Our listener’s choice recaps three topics: Ideal Gas Laws, Solubility and Internal Energy.
Our listener’s choice recaps three topics: Ideal Gas Laws, Solubility and Internal Energy.
We start by recapping what an ideal gas is compared to a real gas (1:28) and review the equation PV=nRT by describing the relationship between selected variables (2:11). We take a closer look at the relationship between particle number and pressure and review partial pressures (3:02). In our solubility recap we review the polarity of the water molecule (4:55) as well as discuss how polarity and intermolecular forces affect solubility and miscibility (5:22). Our third topic starts by defining internal energy (6:17), how the change is calculated (6:41) and the terms exothermic and endothermic (7:06).
Question: Which of the following compounds would be most soluble in H2O? (8:36)
A. NH3
B. Xe
C. He
D. CCl4
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Hi and welcome to the APsolute Recap: Chemistry Edition. Before we begin today’s Listener’s choice episode, I want to let you know that Episode 28 is also a Listener’s choice episode! If you have a topic, skill, or question you want a recap of - please let us know. You can contact us through our website, theapsoluterecap.com, or reach out to us on Instagram, Facebook or Twitter!
Let’s Zoom out:
We are producing this episode at the end of November 2020. And what a school year it has been so far! No matter your mode of learning, if in-person, hybrid or remote - it has been a challenge for all students and teachers! AP Chemistry can be a challenging course in a regular year, but this year's circumstances have certainly brought learning to a new level! But we are here to help! Especially when it comes to topics that you are struggling with. Therefore, today’s episode is a listener’s choice episode and will dive into some of the concepts we need a refresher on. We want to thank everyone who wrote in with topic requests. If you have a topic you’d like a recap on in episode 28, please contact us through email, instagram, or twitter.
Let’s Zoom in:
Our first recap topic comes to us from listener Farthuse A. - asking about the Ideal Gas Law. Let’s start at the term “ideal gas” - what does that mean? Two aspects that distinguish an “ideal gas” from a real gas are: In an ideal gas, the attractive and repulsive forces between gas particles as well as gas particles and the container wall are negligible. Therefore, we are neglecting the intermolecular forces. Furthermore, we assume that we can neglect the volume of the gas particles. So if we have our gas in a container with a volume of 1 liter, we assume that the particles can use 1L.
To describe the macroscopic properties of an ideal gas, we use the ideal gas law: PV = nRT. It is part of the equation sheet, so you don’t have to memorize it, but I am sure many of you have! What does it tell us? It relates pressure, volume, temperature and particle number. Let’s look at some examples describing the relationship: Pressure and Volume are inversely proportional. That means, if you increase the volume, the pressure decreases and vice versa. Pressure and Temperature are directly proportional. If I increase the temperature, the pressure increases. Volume and Temperature are also directly proportional. When increasing the temperature, the volume increases. These relationships can also be represented in graphs that help us to describe the behavior of gases and gas mixtures.
Let’s take a closer look at the relationship between pressure and particle number: According to the ideal gas law, the more particles I have at constant temperature and volume, the higher is the pressure. This is, because we count the frequencies with which particles hit the wall of the container as pressure. If I have more particles, more particles will hit the walls, which will increase the pressure. In gas mixtures, each type of gas has a partial pressure. Let’s take air as an example: We have oxygen, nitrogen, carbon dioxide and some noble gases. If we have a 1L container with air at 25 degrees Celsius, each one of them will contribute to the total pressure, depending on how many particles each gas contributes to the sum of particles. This can be calculated using the mole fraction: mole of the individual gas divided by total moles. The total pressure is therefore the sum of the partial pressures.
Farthuse A. also wants to recap solubility, so let’s dive in! The solubility of a substance depends on intramolecular and intermolecular forces. For a solute to dissolve in solvent they have to have similar intermolecular forces. Let’s talk about our universal solvent water. Water is a polar molecule with a partially positive charge on hydrogen and a partially negative charge on oxygen. It is a dipole. Water can therefore dissolve or dissociate other polar molecules and many ionic compounds by experiencing intermolecular forces between itself and the solute. For example, when NaCl dissolves in water, the partially positive hydrogens in the water molecules are attracted to the chloride anions. At the same time, the partially negative oxygen within the water molecule is attracted to the sodium cations and the ionic lattice is torn apart. Some ionic compounds, however, have ionic bonds that are too strong for the water to overcome, so they are insoluble in water. What about polar and nonpolar substances, like oil and water? They do not have interactions that are strong enough to overcome the hydrogen bonds between water molecules. And so, the water molecules would rather stay amongst themselves. Oil and water are therefore immiscible.
Our last listener’s topic for today’s episode comes from Sariah H. who would like a recap of internal energy, which is part of Unit 6: Thermodynamics. Internal Energy is defined as the sum of all kinetic and potential energies of the components of the system. In a chemical reaction, that is all the interactions and motions of the atoms and molecules but also within the atoms. Our focus in Thermodynamics is the CHANGE in energy, which we calculate by subtracting the final energy minus the initial energy. If this value is positive, the system has gained energy from its surroundings,usually in the form of heat. If it is negative, it has released heat from the system to the surroundings. Energy can be gained or released in the form of heat or work done on or by the system.
In thermodynamics we are mostly interested in heat. If heat is being gained by the system, the reaction is endothermic. “Endo” means “into”, in our case heat that is moving into the system. We see that in reactions where the surroundings get colder, like when using a cold pack for a sport injury. If heat is being lost, for example in combustion reactions, the reaction is “exothermic”. Internal Energy is an interesting and important concept. In AP Chemistry, however, we usually focus on enthalpy and the College Board even excludes the technical distinction between internal energy and enthalpy. Because most reactions on the AP level are under constant pressure, the enthalpy equals the internal energy and therefore also the heat lost or gained by the system.
To recap:
Ideal Gases do not experience intermolecular forces and do not take up volume. The Ideal Gas Law, PV=nRT relates the macroscopic properties of gases to one another. In gas mixtures, each gas contributes partial pressure to the total pressure, depending on its mole fraction. For substances to dissolve or dissociate they need to have similar intermolecular forces. Internal energy is the sum of all potential and kinetic energies within a system. In AP chemistry, we do not distinguish between internal energy and enthalpy.
Coming up next on the APsolute RecAP Chemistry Edition: Midterm Review
Today’s Question of the day is about solubility.
Question: Which of the following compounds would be most soluble in H2O?
A. NH3
B. Xe
C. He
D. CCl4