Intermolecular Forces:relationships between

molecules

Polarity and Shape

Polar bonds do not necessarily always create a polar molecule.

The shape of the molecule and the polarity of each bond is considered before stating if a molecule is considered POLAR or NON-POLAR We will watch a 5 minute explanation to help us understand this

difficult concept

major intermolecular forces

Intramolecular forcesintra = inside

Intramolecular forces: forces that bond the atoms to each other within the molecule.

COVALENT BONDS!!!

Intermolecular Forces inter = between

Forces affecting the relationships between molecules London Forces (Dispersion Forces) AKA van

der Waals forces Dipole-Dipole Interactions Ion-dipole forces Hydrogen Bonding

London (dispersion) Forcesaka van Der Waals forces weakest intermolecular

force occurs between all types

of molecules It is a temporary

attractive force that results when the electrons in two adjacent atoms occupy positions that make the atoms form temporary dipoles

London Dispersion forces in a molecule                          

These instantaneous dipoles may be induced and stabilized as an ion or a polar molecule approaches the non-polar molecule.

Dipole-Dipole Forces

Dipole = polar molecule Molecules with dipoles

will change their direction so that their oppositely charged ends are near to one another.

The electrostatic attraction between the ends is dipole-dipole force

Dipole-Dipole Forces

Attractive forces between polar molecules

Orientation of Polar Molecules in a Solid

11.2

Dipole - Dipole Interactions

Ion Dipole Forces (the reason why ionic compounds dissolve in water) The force of attraction

between an ion and a polar molecule.

NaCl breaks up because the ion dipole with water is stronger than the attraction of Na+ to Cl-

Polar molecules can interact with ions:

Ion - Dipole Interactions

The Hydrogen BondThe Hydrogen Bond

• A hydrogen bond is the strongest form of dipole-dipole interaction.

F—HO—HN—H

• A hydrogen bond is formed between polar molecules that contain hydrogen covalently bonded to the small, highly electronegative atoms, F, O, or N.

• A molecule containing this combination of atoms, will be attracted to another molecule with the same atom combination.

hydrogen bond

covalent bond

covalent bond

• A strong attractive force between the two molecules which is called a hydrogen bond is formed.

water has the highest

melting point

water has the highest

boiling point

water has the highest heat of vaporization

water has the highest heat of fusion

water has the lowest

molar mass

The melting point, boiling point, heat of fusion and heat of vaporization of water are extremely high and do not fit the trend of properties relative to molar mass within Group 17.

Hydrogen bonding in water animation

• hydrogen bonding animation

Water exhibits these unusual properties because of hydrogen bonding between water molecules.

Why do we care about Why do we care about intermolecular forces?intermolecular forces?

• The forces that act on molecules determine their physical properties

• Strong intermolecular forces between molecules increase melting and boiling points because it is these forces that are broken when substances change state.

Polarity and Boiling Point:Polarity and Boiling Point:The polarity of the molecules determines the The polarity of the molecules determines the forces of attraction between the molecules in the forces of attraction between the molecules in the liquid state. liquid state.

Polar molecules are attracted by the opposite Polar molecules are attracted by the opposite charge effect (the positive end of one molecule charge effect (the positive end of one molecule is attracted to the negative end of another is attracted to the negative end of another moleculemolecule)). .

Molecules have different degrees of polarity as Molecules have different degrees of polarity as determined by the functional group present.determined by the functional group present.

The greater the forces of attraction the higher the The greater the forces of attraction the higher the boiling point or the greater the polarity the higher boiling point or the greater the polarity the higher the boiling point.the boiling point.

What is boiling point?What is boiling point?

Boiling point is the Boiling point is the temperature at which a temperature at which a liquid turns into a gas (or liquid turns into a gas (or vapour).vapour).

For a liquid to turn into a For a liquid to turn into a gas, intermolecular gas, intermolecular forces must be broken. forces must be broken.

The stronger the The stronger the intermolecular forces, the intermolecular forces, the higher the boiling point of higher the boiling point of a substance.a substance.

The evidence for hydrogen bondingThe evidence for hydrogen bonding

Many elements form compounds with Many elements form compounds with hydrogen - referred to as "hydrides". hydrogen - referred to as "hydrides". If you plot the boiling points of the hydrides If you plot the boiling points of the hydrides of the Group 4 elements, you find that the of the Group 4 elements, you find that the boiling points increase as you go down the boiling points increase as you go down the group.group.

The increase in boiling point happens because the molecules are getting larger with more electrons, and so van der Waals dispersion forces become greater.

The evidence for hydrogen bondingThe evidence for hydrogen bonding If you repeat this exercise with the If you repeat this exercise with the

hydrides of elements in Groups 5, 6 and hydrides of elements in Groups 5, 6 and 7, something odd happens.7, something odd happens.

Solubility and Solubility and Intermolecular ForcesIntermolecular Forces

Like dissolves likeLike dissolves like– Polar solutes dissolve in polar solventsPolar solutes dissolve in polar solvents

– Nonpolar solutes dissolve in nonpolar Nonpolar solutes dissolve in nonpolar solventssolvents

Molecules with similar intermolecular Molecules with similar intermolecular forces will mix freelyforces will mix freely

Ionic Solute with Ionic Solute with Polar SolventPolar Solvent

Ionic Solute withIonic Solute withNonpolar SolventNonpolar Solvent

Nonpolar Solute withNonpolar Solute withNonpolar SolventNonpolar Solvent

Nonpolar Solute Nonpolar Solute with Polar Solventwith Polar Solvent

Comparison of the Properties of Substances with Ionic, Comparison of the Properties of Substances with Ionic, Covalent, Metallic or Intermolecular BondsCovalent, Metallic or Intermolecular Bonds

IonicIonic CovalentCovalent Metallic Metallic IntermoleculaIntermolecularr

Bond Bond strengthstrength

StrongStrong Very strongVery strong Variable Variable strength, strength, generallymodergenerallymoderateate

WeakWeak

HardnessHardness Moderate to Moderate to highhigh

Insulators in Insulators in solids and solids and liquid statesliquid states

Low to Low to moderate; moderate; ductile, ductile, malleablemalleable

Crystal soft Crystal soft and and somewhat somewhat plasticplastic

Electrical Electrical conductivitconductivityy

Conducts by ion Conducts by ion transport, but transport, but only when liquid only when liquid or dissociatedor dissociated

LowLow Good Good conductors; conductors; conducts by conducts by electron electron transporttransport

Insulators in Insulators in both solid both solid and liquid and liquid statesstates

Melting Melting pointpoint

Moderate to Moderate to highhigh

Very Very highhigh Generally highGenerally high Low Low

SolubilitySolubility Soluble in polar Soluble in polar solventssolvents

Very low Very low solubilitysolubility

Insoluble except Insoluble except in acids or in acids or alkalis by alkalis by chemical chemical reactionreaction

Soluble in Soluble in organic organic solvents solvents

ExamplesExamples Most mineralsMost minerals Diamond, Diamond, oxygen, oxygen, hydrogen, hydrogen, organic organic moleculesmolecules

Cu, Ag, Au, Cu, Ag, Au, other metals other metals

Organic Organic compoundscompounds

Noble gas Noble gas HeliumHeliumneonneonargon argon

HeHeNeNeArAr

-269-269-246-246-186 -186

Nonpolar covalent Nonpolar covalent hydrogenhydrogenoxygenoxygenmethanemethanechlorine chlorine

H2H2O2O2CH4CH4Cl2Cl2

-253-253-183-183-164-164-34 -34

polar covalent polar covalent ammoniaammoniahydrogen hydrogen fluoridefluoridewaterwater

NH3NH3

HFHF

H2OH2O

-33-3319.519.5

100 100

ionic ionic potassium potassium chloridechloridesodium sodium chloridechloridemagnesiumagnesium oxidem oxide

KClKCl

NaClNaCl

MgOMgO

771771

14131413

2826 2826

metallic metallic coppercopperironirontungstentungsten

CuCuFeFeWW

256725672750275056605660

Boiling Point of Various Material (˚C)