Ch 11 States of Matter and Intermolecular Forces

Intramolecular forces (bonds) govern molecular properties. Intermolecular forces are between molecules and determine the macroscopic physical properties of liquids and solids. This chapter:  describes changes from one state of matter to another.  explores the types of intermolecular forces that underlie these and other physical properties of substances. Chapter 11 Preview

Review: Intramolecular Forces Ionic Covalent Metallic

Ionic bond- electron donated and accepted

Covalent- sharing electrons

Metallic-sea of electrons

Ionic Bonds as “Intermolecular” Forces There are no molecules in an ionic solid, and therefore there can’t be any intermolecular forces. These forces increase:  as the charges on the ions increase.  as the ionic radii (size) decrease.

Interionic Forces of Attraction Melting point of NaCl is about 801 o C. Mg 2+ and O 2– have much stronger forces of attraction for one another than do Na + and Cl –. Melting point of MgO is about 2800 o C.

Molecular Forces Compared

States of Matter Compared Intermolecular forces are of little significance; why? Intermolecular forces must be considered. Intermolecular forces are very important.

Intermolecular Forces Hydrogen bonding Dipole-dipole London Forces

Cohesion Attraction for each other  Water  Mercury Boiling point varies based on cohesion

Adhesion A liquids attraction for solid particles  Water’s attraction for glass etc.

WATERMERCURY

Meniscus Formation Water wets the glass (adhesive forces) and its attraction for glass forms a concave- up surface. What conclusion can we draw about the cohesive forces in mercury?

Plant root- capillary action

Surface Tension The forces present in liquids A molecule inside the liquid experiences cohesive forces with other molecules in all directions. A molecule at the surface experiences only downward cohesive forces

Surface Tension There is no force above the molecule on the top of the surface.

Pool ball floating on mercury

Adhesive and Cohesive Forces The liquid spreads, because adhesive forces are comparable in strength to cohesive forces. The liquid “beads up.” Which forces are stronger, adhesive or cohesive?

Cohesive vs. Adhesive Water on plastic Water on metal Water on glass 1 2 3

Intermolecular Forces Hydrogen bonding Dipole-dipole London Forces

Hydrogen bonding A hydrogen is attracted to a highly electronegative atom like O, N, or Cl.

Hydrogen Bonding in Water

Hydrogen Bonding in Ice Hydrogen bonding arranges the water molecules into an open hexagonal pattern. “Hexagonal” is reflected in the crystal structure. “Open” means reduced density of the solid (vs. liquid).

Hydrogen Bonding in Acetic Acid Hydrogen bonding occurs between molecules.

Intermolecular Hydrogen Bonds Intermolecular hydrogen bonds give proteins their secondary shape, forcing the protein molecules into particular orientations, like a folded sheet …

Intramolecular Hydrogen Bonds … while intramolecular hydrogen bonds can cause proteins to take a helical shape.

In which of these substances is hydrogen bonding an important intermolecular force: N 2, HI, HF, CH 3 CHO, and CH 3 OH? Explain.

In which of these substances is hydrogen bonding an important intermolecular force: N 2, HI, CH 3 CHO, and CH 3 OH? Explain. CH 3 CHO and CH 3 OH because of the attraction between the H of one molecule and the O of another. HI would not have hydrogen bonding b/c iodine is not highly electronegative.

Dipole–Dipole Forces A polar molecule has a positively charged “end” (δ+) and a negatively charged “end” (δ–). When molecules come close to one another, repulsions occur between like-charged regions of dipoles. Opposite charges tend to attract one another.

Dipole Forces The more polar a molecule, the more pronounced is the effect of dipole– dipole forces on physical properties.

Dipole–Dipole Interactions Opposites attract!

London Forces- aka Dispersion Forces At first no dipole, like Argon. But the electrons are mobile, and at any one instant they might find themselves towards one end of the molecule, making that end -. The other end will be temporarily short of electrons and so becomes +. An instant later the electrons may have moved up to the other end, reversing the polarity of the molecule.

Induced London Forces What would happen if we mixed HCl with the element argon, which has no dipole? The electrons on an argon atom are distributed homogeneously around the nucleus of the atom. But these electrons are in constant motion. When an argon atom comes close to a polar HCl molecule, the electrons can shift to one side of the nucleus to produce a very small dipole moment that lasts for only an instant.

Dispersion Forces Illustrated (1) At a given instant, electron density, even in a nonpolar molecule like this one, is not perfectly uniform.

Dispersion Forces Illustrated (2) The region of (momentary) higher electron density attains a small (–) charge … When another nonpolar molecule approaches … … the other end of the molecule is slightly (+).

Dispersion Forces Illustrated (3) … this molecule induces a tiny dipole moment … … in this molecule. Opposite charges ________.

Types of forces Click here for an Animation of the forcesAnimation

Molecular Shape and Polarizability Long skinny molecule … … can have greater separation of charge along its length. Stronger forces of attraction, meaning … … higher boiling point. In the compact isomer, less possible separation of charge … … giving weaker dispersion forces and a lower boiling point.

Arrange the following substances in the expected order of increasing boiling point: Carbon tetrabromide, CBr 4 ; Butane, CH 3 CH 2 CH 2 CH 3 ; Fluorine, F 2 ; Acetaldehyde, CH 3 CHO.

Arrange the following substances in the expected order of increasing boiling point: Carbon tetrabromide, CBr 4 ; Butane, CH 3 CH 2 CH 2 CH 3 ; Fluorine, F 2 ; Acetaldehyde, CH 3 CHO. Answer: F 2, CBr 4, CH 3 CH 2 CH 2 CH 3, CH 3 CHO

Vapor Pressure The vapor pressure of a liquid is the partial pressure exerted by the vapor when it is in dynamic equilibrium with the liquid at a constant temperature. vaporization LiquidVapor condensation

Liquid–Vapor Equilibrium More vapor forms; rate of condensation of that vapor increases … … until equilibrium is attained.

Phase Diagrams A phase diagram is a graphical representation of the conditions of temperature and pressure under which a substance exists as a solid, liquid, a gas, or some combination of these in equilibrium. A—B, solid-vapor equilibrium. A—D, solid-liquid equilibrium. A—C, liquid-vapor equilibrium. Triple point

Phase Diagram for CO 2 Note that at 1 atm, only the solid and vapor phases of CO 2 exist.

Phase Diagram for H 2 O

Supercritical Fluid Above the critical temperature and pressure, only one phase exists…a combination of liquid and gas. Properties are in between those of liquids and gases. They act as solvents and dissolve well. They diffuse well like gases. CO 2 and H 2 O- environmentally friendly

The Critical Point At room temperature there is relatively little vapor, and its density is low. At higher temperature, there is more vapor, and its density increases … … while the density of the liquid decreases; molecular motion increases. At T c, the densities of liquid and vapor are equal; a single phase.