Intermolecular Forces Entry Task: Nov. 9 th Friday QUESTION: Which of the following would have the highest boiling point and WHY!! HCl or CH 3 CH 3 YOU

IntermolecularForces

Entry Task: Nov. 9th Friday

QUESTION:

Which of the following would have the highest boiling point and WHY!!

HCl or CH3CH3

YOU HAVE 5 minutes!!


Agenda• Discuss Ch. 11 sec. 1-3

(Should be a review from 1st year)

• HW: What’s the IMF? ws


I can…

• Distinguish the type of Intermolecular forces that exist between substances.

• Explain the relationships between a substances physical properties and IMF.


Chapter 11Intermolecular Forces,

Liquids, and Solids


States of MatterThe fundamental difference between states of matter is the distance between particles.


States of MatterBecause in the solid and liquid states particles are closer together, we refer to them as condensed phases.


The States of Matter

• The state a substance is in at a particular temperature and pressure depends on two antagonistic entities:

The kinetic energy of the particles

The strength of the attractions between the particles


Intermolecular Forces

The attractions between molecules are not nearly as strong as the intramolecular attractions that hold compounds together.



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



These intermolecular forces as a group are referred to as van der Waals forces.


van der Waals Forces

• London dispersion forcesnonpolar molecules (symmetrical) weak

• Dipole-dipole interactionsPolar molecules (asymmetrical) strong

• Hydrogen bondingPolar molecules (asymmetrical- FON) very strong


Ion-Dipole Interactions

• A fourth type of force, 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 are attracted 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 to each other.


Dipole-Dipole Interactions

The more polar the molecule, the higher is its boiling point.


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.



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.



Another helium nearby, then, would have a dipole induced in it, as the electrons on the left 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.



• 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 Forces

• The shape of the molecule affects 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 increased surface area in n-pentane.


Factors Affecting London Forces

• The strength of dispersion forces tends to increase with increased molecular weight.

• Larger atoms have larger electron clouds, which are easier to polarize.


11.1 problemOf Br2, Ne, HCl, and N2, which is likely to have (a) the largest intermolecular dispersion force (b) the largest dipole-dipole attractive force?

(a) Its Br2, why? Heaviest nonpolar molecule there.

(b) Its HCl, why? They are polar!! (H-bonding)


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 Bonding

Hydrogen 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.


11.2 problem• In which of these substances is significant

hydrogen bonding possible: methylene chloride (CH2Cl2), phosphine (PH3), hydrogen peroxide (HOOH), acetone (CH3COCH3)?

CH2Cl2 its polar but not h-bonding- no FON

PH3 it has a pair of electrons- polar but no FON

CH3COCH3 notice that it does have oxygen but it is surrounded by nonpolar CH3

HOOH oxygen is exposed and available for h-bonding


Which Have a Greater Effect:Dipole-Dipole Interactions or Dispersion Forces?

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

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


How Do We Explain This?

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

• The polar series follows the trend from H2Te through H2S, but water is quite an anomaly.


Summarizing Intermolecular Forces


Intermolecular Forces Affect Many Physical Properties

The strength of the attractions between particles can greatly affect the properties of a substance or solution.


Viscosity• Resistance 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 Tension

Surface tension results from the net inward force experienced by the molecules on the surface of a liquid.

Cohesion- the attraction of molecules to each other.


Capillary Action

Capillary action is the ability of a liquid to be drawn up a narrow tube as a result of adhesion and surface tension.

Adhesion- the attraction of molecules to a surface.


11.3 problem• (a) Identify the intermolecular attractions

present in the following substances, and (b) select the substance with the highest boiling point: CH3CH3, CH3OH, and CH3CH2OH.

CH3CH3 has dispersion forces only

CH3OH and CH3CH2OH have dispersion forces & h-bonding

CH3CH2OH its heaviest and has h-bonding


11.8 problem11.8 Consider the following substances: Ar, CHCl3, NH3. Which has a) the largest London dispersion forces; b) the largest hydrogen bonding forces; c) no dipole-dipole forces

CHCl3 greatest mass

NH3 its having FON

Ar monatomic it has dipole-dipole forces, just dispersion


11.14 problem11.14 Which member of the following pairs has the stronger IMF dispersion forces? a) CH3Cl or CH3Br; b) CH3CH2Cl or CH3CH2CH2Cl; c) CH3CH2CH2Cl or (CH3)2CHCl?

a. CH3Br its heavier

b. CH3CH2CH2Cl again- heavier

c. CH3CH2CH2Cl- it does not branch off (stringy)


11.19 problem11.19 Identify the type if IMF that are present in each of the following substances and select the substance in each pair that has a higher boiling point.

a) C6H14 or C8H18

b) C3H8 or CH3OCH3

c) CH3OH or CH3SH

d) NH2NH2 or CH3CH3

Both are dispersion (nonpolar); C8H18 -heavier

C3H8 dispersion, CH3OCH3 disp&dipole2; CH3OCH3 -heavier

Both are disp & dipole2 but CH3OH has h-bonding; CH3OH h-bonding

CH3CH3 dispersion NH2NH2 disp& dipole2; NH2NH2 –h-bonding


HW: What’s the IMF ws

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Intermolecular Forces Entry Task: Nov. 9 th Friday QUESTION: Which of the following would have the highest boiling point and WHY!! HCl or CH 3 CH 3 YOU