BCH 261 Lecture Notes - Lecture 3: Reversible Reaction, Acid Strength, Acid Dissociation Constant
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30 Nov 2020
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Lecture 3 - Weak Acids & Bases
● Weak acid: partially dissociates/donates a H+
○ Proton donors
○ Some compounds, such as acetic acid and ammonium
○ ion, are monoprotic; they can give up only one proton. Others are diprotic
(carbonic acid and glycine) or triprotic (phosphoric acid).
● Weak base: partially ionized to accept H+
○ Proton acceptors
● When a proton donor such as acetic acid (CH3COOH) loses a proton, it
becomes the corresponding proton acceptor, in this case the acetate anion
(CH3COO-). A proton donor and its corresponding proton acceptor make up a
conjugate acid-base pair, related by the reversible reaction :
\
● Conjugate Acid-Base pairs
○
○
● Acid Dissociation Constant, Ka
○ HA ⇔ H+ + A-
○ Weak acids and weak bases reach a chemical equilibrium defined by the
acid constant, Ka: 𝐾𝑒𝑞 = [𝐻+][𝐴−]
[𝐻𝐴]= Ka
■ Stronger the acid, larger Ka value
■ Can rate WA or conjugate acids of WB using Ka
● Henderson Hasselbalch: pH = pKa + log[𝐴−]
[𝐻𝐴]
○ Example: Calculate the pH of a buffer solution prepared by dissolving
363 mg of Tris in 10 mL of 0.2M HCl and diluting to 100 mL with
water. [Tris: mw 121 g/mol and pKa = 8.08 for the conjugate acid]
■