Lec-37-Chap-16-2 - CHE 131 Lecture 37 Lewis Acids and Bases...

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CHE 131 Lecture 37 – Lewis Acids and Bases Chapter 16: pp. 800-802. Acid Dissociation Constant HC 2 H 3 O 2 + H 2 O Ù H 3 O + + C 2 H 3 O 2 - [H 3 O + ][C 2 H 3 O 2 - ] K = [H 2 O][HC 2 H 3 O 2 ] [H 3 O + ][C 2 H 3 O 2 - ] K a = K × [H 2 O] = [HC 2 H 3 O 2 ]
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It is not obvious which base a proton will prefer in an acid-base reaction, for example, which side will be favored in the reaction: HF + CH 3 CO 2 F + HCH 3 CO 2 To address this, we need to know the relative affinity of bases for H + . Affinity : …natural attraction… Water has a stronger affinity for protons than chloride ions. Fluoride ions have a stronger affinity for protons than water. As molecules interact primarily through electrical forces, and as adding or removing a proton changes the charge of a substance, proton affinities play a significant role in molecular interactions. Relative affinities can be identified through values of pK a , a number indicating the relative strength of an acid, and having the characteristics that the smaller the value of pK a , the stronger the acid and the weaker its conjugate base . Acid pKa Conjugate Base HClO 4 –10 ClO 4 HI –10 I H 2 SO 4 –10 HSO 4 HBr – 9 Br HCl – 7 Cl HNO 3 – 2 NO 3 H 3 O + – 1.74 H 2 O HSO 4 + 1.99 SO 4 2– H 3 PO 4 + 2.12 H 2 PO 4 HF + 3.17 F CH 3 CO 2 H + 4.75 CH 3 CO 2 NH 4 + + 9.24 NH 3 H 2 O +15.74 OH CH 3 OH +18 CH 3 O HC CH +25 HCC NH 3 +34 NH 2 CH 2 =CH 2 +36 CH 2 CH CH 3 –CH 3 +42 CH 3 CH 2 More acidic More basic I
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pK a and the Direction of an Acid-Base Reaction When 0.1 M solutions of CH 3 CO 2 H and NaF are mixed, which side of the following reaction is preferred?
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