Intermolecular Forces

Bonding

• Ionic

• Covalent

• Polar covalent

Intermolecular Forces

Intermolecular Vs. Intramolecular Forces

intramolecular forces ( “bonds”)

O

H

H

OH H

OH

H

intermolecular forces

Figure 10.1 The Schematic Representations of the Three States of

Matter

Intermolecular Forces

Vaporize: 40.7 kJ Break O-H Bonds: 467 kJ

Table 10.6 Comparison of Atomic Separations Within Molecules (Covalent Bonds) and Between Molecules (Intermolecular Interactions)

Intermolecular Forces• Covalent and Ionic bonds are intramolecular forces• Attractions between molecules are intermolecular forces• Intermolecular forces are weaker than intramolecular

forces (vaporizing H2O vs. breaking OH bonds)• 3 Possible Different Types of IM forces:

- Dipole-Dipole -Hydrogen bonding - London Dispersion forces

• Properties of pure substances (in liquid or solid form) that depend on the strength of intermolecular forces: Melting point, boiling point, ΔHvap, ΔHfus, Vapor pressure, surface tension

* ALL molecular substances’ molecules will be attracted by London Dispersion forces, but not of equal strength.

* NOT all molecular substances’ molecules will be attracted by Dipole-Dipole or Hydrogen bonding “forces”

Dipole-Dipole Forces

If a molecule is polar, it has a positive and negative end

• If a molecule is polar, the positive end of one molecule will be attracted to the negative end of another– That’s basically dipole-dipole forces!

• If a molecule is nonpolar then it will not have “dipole-dipole” forces of attraction between its molecules

Figure 10.2 a-b (a) The Electrostatic Interaction of Two Polar Molecules (b) The Interaction of Many Dipoles in a Condensed State

Okay, so how do you know if a substance is polar?

• See separate handout….later.– One example now: If substance has only one

bond, and that bond is polar, then the whole molecule is polar

• HCl is polar; ClF is polar

• H2 (H-H) is nonpolar; Cl2 (Cl-Cl) is nonpolar;

• Just remember that you need to first “figure out” if a substance is polar before you conclude it “has” dipole-dipole forces acting between its molecules!

Polarity of Substances

HCl, CO2, NaCl

**Polarity only applies to molecular substances. Ionic compounds are at the “extreme”—the bonding is not considered “polar (covalent)”, but simply “ionic”!

**Ion-ion forces are comparable to intra-molecular forces. VERY STRONG

Figure 10.3 a-b (a) The Polar Water Molecule (b) Hydrogen Bonding Among Water Molecules

Hydrogen Bonding is a Special Case Version of Dipole-Dipole

Hydrogen Bonding

- Strong dipole-dipole forces

- Found in substances in which H is bound to highly electronegative atom (N, O, F)

Why?

- Very polar bond

- H atom is really small

Example

Which substance has the strongest dipole-dipole forces (between its molecules)?

HF, HCl, HBr

Figure 10.4 The Boiling Points of the Covalent Hydrides of the Elements in Groups 4A, 5A, 6A, and 7A

Hydrogen Bonding

London Dispersion Forces

Figure 10.5 (a) An Instantaneous Polarization can Occur on Atom A (b) Nonpolar Molecules Such as H2 also can Develop Instantaneous and Induced Dipoles

Figure 10.4 The Boiling Points of the Covalent Hydrides of the Elements in Groups 4A, 5A, 6A, and 7A

Table 10.2 The Freezing Points of the Group 8A Elements

London Dispersion Forces

• Present between any two molecules

• The only forces present between Nonpolar molecules (& between Noble Gas Atoms)

• London dispersion forces increase as size of atom or molecule increases

London Dispersion Forces

Which substance has the strongest London dispersion forces?

CH4 , CH3CH3, CH3CH2CH3

Properties

• Boiling Point• Melting Point (same as Freezing Point)

• Δ Hvap

• Δ Hmelting

• Vapor Pressure

Boiling Points

Explain the difference in boiling points: 1) n-pentane, C5H12 MW: 72.15 B.P.= 36.2 C Vs. Neopentane, C5H12 MW: 72.15 B.P.= 9.5 C

2) dimethyl ether, CH3OCH3 MW: 46.07 B.P.= -25 C Vs. ethanol, CH3CH2OH MW: 46.07 B.P. = 79 C

3) Napthalene, C10H8 MW: 128.16 B.P.= 218 C Vs. Acetic Acid, CH3CO2H MW: 60.05 B.P.= 118 C

Examples

What type of interparticle forces are present in these substances?Ar, HCl, CaCl2

What is the most important interparticle force in teflon, CF3(CF2CF2)nCF3?

Which substance has stronger intermolecular forces?SeO2 Vs. SO2

Which has the highest boiling point? NaCl or HCl

Which substance has the highest freezing point? H2O, NaCl, or HF

Summary of IM Forces

• All substances have London Dispersion Forces - London Dispersion Forces Increase w/ MW

• Dipole-Dipole Interactions - Stronger when molecules are more polar - Special Case: Hydrogen Bonding (N, O, F)

Take into account all forces as well as the size and shape of molecule when deciding which substance has the strongest IM forces

Intermolecular Force Trends

As IM Forces Increase:

• Boiling Point Increases

• Melting/Freezing Point Increases

• Δ Hvap Increases

• Δ Hmelting Increases

• Vapor Pressure Decreases

Next Lecture

• Vapor Pressure

• Phase Diagrams