Dispersion aka london forces caused by momentary

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Dispersion (aka “London”) Forces – caused by momentary instantaneous oscillations of electric charge (temporarily induced dipoles in molecules that otherwise are nonpolar) Consider a sample of nonpolar molecules with induced dispersion: Dispersion forces are weak, but they exist between ANY molecules/atoms, except for small very polar molecules that are able to form H-bonds. In fact, dispersion forces are the dominant IMF between identical molecules. The strength of dispersion depends on “polarizabilty”….
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Intermolecular Forces Dispersion Forces Continued Polarizability is the ease with which the electron distribution in the atom or molecule can be distorted. Polarizability increases with: greater number of electrons more diffuse electron cloud Dispersion forces usually increase with molar mass ; also molecular shape plays a role (linear, etc.).
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Smaller, more compact molecules/atoms are less polarizable than larger molecules/atoms with electrons further away from the nucleus. Linear molecules are more polarizable than branched nonlinear molecules due to the higher surface area of linear molecules. Polarizability increases with number of electrons and molar mass, so polarizability increases down a group due to increasing atomic size and electron-cloud size. However, polarizability decreases across a period due to increasing Z eff holding electrons more tightly to the nucleus. Cations are less polarizable than parent atoms (smaller than parent atoms), whereas anions are more polarizable than parent atoms (larger than parent atoms).
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Which of the following has the greatest polarizability? a) Br or I ? b) CH 2 CH 2 or CH 3 CH 3 ? c) H 2 O or H 2 Se ? d) Ca 2+ or Ca ? e) CH 3 CH 3 or CH 3 CH 2 CH 3 ? f) n-pentane or neopentane ? Answers: I because it is larger CH 2 CH 2 due to the less tightly-held π electrons H 2 Se because it is larger Ca because it is larger CH 3 CH 2 CH 3 because it is larger n-pentane because of larger surface area
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S O O What type(s) of intermolecular forces exist between each of the following molecules? HBr HBr is a polar molecule: dipole-dipole forces. There are also dispersion forces between HBr molecules. CH 4 CH 4 is nonpolar: dispersion forces. SO 2 SO 2 is a polar molecule: dipole-dipole forces. There are also dispersion forces between SO 2 molecules.
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Intermolecular Forces Hydrogen Bond The hydrogen bond is a special dipole-dipole interaction between the hydrogen atom in a polar N-H, O-H, or F-H bond and an electronegative O, N, or F atom (note the lone pair(s)). A H B A H A or A & B are N, O, or F
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Which substance exhibits H-bonding? a) H 2 O (water) ------ (yes) b) H 2 Se (hydrogen selenide) ------ (no) c) NH 3 (ammonia) ------ (yes) d) PH 3 (phosphine) ------ (no) e) C 2 H 6 (ethane) ------ (no, just dispersion forces) f) CH 3 OH (methanol) ------ (yes) g) CH 3 CONH 2 (acetamide) ------ (yes) a) CH 3 COCH 3 (acetone) ------ (no) b) CH 3 COOH (acetic acid) ------ (yes) c) H 3 PO 4 (phosphoric acid) ------ (yes)
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Why is the hydrogen bond considered a “special” dipole-dipole interaction?
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