University:
Purdue University
Course:
CHM 11500 | General Chemistry
Academic year:

2022
Views:

493

Pages:

4
Author:

Elliott James

4/28/05 Lecture - CHM 123 (Monday, 10:30am) Quiz 6 - next week, 10/10/05 and 12/8/05 (Tuesday) (Thursday) Molecular Polarity Bonds between atoms can be polar. A molecule with polar bonds may or may not be polar. If the polar bonds line up so that they cancel each other out, the molecule is nonpolar. e.g., O=C=O - Although C=O bond is polar (difference in electronegativity between C and O), the molecule is nonpolar because the 2 polarity vectors are equal in magnitude but opposite in direction and cancel each other out. If the polar bonds do not line up so that they don't cancel each other, the molecule is polar. e.g., Cl-Cl H-Cl nonpolar polar molecule, HCl. molecule General trends regarding dipole moments: A central atom with no lone pairs surrounded by other atoms of all the same type will NOT have a dipole moment (CH4, CO2, SF6). Central atom with no lone pairs surrounded by other atoms of not all the same type WILL usually have a dipole moment (CH3Cl, NO2Cl, OCN-). Central atom with lone pairs surrounded by other atoms of any type WILL almost always have a dipole moment (H2O, NH3, ICl3). There are exceptions to this trend. Diatomic molecules containing the same atoms will NOT have a dipole moment (O2); a diatomic molecule containing different atoms WILL have a dipole moment (CO). Exceptions to trend #3 - Molecule has no dipole moment. - Symmetrically distributed lone pairs. Intermolecular Interactions Responsible for the internal 'structure' of liquids and solids. Always weaker than chemical bonds. Less directional than covalent bonds but more directional than ionic bonds. Operate at longer ranges than covalent bonds but at shorter ranges than ionic bonds. Stronger the intermolecular interactions, the higher the m.p (melting point) and b.p (boiling pt.) of a compound. Types of Intermolecular Interactions (a) London (Dispersion Forces): Occur in all molecular substances. Result from attraction between positive and negative ends of induced dipoles in adjacent molecules. Induced dipoles - instantaneous dipoles formed because of the movement of electrons in a molecule (polarization). Increase with increasing number of electrons in molecule. e.g. I2 should have stronger dispersion forces than F2. (b) Dipole-Dipole Interactions: Exist between polar molecules. Result from attraction between positive and negative ends of permanent dipoles in adjacent molecules. Usually stronger than London forces.