TOPIC 4 CHEMICAL BONDING AND STRUCTURE 4.4 INTERMOLECULAR FORCES

ESSENTIAL IDEA The physical properties of molecular substances result from different types of forces between their molecules. NATURE OF SCIENCE (2.2) Obtain evidence for scientific theories by making and testing predictions based on them – London dispersion forces and hydrogen bonding can be used to explain special interactions. For example, molecular covalent compounds can exist in the liquid and solid state. To explain this, there must be attractive forces between their particles which are significantly greater than those that could be attributed to gravity.

INTERMOLECULAR FORCES Intermolecular forces are forces between molecules. Intramolecular forces are forces within the molecule such as covalent, ionic and metallic bonding. The strength of intermolecular forces determines the volatility of a substance. The stronger the forces, the higher the melting and boiling points.

UNDERSTANDING/KEY IDEA 4.4.A Intermolecular forces include London dispersion forces, dipole-dipole forces and hydrogen bonding.

LONDON DISPERSION FORCES These are the weakest of the intermolecular forces . Substances held together with London dispersion forces have low melting and boiling points and are often gases at room temperatures. These forces occur when electron clouds shift to form temporary dipoles which then induce dipoles in neighboring molecules so that they can attract each other. The larger the electron cloud, the stronger the London dispersion forces because there is a higher probability of temporary dipoles forming.

London dispersion forces are responsible for the fact that non-polar molecules can be condensed to form liquids and sometimes solids. London dispersion forces are also present in polar molecules but are often overlooked because they are so much weaker than dipole-dipole forces.

DIPOLE-DIPOLE ATTRACTION These are stronger than London dispersion forces and the strength depends upon the degree of polarity. These intermolecular forces are caused when molecules with permanent dipoles (areas of pos/neg charge) attract each other. The stronger the dipole attraction – the higher the boiling point.

HYDROGEN BONDING This is the strongest of the intermolecular forces . Hydrogen bonding is a type of dipole-dipole attraction. Hydrogen bonds form when hydrogen bonds to either nitrogen, fluorine or oxygen. The strength of the hydrogen bond is due to the small size of hydrogen and the large electronegativity of N, O and F. Water is a unique substance with hydrogen bonding in that the solid is less dense than the liquid.

UNDERSTANDING/KEY IDEA 4.4.B The relative strengths of these interactions are London dispersion forces < dipole-dipole forces < hydrogen bonds.

APPLICATION/SKILLS Be able to deduce the types of intermolecular forces present in substances, based on their structure and chemical formula.

GUIDANCE The term “London dispersion forces” refers to instantaneous induced dipole- induced dipole forces that exist between any atoms or groups of atoms and should be used for non-polar entities. The term “van der Waals forces” is an inclusive term, which includes dipole-dipole, dipole-induced dipole and London dispersion forces.

All molecules will have some type of van der Waal’s force. Non-polar molecules have only London dispersion forces. Polar molecules have dipole-dipole forces and London dispersion forces. Hydrogen bonding exists when the positive hydrogen bonds with lone pairs of electrons on nitrogen, oxygen and fluorine.

van der Waals’ forces The umbrella term “van der Waals’ forces is used to include both London dispersion forces and dipole-dipole attractions. It also covers the less common type of attraction known as the dipole-induced dipole. It refers to all forces between molecules that do not involve electrostatic attractions between ions or bond formation. London Dispersion Force: Cl2 – Cl2 Dipole – Dipole attraction: HCl – HCl Dipole-induced dipole: HCl – Cl2

APPLICATION/SKILLS Be able to explain the physical properties of covalent compounds (volatility, electrical conductivity, and solubility) in terms of their structure and intermolecular forces.

Volatility, solubility and conductivity can all be predicted and explained from knowledge of the nature of the forces between molecules.

VOLATILITY The weaker the intermolecular force, the higher the volatility. London dispersion > dipole-dipole > hydrogen bonding Ionic compounds and giant covalent compounds have low volatility. Ionic, giant covalent< polar covalent < non polar

ELECTRICAL CONDUCTIVITY Covalent compounds do not contain ions; therefore, they do not conduct electricity in the solid or liquid state. Some polar covalent compounds, such as HCl which can ionize in water, will conduct electricity. Ionic compounds conduct electricity in the molten or aqueous state. Giant covalent structures are generally non-conductors except for graphite, graphene, fullerene and Si.

SOLUBILITY Non-polar compounds dissolve in non-polar solvents. Polar compounds dissolve in polar solvents. Solubility is reduced in larger molecules where the polar bond is only part of the total structure. Ionic compounds are very soluble in water and non-soluble in non-polar solvents. Giant covalent structures are non-soluble in both polar and non-polar solvents due to the very strong covalent bonds within their structure.

BOILING/MELTING POINT COMPARISON The stronger the force, the higher the boiling and melting points. Hydrogen bonding > dipole-dipole > London dispersion forces