2022 Live Review 7 | AP Chemistry | Bonding and the Condensed State

Dina Leggett, a chemistry teacher from Franklin High School, welcomes students to session seven of the AP Chemistry review course. She emphasizes the importance of hard work and its benefits for college, regardless of the test score. The session focuses on bonding in condensed states, specifically looking at liquid and solid properties, and the principle that 'structure determines function.' Leggett plans to cover topics including melting points, vapor pressures, electrical conductivity, malleability, ductility, Coulomb's law, intermolecular forces, and Lewis dot structures. She also discusses classifying substances as network covalent, metallic, ionic, or molecular covalent, starting with a review of network covalent materials and their properties.

Leggett delves into the properties of network covalent materials, which are hard, rigid, and typically have high melting points due to the strength of covalent bonds. She uses silicon and silicon dioxide as examples, noting that graphite is an exception due to its delocalized electrons allowing layers to slide. The discussion also touches on the allotropes of carbon, including diamond, graphite, and fullerenes, and how their structures (sp2 hybrid for graphite and sp3 hybrid for diamond) contribute to their distinct properties.

The video script explains metallic bonds through the 'sea of electrons' model, where delocalized valence electrons move freely among positively charged metal cations. This delocalization is responsible for the conductivity and malleability of metals. Leggett discusses the wide range of melting points in metals and the ability of metals to form alloys, which are mixtures of metals or metals with non-metals that can have varying properties depending on the size and type of the atoms involved.

Leggett describes ionic bonding as an alternating lattice of cations and anions held together by the force of attraction as described by Coulomb's law. Ionic compounds are generally rigid, brittle, and have high melting points due to the strength of the ionic bonds. They can conduct electricity when melted or dissolved in water because the ions become mobile. The strength of the ionic bond and thus the melting point is influenced by the charge of the ions and the distance between them.

The script shifts to molecular covalent substances, where the focus is on intermolecular forces rather than intramolecular bonds. It differentiates between polar and nonpolar covalent molecules and discusses London dispersion forces, dipole-dipole interactions, and hydrogen bonding. Leggett emphasizes that increasing the size of a molecule enhances London dispersion forces, leading to higher melting points. She also clarifies misconceptions about bond breaking during phase changes, stressing that only intermolecular forces are broken.

Leggett explains the relationship between vapor pressure and intermolecular forces. She notes that substances with weaker intermolecular forces have higher vapor pressures because the molecules are more likely to evaporate and enter the gas phase. Using a vapor pressure curve, she illustrates how temperature affects vapor pressure and how the structure of molecules, such as hexane and cyclohexane, influences their vapor pressures due to differences in intermolecular forces.

The video script concludes with a reminder to avoid common misconceptions when studying chemical bonding. Leggett stresses that during phase changes, molecular covalent substances only break intermolecular forces, not intramolecular bonds. She also reiterates that hydrogen atoms bonded to carbon do not participate in hydrogen bonding. The summary advises students to classify substances, read through all questions, answer easy questions first to secure points, and use the claim-evidence-reasoning framework for structured answers.