Chapter 11: Liquids and Intermolecular Forces

Units 11.1-11.5

Intermolecular Forces

The forces that exist between molecules

• Electrostatic — Attractions between negative and positive ends

What is the relationship between the kinetic energy of particles and the energies of particle—particle attractions for each state of matter?

Gases — K_E of particles > energy of particle-particle attraction

• Particles are spread out

• Experience complete freedom of motion

Liquids — K_E of particles ~ energy of particle-particle attraction

• Particles are packed together but randomly oriented

• Have freedom of motion

Solids — K_E of particles < energy of particle-particle attraction

• Particles are organized into a closely packed array

• Particles’ positions are fixed in place

Kinetic Energy vs. Interparticle Attractions

• K_E — Depends on the temperature and is the separating force

• Interparticle Attractions — Draws particles together

How does one change the state of a substance?

Through heating and cooling

Intermolecular Forces vs. Intramolecular Forces

Intermolecular — Between molecules

Intramolecular — Between the atoms that constitute the molecule

• It’s the reason why phase changes keep the molecular compounds intact

• Boiling point depends on the strength of intermolecular forces

  • Stronger IMFs → More energy needed to separate the particles

    • molecules want to stay together more → higher boiling(liquids) point and higher melting point(solids)

• Substances with chemical bonds(i.e. covalent/ionic/metallic where the bonds are responsible for their structure) have much higher melting and boiling points than molecules only held together by IMFs (i.e. N₂)

What are the three types of intermolecular attractions that exist between electrically neutral molecules

1. dispersion forces

2. dipole-dipole attractions

3. hydrogen bonding

Which two intermolecular attractions are called van der Waals forces?

• dispersion forces

• dipole-dipole interactions

Electrostatic attractions

• Stronger as the magnitude of the charges increases

• Weaker as the distance between charges increases

Why are IMFs weaker than ionic bonds

• The distance between molecules are generally larger than the distance between atoms

• The charges responsible for intermolecular forces are generally much smaller than those of ionic compounds

Dispersion Forces

AKA — London Dispersion Forces; Induced Dipole-Induced Dipole Interactions

• Present in all substances, only force present in nonpolar molecules

• Based on the fact that the instantaneous distribution of electrons can yield a polar atom

  • Causes an instantaneous dipole moment — Influences the motion of electrons in neighboring atoms; hence why it’s dipole induced

• Only taken into account when molecules are close together

Polarizability

• The ease with which the charge distribution of a molecule can be distorted to induce an instantaneous dipole.

• More polarizable molecules have larger dispersion forces

• Polarizability increases as a result of the following:

  • numerous—as the number of electrons in an atom or molecule increases

  • bigger—as the size of the atomic or molecule increases

  • heavier—increase as the molecular weight of a molecule increases

Dipole-Dipole Interactions

• Electrostatic interaction between the negative side and positive side of molecules

• Caused by the permanent dipoles present in polar molecules

• Only effective when molecules are very close together

• Strength of intermolecular attraction increases with increasing polarity.

Hydrogen Bond

• An attraction between a hydrogen atom attached to a highly electronegative atom

  • Usually F, O, or N

• Involves a lone pair of electrons on the acceptor atom.

  • Can’t form with atoms that lack lone pairs (i.e. Carbon)

• Considered a “special dipole-dipole interaction”

• Hydrogen is a small atom, allowing the nearly completely positive atom to interact very closely and therefore strongly with an electronegative atom. (i.e. F, O, or N)

Ion-Dipole Forces

• Between an ion and a polar molecule

• Important for ionic substances in polar liquids

Generalizations of comparing relative strengths of intermolecular attractions

1. When the molecule of two substances have comparable molecular weights and shapes, dispersion forces are approximately equal in the two substances.

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2. When the molecules of two substances differ widely in molecular weights, and there is no hydrogen bonding, dispersion forces tend to determine which substance has the stronger intermolecular attractions.

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3 properties of liquids

1. Viscosity

2. Surface Tension

3. Capillary Action