Intermolecular Forces, Liquids, and Solids

By: Ms. Buroker

11.1: A Molecular Comparison of Liquids and Solids

There is a fundamental difference between the three states of matter

and that is due to the space between the particles.

Review of Gases A) Compressible - gases occupy ALL the

available volume:

There is a large space between the molecules. The volume occupied by the gas is much larger than the space occupied by the molecules themselves.

B) Intermolecular forces are quite WEAK at high temperature and low pressure. Under these conditions the molecules are far apart and act independently of one another. They are moving fast and the molecular size and _________________ can be ignored.

States of MatterBecause in the solid and liquid

states particles are closer together, we refer to them as

condensed phases.

The state a substance is in at a particular temperature and pressure depends on two

things:1.) The kinetic energy of the particles and2.) The strength of attractions between the

particles

11.2: Intermolecular Forces

The attractions between molecules are not nearly as strong as the intramolecular

attractions that hold compounds together.

Intramolecular

11.2: Intermolecular Forces

They are, however, strong enough to control physical properties such as boiling and melting points, vapor pressures, and viscosities.

Intermolecular ForcesThere are different types of attractions between

molecules/ions: Ion-Ion; Ion-Dipole; Dipole-Dipole; London Forces, etc.

• Electrostatic attractions are determined by Coulomb’s Law:

F = kq1q2/r2

• Attractive forces depend upon three factors: 1.) The distance between the ion and the dipole.

*The closer the ion and dipole, the stronger the attraction2.) The charge on the ion.

*The higher the ion charge, the stronger the attraction.3.) The magnitude of the dipole.

*The greater the magnitude of the dipole, the stronger the attraction

Effects of intermolecular forces

1.) Affects solvation energy (enthalpy of hydration):

2.) Dissolving substances in one another

* In general: Like substances dissolve like substances polar in polar (ionic); nonpolar in nonpolar, etc.)

3.) Permanent dipole-attractions: DHvap increases with polarity and molar mass

“Like Dissolves Like”

Ion- Dipole Forces

Ion-dipole interactions are an important force in solutions of ions. The strength of these forces are what make it possible for ionic substances to dissolve in polar solvents.

Dipole- Dipole Interactions

Molecules that have permanent dipoles areattracted to each other. * The positive end of one is attracted to the negative end of the other and vice versa. * These forces are only important when the molecules are close toeach other.

Dipole- Dipole Forces

Dipole- Dipole Interactions

In terms of polarity … would the more or less polar molecule have the higher boiling point?

London Dispersion Forces

1.) Produce “momentary dipoles” that hold the molecules together

2.) Caused by the temporary shifting of theelectron clouds through interactions betweenMolecules

3.) For an homologous series (CH4, C2H6, C3H8, etc.) polarizability increases with molar mass (greater electron cloud shifting)

London Dispersion Forces

While the electrons in the 1s orbital of helium would repel each other (and, therefore, tend to stay far away from each other), it does happen that they occasionally wind up on the same side of the atom.

London Dispersion Forces

At that instant, then, the helium atom is polar, with an excess of electrons on the left side and a shortage on the right side.

London Dispersion Forces

Another helium nearby, then, would have adipole induced in it, as the electrons on theleft side of helium atom 2 repel the electrons in the cloud on helium atom 1. London dispersion forces, or dispersion forces, are attractions between an instantaneous dipole and an induced dipole.

London Dispersion Forces

*These forces are present in all molecules,whether they are polar or nonpolar.

* The tendency of an electron cloud to distort in

this way is called polarizability.

Factors Affecting London Dispersion Forces

The shape of the moleculeaffects the strength of dispersion forces: long, skinny molecules (like n-pentane tend to have stronger dispersion forces than short, fat ones (like neopentane).

* This is due to the increasedsurface area in n-pentane.

Factors Affecting London Dispersion Forces

* The strength of dispersion forces tends to increase with increased molecular weight.* Larger atoms have larger electron clouds, which are easier to polarize.

Which Have a Greater Effect: Dipole- Dipole

Interactions or Dispersion Forces?

1.) If two molecules are of comparable size and shape, dipole-dipole interactions will likely be the dominating force.

2.) If one molecule is much larger than another, dispersion forces will likely determine its physical properties.

* The nonpolar series (SnH4 to CH4) follow the expected trend.

* The polar seriesfollows the trendfrom H2Te throughH2S, but water isquite an anomaly.

Hydrogen Bonding

The dipole-dipole interactions experienced when H is bonded to N, O, or F are unusually strong.

* We call these interactions hydrogen bonds.

Hydrogen BondingHydrogen bonding

arises in part from the

high electronegativity

of nitrogen, oxygen,

and fluorine.Also, when hydrogen is

bonded to one of those very electronegative elements, the hydrogen nucleus is exposed.

Just to Review …

Intermolecular Forces AffectMany Physical Properties

The strength of the attractions

between particles can greatly affect the properties of a substance

or solution.

ViscosityResistance of a

liquid to flow is called viscosity.

It is related to the ease with which molecules can move past each other.

Viscosity increases with stronger intermolecular forces and decreases with higher temperature.

Surface TensionSurface tension

results from the net inward force

experienced by the molecules on the

surface of a liquid.

The "skin" on the surface of a liquid caused by the increase in downward force on surface molecules from by the molecules that exist below the surface. (i.e. there is no counter force from above).

By definition, surface tension (γ), is the amount of work required toextend a liquid surface. Units are usually expressed in J/m2.Surface tension decreases with an increase in temperature.Higher

Energy

Lower Energy

Phase Changes

Energy Changes Associated

with Changes of State

Heat of Fusion: Energy required to change a solid at its melting point to a liquid.

Heat of Vaporization: Energy required to change a liquid at its boiling point to a gas.

Energy Changes Associated

with Changes of State

* The heat added to thesystem at the melting andboiling points goes intopulling the moleculesfarther apart from eachother.

* The temperature of thesubstance does not riseduring the phase change.

How do we calculate heats of vaporization

and fusion? DH = (specific heat) x ( grams of substance) x DT

Vapor PressureAt any temperature, some

molecules in aliquid have enough energy to

escape.• As the temperature rises, the

fraction ofmolecules that have enough energy

toescape increases.

Vapor Pressure

As more moleculesescape the liquid,the pressure they exert increases.

The liquid and vapor reach a state of dynamic equilibrium: liquid molecules evaporate and vapor molecules condense at the same rate.

The boiling point of a liquid is the temperature at whichits vapor pressureequals atmosphericpressure.The normal boilingpoint is the temperature at which its vapor pressure is 760 torr.

Critical Temperature= the highest temperature at which a substance can exist as a liquidCritical Pressure= the pressure required to bring about the liquefaction