Unit 7.5 Comparing Intermolecular Forces

Unit 7.5 Comparing Intermolecular Forces

Teacher: Dr. Van Der Sluys

Objectives

• To determine– Relative melting and boiling points– Relative solubility of various combinations

of compounds

Vocabulary

• Solution - a homogeneous mixture of two or more compounds.

• Solvent - The compound that comprises the majority of a solution.

• Solute - A minor component of a solution.

• Aqueous solution - A mixture of liquid water and one or more solutes.

Types of BondingIntramolecular Intermolecular

Nonpolar Covalent London Dispersion Forces

Polar CovalentLondon Dispersion when symmetric

Dipole-Dipole when asymmetric

Extremely Polar Covalent: H-N,

H-O or H-F bondsHydrogen Bonding

Weakly Ionic:

NaCl

Soluble in water due to

ion-dipole interactions

Extremely Ionic:

Fe2O3

Insoluble in water due to high crystal lattice energies

Like Dissolves Like

• In order for a mixture to combine and become homogeneous on the molecular level, the compounds must have similar intermolecular bonding properties, i.e. ethanol and water.

• Two compounds that have very different intermolecular forces will not mix and will produce a heterogeneous mixture, i.e. Italian salad dressing.

Comparing Intermolecular Forces

Compound London Dispersion

Dipole-Dipole

(AXE?)

Hydrogen Bonding

(H-F, H-N, H-O)

Weakly Ionic

(Ions with low

charges)

Strongly ionic

(Both ions have

charges >2

Usually Insoluble in

H2O)



Dipole-Dipole

(AXE?)

Hydrogen Bonding

(H-F, H-N, H-O)

Weakly Ionic

(Ions with low

charges)

Strongly ionic

(Both ions have

charges >2


H2O)

CH4

methane

H2O

water



Dipole-Dipole

(AXE?)

Hydrogen Bonding

(H-F, H-N, H-O)

Weakly Ionic

(Ions with low

charges)

Strongly ionic

(Both ions have

charges >2


H2O)

NH3

ammonia

H2O

water



Dipole-Dipole

(AXE?)

Hydrogen Bonding

(H-F, H-N, H-O)

Weakly Ionic

(Ions with low

charges)

Strongly ionic

(Both ions have

charges >2


H2O)

Fe2O3

Iron(III) oxide

C8H18

octane



Dipole-Dipole

(AXE?)

Hydrogen Bonding

(H-F, H-N, H-O)

Weakly Ionic

(Ions with low

charges)

Strongly ionic

(Both ions have

charges >2


H2O)

CO2

Carbon dioxide

H2O

water

Melting and Boiling Points

• The relative melting and boiling of various compounds are due to the strength of intermolecular forces.

• If two compounds have the same types of intermolecular forces, the total number of electrons can usually be used to predict the degree of London dispersion forces. The compound with more electrons usually has the higher melting and boiling points.

• For compounds with hydrogen bonding, the ratio of hydrogen atoms to lone pairs can be important.


Compound

London Dispersion

(Total number of electrons?)

Dipole-Dipole

(AXE?)

Hydrogen Bonding

(H-F, H-N, H-O)

Weakly Ionic

(Ions with low

charges)

Strongly ionic

(Both ions have

charges >2


H2O)

CH4

methane

C2H6

ethane


Compound

London Dispersion


Dipole-Dipole

(AXE?)

Hydrogen Bonding

(H-F, H-N, H-O)

Weakly Ionic

(Ions with low

charges)

Strongly ionic

(Both ions have

charges >2


H2O)

CO2

Carbon dioxide

H2O

water


Compound

London Dispersion


Dipole-Dipole

(AXE?)

Hydrogen Bonding

(H-F, H-N, H-O)

Weakly Ionic

(Ions with low

charges)

Strongly ionic

(Both ions have

charges >2


H2O)

NaClSodium chloride

C6H14

Hexane


Compound

London Dispersion


Dipole-Dipole

(AXE?)

Hydrogen Bonding

(H-F, H-N, H-O)

Weakly Ionic

(Ions with low

charges)

Strongly ionic

(Both ions have

charges >2


H2O)

CO2

Carbon dioxide

H2O

water

Summary

• If compounds have similar intermolecular forces they tend to form homogeneous solutions, “like dissolves like.”

• The strength of intermolecular forces can be used to predict relative melting and boiling points

Unit 7.5 Comparing Intermolecular Forces

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