higher because two molecules of ethanoic acid can form two bonds between them. hydrogen (through the O and OH groups), while two molecules of butan-1-ol establish only one hydrogen bond to each other (through the OH group).
By Jennifer Fogaça
Graduated in Chemistry
Source: Brazil School - https://brasilescola.uol.com.br/quimica/comparacao-entre-pontos-ebulicao-das-substancias.htm
