Episode 27 reviews Units 1-4 of the AP Chemistry curriculum. Starting off with Unit 1 it gives an overview over counting atoms and isotopes.
Episode 27 reviews Units 1-4 of the AP Chemistry curriculum. Starting off with Unit 1 it gives an overview over counting atoms and isotopes (1:28). Diving deeper, we review atomic structure and electron configurations (2:17) as well as Coulomb’s Law (3:04). In Unit 2 the episode focuses on the types of bonds (3:33) and how they are determined. The second half of the Unit takes a closer look at covalent compounds by describing Lewis Diagrams and VSEPR Theory (5:26). In unit 3 we recap the intermolecular forces (6:05), which lays the foundation for gas laws (7:03) and solutions (7:30). Unit 4 finally gets us to some chemical reactions! It introduces types of chemical equations (8:01), which we can use for stoichiometric calculations (8:43) as well as three types of reactions (8:52): acid-base reactions, redox reactions and precipitation reactions.
Question: What determines the strength of London-Dispersion Forces of two nonpolar compounds?
A. Molecular weight
B. Number of protons
C. Number of electrons
D. Molecular geometry
Thank you for listening to The APsolute RecAP: Chemistry Edition!
(AP is a registered trademark of the College Board and is not affiliated with The APsolute RecAP. Copyright 2020 - The APsolute RecAP, LLC. All rights reserved.)
Website:
EMAIL:
Follow Us:
Hi, and welcome to the APsolute Recap: Chemistry Edition. Before we start today’s midterm review episode, I want to let you know that Episode 28 is 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:
As of today’s published episode, there are 144 days until the 2021 AP exam. And, depending on when you started school, you are about 120 days into the school year. Almost half-time! We therefore want to use today’s episode to give you an overview over the first four units: Atomic Structure, Chemical Bonding, Intermolecular Forces and Chemical Reactions. While listening, try to “tick off” the terms we are using in your head or on our study guide. If there are terms that you don’t know what they mean, pause and make a note so you can go back and review.
Let’s Zoom in:
Literally - we are zooming ALL the way in and look at Atomic Structure. The unit starts off by counting atoms using Avogadro’s number: 6.022 x 10 to the 23rd and connecting moles, mass and molar mass. Differences in atomic weight of elements can be explained by the concept of isotopes: elements that have the same number of protons but different number of neutrons. Mass spectroscopy helps us to identify isotopes. We can distinguish between pure substances and use the elemental composition by mass to determine the empirical formula and the composition of mixtures in which at least two pure substances are combined in varying proportions. The purity of a sample can be determined by using elemental analysis.
But let’s talk a bit more about atomic structure: atoms are composed of positively charged protons as well as neutrons in the nucleus. The negatively charged electrons surround the dense nucleus. To describe the distribution of electrons in shells and orbitals, we are using electron configurations and distinguish between core electrons and valence electrons. The number of valence electrons can also be determined by the position of the element in the periodic table. To determine the electron configuration of an element, chemists use photoelectron spectroscopy in which they measure the amount of energy it takes to remove an electron. This energy correlates with the shells and subshells of the atomic structure and is determined by the interaction between electrons and nucleus. This interaction can be quantified using Coulomb’s Law, which states that the force of attraction increases with increasing charge and decreases with increasing distance between the particles. This law is suuuuuper important. It also helps us to explain our periodic trends: ionization energy, atomic and ionic radii, electron affinity and electronegativity.
In Unit 2 we leave the individual, atomic level and focus on molecular and ionic compound structure and their properties. We distinguish between ionic bonds, polar and nonpolar covalent and metallic bonds. There are three approaches we can take to identify the type of bond: 1)The difference in electronegativity between the elements. 2) The element type: nonmetal and metal form ionic bonds, nonmetal and nonmetal covalent and metals have metallic bonds. And 3) Properties of the substances: melting points, boiling points and conductivity. The strength of covalent bond is determined by the atom’s size and the bond order. To understand the strength of ionic bonds, we can, again, use Coulomb's Law. For metals we can distinguish between substitutional and interstitial alloys.
But let’s zoom in to covalently bonded substances: We can represent the structure of a covalently bonded molecule using Lewis Diagrams. Some compounds have to be represented using resonance structures and to determine the dominant Lewis Structure if there are multiple possible, we calculate formal charges. The Lewis diagram, however, doesn’t give us information about the arrangement of the molecule in a 3D space. For that, we use the VSEPR theory and determine the molecular shape, bond angles, and polarity. These concepts are closely related to the hybridization of a molecule and the sigma and pi bonds.
Now that we have looked at the individual atoms as well as the bonds within compounds, we are looking at the forces BETWEEN molecules in Unit 3: Intermolecular Forces. This is a big one! The foundation is, of course, our intermolecular forces: London Dispersion Forces, dipole-dipole forces, hydrogen bonding and ion-dipole forces. These forces directly determine properties of solids, liquids, solutions and gases. In solids, for example, the intermolecular forces can be used to explain differences in melting points and vapor pressures between substances. We distinguish between four different types of solids: ionic, covalent, network covalent and metallic. When moving to liquids and finally gases by overcoming the intermolecular forces, the arrangement of particles gets less and less orderly and the particle motion increases. Which brings us to the Ideal Gas Law. The ideal gas law, also known as pivnerd, helps us to connect the properties of pressure, temperature, particle number and volume of a gas. The Kinetic Molecular Theory provides a framework to explain the relationship between these variables and the particle motion. And because nothing is ever ideal, we can use volume and intermolecular forces to explain deviations from the ideal gas law.
Switching our focus to solutions, Unit 3 introduces us to the calculation of Molarity. To determine the concentration of a colored solution experimentally, we can use the Beer Lambert Law and measure the molar absorptivity. Particle diagrams can be helpful to visualize the forces between solute and solvent. The intermolecular forces between substances also help us to determine the solubility of a substance. But what if you want to separate them? Then you can use chromatography and distillation!
In Unit 4 we are taking the next step: Finally some “real” chemistry: Chemical Reactions. To communicate our chemical reactions we use balanced chemical equations. There are three different types of equations: molecular equations, complete ionic equations and net ionic equations. Each are used depending on the context and the message you want to convey. But what is a chemical reaction? In a chemical reaction, compared to a physical change, the bonds are being broken and new bonds are formed. This leads to change in identity. The coefficients in the balanced chemical reaction are an important part of our chemical calculations: Stoichiometry. Using the mole ratio and the law of conservation of mass we can relate the amounts of reactants and products to one another. Unit 4 introduces us to three types of chemical reactions: Number 1) acid-base reactions in which a proton is transferred and connects it to the concentration of a solution via titration. Number 2) Oxidation-Reduction Reactions, also known as redox reactions, which involve the transfer of one or more electrons between species and number 3) precipitation reactions in which we form an insoluble compound.
To recap:
Unit 1 starts us off with Atomic Structure by focusing on the structure of an atom, electron configurations and how we quantify atoms. In Unit 2 we connect our atoms to form chemical bonds: ionic, covalent and metallic. In Unit 3, we look at the forces between molecules, the intermolecular forces. These determine properties of solids, liquids and gases as well as their solubility. Finally some chemistry in Unit 4: We are introduced to chemical reactions and focus on chemical quantities as well as different types of chemical reactions.
Coming up next on the APsolute RecAP Chemistry Edition: Listener’s Choice II
Today’s Question of the day is about Intermolecular Forces.
Question: What determines the strength of London-Dispersion Forces of two nonpolar compounds?
A. Molecular weight
B. Number of protons
C. Number of electrons
D. Molecular geometry