intermolecular forces

intermolecular forces - Kinetic Molecular Theory and...

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Unformatted text preview: Kinetic Molecular Theory and Condensed Phases Phase: Gas Liquid Solid Property intermolecular weak medium strong forces molecular translational rotational vibration motion rotational vibrational vibrational energy terms KE KE & PE PE compressibility high low low density low high high molecular none moderate high order thermal expansion large small small Theory of Intermolecular Forces, and Types 1 Cause attractive interactions between molecules (or ions or atoms) of a substance, or within mixtures of substances. Based on electrostatic attractions and repulsions A dipole results when positive and negative charges are separated in the same molecule. e.g. water molecule. Ions have either positive or negative charge. Dipoles have partial positive and negative charge regions Opposite charges attract; like charges repel. Strength of attraction depends on : distance between the opposite charges magnitude of the charges involved Coulomb’s Law: 2 2 1 r q q k F = energy is released as opposite charges form a bond. Ion-ion interactions and ion-dipole interactions Water as dipolar molecule: both positive and negative poles 2 poles have partial electron charge water bonds with ions: releases “energy of hydration” other polar solvents: release “energy of solvation” Example: List the following ions in order of their enthalpy of hydration, ∆ H hydr , from least to greatest. Na + , d = 116 pm; Mg 2+ , 86 pm; Cs + , 181 pm Molecules with Permanent Dipoles: Bond between atoms of different electronegativity Asymmetrical arrangement within the molecule. Predicting polar vs. non-polar: Start with Lewis structure Examples: Hydrogen Bonding: Special case of dipole − dipole interaction H on N, O or F bonds with other N, O or F 3 HF H 2 O NH 3 H-bond receivers and donors Dispersion Forces (also called “London Forces”): Induced dipole: Temporary asymmetry in electron distribution causing a temporary dipole to develop. Most common type of charge separation: found in all molecules 4 An induced dipole tends to be weaker than a permanent dipole for similar sized molecules. Momentary dipoles can be induced by an ion, a dipole or another non-polar species. Electrons repel each other! Induced dipoles are reason substances like I 2 and O 2 dissolve in water. Strength of interaction depends on polarizability of the molecule(s) involved. (How readily e- move around.) Factors involved in strength of interaction: increasing effect with increased mass inc. if greater number of electrons involved inc. if electrons less tightly held electron cloud more deformable ("nephelauxetic") When induced dipole - induced dipole interactions are predominant: Interactions between non-polar molecules No participant has permanent dipole Strength of interaction increases with size and mass 5...
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intermolecular forces - Kinetic Molecular Theory and...

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