Lecture 11- Intermolecular Forces

General Chemistry Course # 111, two credits Second Semester 2009

King Saud bin Abdulaziz University for Health Science Textbook: Principles of Modern Chemistry by David W. Oxtoby, H. Pat Gillis, and Alan Campion (6 edition; 2007)

Dr. Rabih O. Al-Kaysi Ext: 47247 Email: [email protected]

Lecture 11

Intermolecular Forces

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A Molecular Comparison of Liquids and Solids

• Physical properties of substances understood in terms of kinetic molecular theory: • Liquids are almost incompressible, assume the shape but not the volume of container: – Liquids molecules are held closer together than gas molecules, but not so rigidly that the molecules cannot slide past each other.

• Solids are incompressible and have a definite shape and volume: – Solid molecules are packed closely together. The molecules are so rigidly packed that they cannot easily slide past each other.

Comparison of Liquids and Solids Cont.

Intermolecular Forces •These are the forces holding solids and liquids together are called intermolecular forces.

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What are Intermolecular 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). • However, when a substance condenses, intermolecular forces are formed.

Vaporizing HCL intermolecular forces are broken

Vapor phase

Formation intermolecular forces

Heating Liquid Solutions

Liquid phase

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Inter- vs. Intramolecular Forces

The covalent bond holding a molecule together is an intramolecular force.

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Properties Reflecting Molecular Force Strengths • Boiling and melting points reflect the strengths of intermolecular forces. • High boiling points indicate strong attractive forces between molecules. - For example, HCl boils at -85oC at room temperature due to its weak attractive forces.

• Melting points increase with increasing attractive forces (i.e., molecules become harder to separate).

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Types of Molecular Forces • There are four types of molecular forces: • • • •

Ion-dipole Forces Dipole-dipole Forces London Dispersion Forces Hydrogen Bonding Forces

• The lateral three forces are general called van der Waals forces (developed by Johannes van der Waals) and exist between neutral molecules • The ion-dipole forces exist between ions and polar molecules.

Water (H2O)

Molecular Polarity

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

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Dipole-dipole Forces • Dipole-dipole forces exist between neutral polar molecules. • Only effective when polar molecules are close together. • These forces are weaker than ion-dipole forces. • There is a mix of attractive and repulsive dipole-dipole forces as the molecules tumble (free flow in liquids) • If two molecules have about the same mass and size, then dipole-dipole forces increase with increasing polarity.

Dipole-dipole Forces Schematic

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London Dispersion Forces • Weakest of all intermolecular forces. • Primary property that cause nonpolar substances to condense to liquids and to freeze into solids at low temperatures. • Form when electrons occupy positions around the nucleus in two adjacent atoms causing a temporary dipole. • The nucleus of one molecule (or atom) attracts the electrons of the adjacent molecule (or atom).

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Formation of London Dispersion Forces

• 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).

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Properties Effecting London Dispersion Forces

• Dispersion forces are present in all molecules whether polar or nonpolar • The larger the molecule (the greater the number of electrons) the more polarizable. • London dispersion forces increase as molecular weight increases. • London dispersion forces depend on the shape of the molecule. • Example: neopentane (gas at 25oC), n-pentane (liquid at 25oC)

Trends in London Dispersion Forces

Notice that as the molecular weight increases the boiling points of the halogen increases, indicating greater London dispersion forces between atoms.

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Hydrogen Bonding • By experiments: boiling points of compounds with HF, H-O, and H-N bonds are abnormally high. • In the case of NH3, H2O, and HF, additional intermolecular forces must be present which increases the amount of heat energy needed to separate the atoms. • These additional intermolecular forces are called hydrogen bonds.

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The Origin of Hydrogen Bonding

• Notice that the hydrogen is attached to the most electronegative elements. Thus, causing the hydrogen to acquire a significant amount of positive charge.

Hydrogen Bonding Schematic

Hydrogen Bonding in H2O • Hydrogen bonds are responsible for: • Ice Floating – 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. – Each δ + H points towards a lone pair on O.

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Comparing Intermolecular Forces • Dispersion forces are found in all substances. • Their strength depends on molecular shapes and weights.

• Dipole-dipole forces add to the effect of dispersion forces. • They are found only in polar substances.

• H-bonding is a special case of dipole-dipole interactions. • Strongest of the intermolecular forces involving neutral species. • Most important for hydride compounds (NH3, H2O, etc.).

• Ion-dipole forces are interactions between ionic and polar molecules. • Ion-dipole are stronger than H-bonds.

• Covalent bonds are stronger than any of these reactions.

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Intermolecular Forces Chart