CHEM301 F08 CA3BrevKey

CHEM301 F08 CA3BrevKey - CHEM301, Fall 2008 Dr. Ruder 1...

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CHEM301, Fall 2008 Dr. Ruder 1 Class Activity 3B Acids and Bases Part B : Predicting Acid/Base Strength Without pKa Values Model 1: Stability of Conjugate Base The strength of an acid can be predicted by estimating the stability of the conjugate base formed. The stability of a base is affected by the following factors: formal charge , electronegativity, inductive electron withdrawing effect , size , and resonance stabilization. H + A acid conjugate base H A Questions: (Refer to the Lewis acid/base definition for the following questions) 1. Consider the reverse direction of the reaction in Model 1 : (a). If the base A- is stable, is it more or less likely to donate an electron pair (to form HA)? (b). If the base A- is unstable, is it more or less likely to donate an electron pair (to form HA)? (c). Is an unstable base more reactive or less reactive? 2 . In the forward direction of the reaction in Model 1 : (a). If the conjugate base A- is stable, would the acid be more or less likely to accept an electron pair? (b). If the conjugate base is unstable, would the acid be more or less likely to accept an electron pair? (c). The more stable the conjugate base, then is the acid it is derived from stronger or weaker? 3. (a) . Draw the conjugate base produced on dissociation of the acids in the following two reactions. Calculate the formal charge of oxygen for each conjugate base. O H O H H H H (b). Which conjugate base above would you expect to be more stable and why? (c). Based on the stability of the conjugate base, predict the direction of the equilibrium for each reaction. (d). Based on the stability of the conjugate base, which acid would be the strongest, H 3 O+ or H 2 O? Explain.
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CHEM301, Fall 2008 Dr. Ruder 2 4. (a). Draw the conjugate base produced on dissociation of CH 4 shown below. Calculate the formal charge of carbon in the conjugate base. C H H H H (b). Which would you expect to be more stable, CH 4 or the conjugate base? Why? (c). Predict the direction of the equilibrium based on your answer above. (d).
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This note was uploaded on 02/17/2011 for the course CHEM 301 taught by Professor Sahli during the Fall '07 term at VCU.

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CHEM301 F08 CA3BrevKey - CHEM301, Fall 2008 Dr. Ruder 1...

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