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Bond
INTERMOLECULAR
FORCES (IMF)
Have you noticed? Acetone never last….it can never be stored for long period….. In cooking, experienced cook will boil water first before starting cooking meat and adding condiments…. There are solids that can be hammered to thin sheets while other solids can’t…. Types of Intermolecular forces 1. London Dispersion Forces (LDF) 2. Dipole-Dipole Forces 3. Hydrogen Bond 4. Ion-Dipole forces 1. London Dispersion Forces (LDF) • exist for all substances, whether composed of polar or nonpolar molecules • arise from the formation of temporary instantaneous polarities across a molecule from the circulations of electrons • an instantaneous polarity in one molecule may induce an opposing polarity in an adjacent molecule, resulting in a series of attractive forces among neighboring molecules • a weak attractive force 2. Dipole-Dipole Forces (DDF) • exist between molecules that are polar • This uneven sharing gives one side of the molecule a partial positive charge (δ+) and the other side a partially negative charge (δ-) • Substances with dipole-dipole attractions tend to have higher melting and boiling points compared to nonpolar molecules, which only have LDF. Polarity is the key…… • Determining the polarity of a molecule is essential in identifying IMF existing between molecules • To determine polarity of molecule: 1. Identify the bonds between atoms in a molecule (polar bond or non polar bond) 2. Examine symmetry of molecular structure (symmetry may cancel poles resulting to a nonpolar body or molecule) Identify bond type: Polar bond or Non polar bond 1. Identify the electronegativity (EN) values of bonding atoms. 2. Get the absolute difference of EN values of bonding atoms. /EN-EN/ 3. Interpret the difference: • 0.0 to 0.3 – non polar bond • > 0.3 – polar bond e.g. Carbon monoxide (CO) = /2.5 – 3.5/ = 1.0 polar bond؞ Let’s practice: Determine the bonds in the following molecules. 1. MgCl2 – Magnesium chloride 2. CH3OH - Methanol Answer: #1 Mg-Cl (2 bonds) = /EN – EN/ = /1.2 – 3.0/ = 1.8 ؞ polar bond Answer: #2 * C-H (3 bonds) * C-O * O-H = /EN – EN/ = /EN – EN/ = /EN – EN/ = /2.5 – 2.1/ = /2.5 – 3.5/ = /3.5 – 2.1/ = 0.4 = 1.0 = 1.4 ؞ polar bond ؞ polar bond ؞ polar bond Let’s practice
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Polar molecule or Non polar molecule POLAR molecule: polar bond exists and the structure is asymmetrical NON POLAR molecule: no polar bond exists or 2 or more polar bonds and structure is symmetrical Let’s try this Molecule Bond type (polar bond or non polar bond) Symmetry (symmetrical or asymmetrical) Polarity of molecule (Polar or Non polar) Acetone Methane asymmetrical C-H : polar bond (6) C-O : Polar bond (1) C-C : non polar bond (2) C-H: polar bond (4) Symmetrical POLAR molecule NON POLAR molecule
