Ch. 12: Liquids, Solids, and Ch. 12: Liquids, Solids, and Intermolecular ForcesIntermolecular Forces

Dr. Namphol Sinkaset

Chem 152: Introduction to General Chemistry

I. Chapter OutlineI. Chapter Outline

I. Introduction

II. Liquids and Solids

III. Surface Tension and Viscosity

IV. Types of Intermolecular Forces

I. Electrostatic ForcesI. 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 ForcesI. 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?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 ForcesI. 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 SolidsII. 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 LiquidsII. 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 SolidsII. 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 III. Effects of Intermolecular ForcesForces

• 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 TensionIII. Surface Tension

• More dense objects float??

III. Surface TensionIII. Surface Tension

III. Attractions Pull Surface InIII. 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. ViscosityIII. Viscosity

• Viscosity is the resistance of a liquid to flow.

• Viscosity is the result of intermolecular attractions.

IV. Types of IM ForcesIV. 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 ForceIV. Dispersion Force

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

IV. Instantaneous DipolesIV. Instantaneous Dipoles

• Charge separation in one creates charge separation in the neighbors.

IV. Dispersion Force StrengthIV. 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 PointsIV. Noble Gas Boiling Points

IV. Dipole-Dipole ForceIV. Dipole-Dipole Force

• Occurs in polar molecules which have permanent dipoles, so attraction between molecules is always present.

IV. Effect of Dipole-Dipole IV. Effect of Dipole-Dipole ForceForce

• Polar molecules have dispersion forces and dipole-dipole forces.

• Effects can be seen in boiling and melting points.

IV. “Like Dissolves Like”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”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 & IV. H “Bonding” in Ethanol & WaterWater

IV. Effect of H “Bonding”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 LifeIV. H “Bonding” and Life

IV. Summary of IM ForcesIV. Summary of IM Forces

IV. Sample ProblemIV. Sample Problem

• Determine the kind(s) of intermolecular forces present in the following substances. HBr I2

CO NF3

SiCl4