Ch. 12: Liquids, Solids, and Intermolecular Forces Dr. Namphol Sinkaset Chem 152: Introduction to General Chemistry

I. Chapter Outline Introduction Liquids and Solids Surface Tension and Viscosity Types of Intermolecular Forces

I. Electrostatic Forces Every molecule in a sample of matter experiences two types of electrostatic forces. Intramolecular forces: the forces that exist within the molecule (bonding). These forces determine chemical reactivity. Intermolecular forces: attractive forces that exist between molecules. These forces determine physical properties.

I. Intermolecular Forces IM forces originate from interactions between charges, partial charges, and temporary charges on molecules. IM forces are relatively weak because of smaller charges and the distance between molecules.

I. Solid, Liquid, or Gas? Whether a substance exists as a solid, liquid, or gas depends on the relationship between the intermolecular forces and the kinetic energy of the molecules. It’s a battle – which dominates? The KE or the IM attractions? Recall that the average KE of a sample is related to its temperature, called the thermal energy.

I. KE vs. IM Forces Gas: the kinetic energy of the molecules is much greater than the intermolecular attractions. Liquid: the kinetic energy of the molecules is moderately greater than the intermolecular attractions. Solid: the kinetic energy of the molecules is less than the intermolecular attractions.

II. Gases, Liquids, and Solids Major difference between gases and condensed phases is the distance between particles. Major difference between liquid and solid is the freedom of motion.

II. Properties of Liquids High densities relative to gases. Indefinite shape; liquids take the shape of their container. Definite volume; liquids cannot be compressed easily.

II. Properties of Solids High densities compared to gases. Definite shape; solids do not take shape of their container. Definite volume; solids are not easily compressed. May be crystalline (ordered) or amorphous (unordered).

III. Effects of Intermolecular Forces Without IM forces, only the gas phase would exist! Before looking at different types of IM forces, we look at two of their macroscopic effects. Surface tension Viscosity

III. Surface Tension More dense objects float??

III. Surface Tension

III. Attractions Pull Surface In Molecules on the surface are attracted to molecules in the bulk. This attraction pulls the surface molecules inward, minimizing surface area. The result is a “skin” which resists penetration.

III. Viscosity Viscosity is the resistance of a liquid to flow. Viscosity is the result of intermolecular attractions.

IV. Types of IM Forces There are different kinds of IM forces, each with a different level of strength. Dispersion force Dipole-dipole force *Hydrogen “bonding”

IV. Dispersion Force Dispersion force (London force) is present in all molecules and atoms and results from changes in e- locations.

IV. Instantaneous Dipoles Charge separation in one creates charge separation in the neighbors.

IV. Dispersion Force Strength The ease with which e-’s can move in response to an external charge is known as polarizability. Large atoms with large electron clouds tend to have stronger dispersion forces. Large molecules tend to have stronger dispersion forces.

IV. Noble Gas Boiling Points

IV. Dipole-Dipole Force Occurs in polar molecules which have permanent dipoles, so attraction between molecules is always present.

IV. Effect of Dipole-Dipole Force Polar molecules have dispersion forces and dipole-dipole forces. Effects can be seen in boiling and melting points.

IV. “Like Dissolves Like” Polar liquids are miscible with other polar liquids, but not with nonpolar liquids. Miscible: mix without separating into two phases.

IV. Hydrogen “Bonding” This IM force is a misnomer since it’s not an actual bond. Occurs between molecules in which H is bonded to a highly electronegative element (N, O, F), leading to high partial positive and partial negative charges. It’s a “super” dipole-dipole force.

IV. H “Bonding” in Ethanol & Water

IV. Effect of H “Bonding” Hydrogen “bonding” is a very strong intermolecular force. Molecules with H “bonding” have much higher than expected melting and boiling points.

IV. H “Bonding” and Life

IV. Summary of IM Forces

IV. Sample Problem Determine the kind(s) of intermolecular forces present in the following substances. HBr I2 CO NF3 SiCl4