IMFA s. intermolecular forces of attraction Chez Chem, LLC All rights reserved.

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1 IMFA s intermolecular forces of attraction 2014 Chez Chem, LLC All rights reserved.

2 **London Dispersion Forces Also know as Van der Waals forces A momentary non symmetrical electron distribution that can develop giving a temporary dipole arrangement of charge called an INSTANTANEOUS DIPOLE Weak and short lived Noble gases freezing point goes up as you increase atomic number because of the size of the atoms and the number of electrons. What affects the strength of a dispersion force? Molecules must be very close together for these attractive forces to occur. 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 it is. LD forces increase as molecular weight increases. LD forces depend on the shape of the molecule. The greater the surface area available for contact, the greater the dispersion forces. London dispersion forces between spherical molecules are smaller than between more cylindrically shaped molecules.

3 Strength of LD forces What affects the strength of a dispersion force? Molecules must be very close together for these attractive forces to occur. 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 it is. LD forces increase as molecular weight increases because there are more electrons. LD forces depend on the shape of the molecule. The greater the surface area available for contact, the greater the dispersion forces.

4 Hydrogen bonding A special type of bonding between hydrogen and the NOFs or FONs nitrogen, oxygen and fluorine A special type of dipole dipole Occurs with atoms of high electro negativity and small size Cause of the spaces in crystal lattice of water so that water as a solid EXPANDS most solids do not. Dipole-dipole forces exist between neutral polar molecules. Polar molecules attract one another. The partially positive end of one molecule attracts the partially negative end of another. Polar molecules need to be close together to form strong dipoledipole interactions. Dipole-dipole forces are weaker than ion-dipole forces. If two molecules have about the same mass and size, then dipoledipole forces increase with increasing polarity.

5 Crystals Crystal Lattice Energy: Solids have a crystal lattice Smallest repeating unit is a UNIT CELL Determined by X-ray diffraction Coulombs law Attractive and repulsive forces lattice energy INCREASES as the charges on the ions increase and as their radii decrease.

6 Type of Solid Ionic Molecular Nonpolar Melting Point Very high Types of Solids Conductivity Electrical and thermal conductors Physical Properties Hard, malleable, ductile Unit at Lattice Sites + and - ions Very low insulators soft Non polar molecules Polar moderate insulators intermediate Polar molecules Network covalent Metallic Atomic Extremely high Insulators or semiconductors Extremely hard atoms variable conductors variable Positive ions Force between Units Coulombs, electrostatic forces London dispersion forces (LD) LD, H- bonds, dipole Covalent bonds Sea of electrons Eg. salts CO 2, HCs Water, sugar Diamond, quartz, sand Cu, brass, alloys Extremely low insulators atoms LD Ne, He

7 Polar Liquids Surface tension the resistance of a liquid to an increase in surface area sphere has least amount of surface area Viscosity the measure of a liquid s resistance to flow Capillary action - Spontaneous rising of liquid in a narrow tube

8 Cohesive forces are intermolecular forces that bind molecules to one another. Adhesive forces bind molecules to a surface Illustrate this by looking at the meniscus in a tube filled with liquid. The 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 to the bulk molecules. Therefore, the meniscus is U-shaped (e.g., water in a glass). If cohesive forces are greater than adhesive forces, the meniscus is curved downward (e.g., Hg(l) in glass)

9 Volatile Liquids Weak imfa s Low boiling point Evaporates readily High vapor pressure Liquid A is volatile normal boiling point is at 68 o C Liquid C is not volatile normal boiling point is at 100 o C

10 Strength of imfa s Rated from strongest to weakest: Network covalent Ionic Metallic - large variety Molecular - hydrogen bonding (a special type of dipole force) Dipole LD * can be stronger than dipole forces Molecular non polar Atomic