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ASSIGNMENT 10(ON INTERMOLECULAR FORCES AND STRENGTH OF ACIDS) (DRAW Lewis structures and 3-D for all molecules before you answer the question; A useful summary for INTERMOLECULAR FORCES: DISPERSION forces are important in all large molecules.(not just in non polar moleccules) The following flow chart may help you. Intermolecular forces ------------------------------------------------------------------------------------------------------ IONIC COVALENT (Strongest intramoleculer force) ions only ions in aq.soln. 2 polar molecules polar+nonpolar non polar Ion-ion NaCl Ion-dipole NaCl in water Dipole- Dipole H 2 O + alcohol Dipole +induced dipole Water +hexane Induced dipole- induced dipole I 2 ---I 2 Note: Always ask after the above is considered, is there a N, O or F present. Then there is H bonding. A special case of dipole-dipole. The above comes into play when substances of similar sizes are considered. If not, Size is the primary consideration, especially when dipole is not present to a significant extent . Eg: As you go down the period, size becomes larger and dispersion forces are higher. Remember to draw 3-D structures before you decide on polarity. In fact make it a rule to draw them out. SUMMARY OF IMPORTANT POINTS FOR ACIDS AND BASES: 1. To determine strength of acids, Ka is the determining factor. Learn to write expressions for Ka and understand the significance of the energy diagrams in your notes. 2. When asked to compare the strengths of acids, consider the stability of the negative charge on the conjugate base. Draw the structure of the conjugate base and see what factor delocalises the charge on the conjugate base. 3. Remember more de-localised the charge, more stable the conjugate base, less reactive the conjugate base and stronger the acid. The ka being high, Pka is low.
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Remember the significance of Pka on titration curves and draw them properly and assign the Pka’s correctly. 4. The factors that delocalises the charge on conjugate base: Size( ex; HF, HCl, HBr, HI; (b) Electronegativity(NH 3 , H 2 O, HF) (c) presence of electronegative groups like, F, Cl Br. (d) Presence of methyl groups, which are weal electron donators, localises the charge and make them unstable.(e) Resonance is one of the more important factors that delocallises the charge/ Draw Resonance structures of the conjugate base to illustrate this. USUALLY THE H atom attached to a carbon or oxygen or anything else attached to a sp2 or sp3 hybridised atom(or carbon) is acidic. For Bases: Remember bases as electron donors. What are the important factors that prevent the basic part from donating the lone pair of electrons. These would make the base weaker. a) Resonance delocalises the lone pair and the base is weaker. b) Presence of an electronegative atom draws the lone pair from the basic atom and
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This note was uploaded on 11/15/2010 for the course CHEE 638 taught by Professor Hampton during the Spring '10 term at MO Southern.

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