• Document: INTERMOLECULAR FORCES
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___________________________INTERMOLECULAR FORCES___________________________ Intermolecular forces- forces of attraction and repulsion between molecules that hold molecules, ions, and atoms together. Intramolecular - forces of chemical bonds within a molecule Boiling Point and Electronegativity Boiling Point - the temperature at which the liquid form of a compound vaporizes into a gas. - the attractive forces in the liquid must be overcome to become a gas. - boiling point of 0 = no attractive force Trends: Boiling Point example from Bruice 5.2 Explain trend in boiling points of H Halides: HCl, - 85`C; HBr, -67`C; HI, -15`C Answer: Stronger intermolecular forces result in higher boiling points. The dipole moments increase with the polarity of the H-X bond (the greater the difference in EN between the H and halogen atoms, the stronger the dipole-dipole attractions.) The strength of London forces increases with the number of electrons Cl is more EN than I so HCl should have a greater boiling point but this data suggests that London forces dominate dipole-dipole interactions for these molecules since Iodine is larger than Cl. Relative Boiling Points amide > carboxylic acid> nitrile > > Ester ~acyl chloride ~aldehyde~ keytone Amides have highest boiling points because they have strong dipole-dipole interactions because of resonance. Electronegativity - the attractive force that an atom exerts on an electron (e-) - closer e- to nucleus = more electronegative - more protons, more positive, more attractive to e- Electronegative difference:  Ionic Bonds = >2.0  Polar Covalent Bonds = 0.5 - 2.0  Nonpolar Covalent Bonds (equal sharing of e-) = < 0.5 - Electrostatic Attraction (common in every noncovalent molecular force)= when opposite charges with permanent dipoles attract bonding IONIC_________________________________________________________________________________ Ionic Bond (strongest bond)  a complete transfer of one or more e- occurs when one interacting atom is much more electronegative than the other.  one gives up e- (cation) and one takes e- (anion) making ions that are electrostatically attracted.  It takes a lot of energy to break ionic bonds and to turn the molecule into a gas STRENGTH: strongest bond TREND: The greater the charge difference, the stronger the attraction DIPOLE-DIPOLE_______________________________________________________________________ - sharing of electrons in a bond is covalent but not completely even (Polar Covalent)  There is a polar covalent bond between the Br and Cl atoms. The electron density shifts towards chlorine since it is more electronegative than bromine. Chlorine thus has a slight negative dipole and bromine has a slight positive dipole.  The negative dipole of the chlorine is attracted to the positive dipole of the bromine of another BrCl molecule. This dipole-dipole attraction accounts for the BrCl b.p. of 5`C TRENDS:  Electronegative Difference. Polarity is amplified when electronegativity difference is large and minimized when the difference is small.  Bond Length (primarily atomic radii of the bonding partners)  Increasing bond length decreases dipole HYDROGEN BONDING_________________________________________________________________  A dipole-dipole interaction where the H atom is noncovalently attracted to an electronegative atom  The H must have a large positive delta and the other atom must have a source of electrons to be attracted to H  Usually negatively charged F, O, N is attracted to the positively charged H  Very important biologically STRENGTH:  a H bond is a weak bond and is dynamic [3x10^-12 lifetime]  In solid phase H-bond is static  H-bonds between molecules continually form and break in liquid.  Every water molecule forms 3.4 H bonds with other water molecules TREND:  The more electronegative the atom attracted to H, the stronger the bond LONDON / VAN DER WAALS FORCES___________________________________________________ Principle : Electron can respond to changes must faster than nucleus London Dispersion Forces/ Van der Waals Forces  The attraction between instantaneous electric dipoles on adjacent molecules  Present in all molecules because everything has an electron cloud STRENGTH:  Weak because temporary TRENDS:  Strength increases with # of electrons in molecule  strength depends on the polarizability [ease with which their electron clouds can be distorted]  Molecules with lesser EN and larger radii have higher polarizability.  Higher polarizability = stronger attraction. _______________________________________________________________________________________________ TRENDS: 1. The smaller the atom, the harder it is to dist

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