CHEM301 F10 CA3Bkey

CHEM301 F10 CA3Bkey - CHEM301, Fall 2010 Dr. Ruder 1 Class...

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CHEM301, Fall 2010 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 AΘ is stable, is it more or less likely to donate an electron pair (to form HA)? (b). If the base AΘ 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 conj. base AΘ is stable, would HA be more or less likely to accept an electron pair? (b). If the conj. base AΘ is unstable, would HA 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? H2O because it is neutral with a formal charge of zero on oxygen. (c). Based on the stability of the conjugate base, predict the direction of the equilibrium for each reaction. (see arrows above) (d). Based on the stability of the conjugate base, which acid would be the strongest, H 3 O+ or H 2 O? Explain. H 3 O+ is the stronger acid because it forms the more stable, neutral conjugate base. 4. (a). Draw the conjugate base produced on dissociation of CH 4 shown below. Calculate the formal charge of carbon in the conjugate base.
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CHEM301, Fall 2010 Dr. Ruder 2 C H H H H (b). Which would you expect to be more stable, CH 4 or the conjugate base? Why? CH4 is more stable because it is neutral. (c). Predict the direction of the equilibrium based on your answer above. (see arrows above) (d). The pKa for methane (CH 4 ) is approximately 50. Does this agree with your conclusions on the stability of the conjugate base? Explain. Yes, the conjugate base is unstable with the negative charge on the less electronegative carbon. Thus, the acid is weak and has a high pKa. 5
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This note was uploaded on 04/30/2011 for the course CHEM 301 taught by Professor Sahli during the Spring '07 term at VCU.

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CHEM301 F10 CA3Bkey - CHEM301, Fall 2010 Dr. Ruder 1 Class...

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