Our episode recaps two connected concepts: Boiling points and the strength of intermolecular forces.
Our episode recaps two connected concepts: Boiling points and the strength of intermolecular forces. We start with the chemistry definition of boiling points (0:53) and take a closer look at what happens at the start of heating (1:13), while heating (1:30) and at the boiling point of a pot of water (1:41). The episode then takes a closer look at the connection between atmospheric pressure and boiling point (2:02). For our second topic, we review the five types of intermolecular forces (4:18) and compare three pairs: hexane and pentanol (4:48), methane and hexane (5:00) as well as water and iodine (5:25).
Question: If you cook an egg on the Mount Everest, would it take less time, the same time or more time to be hard boiled as at sea level?
A. Less time
B. Same time
C. More time
Thank you for listening to The APsolute RecAP: Chemistry Edition!
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Hi and welcome to the APsolute Recap: Chemistry Edition. Today’s episode is our second listener’s choice episode.
Let’s Zoom out:
In today’s listener’s choice episode, you will learn how to make the perfect egg on Mount Everest! What does that have to do with Chemistry? Well, just wait for it! Today’s episode will review boiling points as well as the strength of intermolecular forces!
Let’s Zoom in:
Our first recap topic comes to us from listener Farthuse A. - asking about the boiling points. What is a boiling point? You might say: “Well, that’s the temperature at which a liquid becomes a gas”. And that’s not wrong! In chemistry, we are a bit more detailed, though. The boiling point is “the temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid and the liquid changes into a vapor.” What does that mean?
Think about boiling water in a pot to cook a nice hard boiled breakfast egg. Starting at room temperature, only a small portion of the particles have enough energy to overcome the intermolecular forces holding them in the liquid and leave. These particles form the “vapor pressure”. While heating, more and more particles escape, but the atmospheric pressure is greater than the vapor pressure and crushes all attempts of particles in the bubbles to escape. Ugh.
But when you are close to and at the boiling point, you can see more and more bubbles rising from the bottom of the pot. The vapor pressure increases and is eventually equal to the atmospheric pressure. And the bubbles can no longer be crushed! Yeah! They rise to the surface and the particles leave the liquid and join the atmosphere!
The boiling point depends on, among other factors, the atmospheric pressure. The value you usually see in Chemistry class is for 1 atm, atmospheric pressure. What if I cook my egg on top of a mountain? Well, the atmospheric pressure is lower, so the point at which the vapor pressure is equal to the atmospheric pressure is reached at a lower temperature. For example, water has a normal boiling point at 100℃. On Kilimandscharo, which is 5895 meters above sea level, water boils at 80.33℃. And on the highest mountain on earth - Mount Everest? It boils at 69.94℃.
Our last listener’s topic for today’s episode comes from Rosie, who wants to recap the strength of intermolecular forces. That actually ties in really neatly with Farthuse’s questions! Let’s start by recapping intermolecular forces: Intermolecular forces are the forces BETWEEN the molecules. Boiling points can be a good indicator of the strength of intermolecular forces. The stronger the intermolecular forces, the higher the boiling point, because we have to overcome all intermolecular forces to go from a liquid to a gas.
We distinguish between five types of intermolecular forces: London Dispersion forces, dipole-induced dipole interactions, dipole-dipole interactions, hydrogen bonding and ion-dipole interactions. They are usually listed in that order, which indicates the strength of the intermolecular force. Generally speaking, we say that London Dispersion forces are weaker than dipole-dipole forces, which are weaker than hydrogen bonding. Therefore, when we, for example, compare a nonpolar molecule like hexane with a molecule that exhibits hydrogen bonding, like pentanol, pentanol has a much higher boiling point.
But there are other factors we have to take into account. What if we have two nonpolar molecules, for example methane and hexane? Both of these are experiencing London Dispersion Forces, but hexane has a higher boiling point. One reason is that hexane has more electrons. More electrons lead to greater polarizability, which increases the strength of the London Dispersion forces.
This wouldn’t be chemistry, if there weren’t any exceptions to it. As an example, let’s compare water and iodine. At room temperature, water is a liquid, iodine is a solid. This indicates that iodine has stronger intermolecular forces than water. But water experiences hydrogen bonding, whereas iodine only has London Dispersion forces! Therefore, our “general rule” is more of a guideline.
On the AP Exam, in those instances you will usually be told that one is a liquid and the other one is a solid and you then have to argue that, more or less obviously, the London Dispersion forces of the solid are stronger than the hydrogen bonding in water.
To recap:
The boiling point is defined as the temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid. Therefore, it depends on the vapor pressure of the liquid as well as the atmospheric pressure. The boiling point can also be used as a measurement for the strength of intermolecular forces, because all intermolecular forces have to be overcome when changing from a liquid to a gas. Even though we organize the five types of intermolecular forces in the order of increasing strength, there are always exceptions!
Coming up next on the APsolute RecAP Chemistry Edition: Energy of Chemical Reactions
Today’s Question of the day is about making an egg!
Question: If you cook an egg on Mount Everest, would it take less time, the same time or more time to be hard boiled than at sea level?
A. Less time
B. Same time
C. More time