CHEMISTRY

Intermolecular Forces

The Central Science 9th Edition

Chapter 11 Intermolecular Forces, Liquids and Solids Dr. Doug Harris Prentice Hall © 2003

• The covalent bond holding a molecule together is an intramolecular forces. • The attraction between molecules is an intermolecular force. • Intermolecular forces are much weaker than intramolecular forces (e.g. 16 kJ/mol vs. 431 kJ/mol for HCl). • When a substance melts or boils the intermolecular forces are broken (not the covalent bonds). Prentice Hall © 2003

Chapter 11

A Molecular Comparison of Liquids and Solids

Chapter 11

Intermolecular Forces

Figure 11.2 Prentice Hall © 2003

Chapter 11

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A Molecular Comparison of Liquids and Solids Figure 11.1

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Chapter 11

Chapter 11

Intermolecular Forces Ion-Dipole Forces • Interaction between an ion and a dipole (e.g. water). • Strongest of all intermolecular forces.

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Chapter 11

Intermolecular Forces Dipole-Dipole Forces • Dipole-dipole forces exist between neutral polar molecules. • Polar molecules need to be close together. • Weaker than ion-dipole forces. • There is a mix of attractive and repulsive dipole-dipole forces as the molecules tumble. • If two molecules have about the same mass and size, then dipole-dipole forces increase with increasing polarity. Prentice Hall © 2003

Chapter 11

Intermolecular Forces London Dispersion Forces • Weakest of all intermolecular forces. • It is possible for two adjacent neutral molecules to affect each other. • The nucleus of one molecule (or atom) attracts the electrons of the adjacent molecule (or atom). • For an instant, the electron clouds become distorted. • In that instant a dipole is formed (called an instantaneous dipole). Prentice Hall © 2003

Intermolecular Forces Dipole-Dipole Forces

Chapter 11

Intermolecular Forces London Dispersion Forces • One instantaneous dipole can induce another instantaneous dipole in an adjacent molecule (or atom). • The forces between instantaneous dipoles are called London dispersion forces. Figure 11.5

Figure 11.4 Prentice Hall © 2003

Intermolecular Forces Dipole-Dipole Forces

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Chapter 11

Chapter 11

Intermolecular Forces London Dispersion Forces • Polarizability is the ease with which an electron cloud can be deformed. • The larger the molecule (the greater the number of electrons) the more polarizable. • London dispersion forces increase as molecular weight increases. • London dispersion forces exist between all molecules. • London dispersion forces depend on the shape of the molecule. Prentice Hall © 2003

Chapter 11

Intermolecular Forces London Dispersion Forces • The greater the surface area available for contact, the greater the dispersion forces. • London dispersion forces between spherical molecules are lower than between sausage-like molecules.

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Chapter 11

Intermolecular Forces Hydrogen Bonding • Special case of dipole-dipole forces. • By experiments: boiling points of compounds with H-F, H-O, and H-N bonds are abnormally high. • Intermolecular forces are abnormally strong.

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Intermolecular Forces London Dispersion Forces

Chapter 11

Intermolecular Forces Hydrogen Bonding • H-bonding requires H bonded to an electronegative element (most important for compounds of F, O, and N). – Electrons in the H-X (X = electronegative element) lie much closer to X than H. – H has only one electron, so in the H-X bond, the δ+ H presents an almost bare proton to the δ- X. – Therefore, H-bonds are strong.

Figure 11.6 Prentice Hall © 2003

Hydrogen Bonding

Intermolecular Forces London Dispersion Forces

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Chapter 11

Chapter 11

Figure 11.8

Some Properties of Liquids

Intermolecular Forces Hydrogen Bonding • Hydrogen bonds are responsible for: – Ice Floating • • • • • • • •

Solids are usually more closely packed than liquids; Therefore, solids are more dense than liquids. Ice is ordered with an open structure to optimize H-bonding. Therefore, ice is less dense than water. In water the H-O bond length is 1.0 Å. The O…H hydrogen bond length is 1.8 Å. Ice has waters arranged in an open, regular hexagon. Each δ+ H points towards a lone pair on O.

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Chapter 11

Viscosity • Viscosity is the resistance of a liquid to flow. • A liquid flows by sliding molecules over each other. • The stronger the intermolecular forces, the higher the viscosity. Surface Tension • Bulk molecules (those in the liquid) are equally attracted to their neighbors. Prentice Hall © 2003

Chapter 11

Some Properties of Liquids

Intermolecular Forces Hydrogen Bonding

Viscosity

Figure 11.10 Prentice Hall © 2003

Intermolecular Forces

Chapter 11

cP − centipoise poise-1g/cm·s

Surface Tension

Figure 11.12

Figure 11.14

Some Properties of Liquids Surface Tension • Surface molecules are only attracted inwards towards the bulk molecules. – Therefore, surface molecules are packed more closely than bulk molecules.

• Surface tension is the amount of energy required to increase the surface area of a liquid. • Cohesive forces bind molecules to each other. • Adhesive forces bind molecules to a surface. Prentice Hall © 2003

Chapter 11

Some Properties of Liquids Surface Tension • Meniscus is the shape of the liquid surface. – If adhesive forces are greater than cohesive forces, the liquid surface is attracted to its container more than the bulk molecules. Therefore, the meniscus is U-shaped (e.g. water in glass). – If cohesive forces are greater than adhesive forces, the meniscus is curved downwards.

• Capillary Action: When a narrow glass tube is placed in water, the meniscus pulls the water up the tube. Prentice Hall © 2003

Chapter 11