The Acidity Constant, K a

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1 The Acidity Constant, K a The strength of an acid is represented by its ionization constant (acidity constant), K a K a = product of concentrations of ionized species concentration of intact acid K a = The Acidity Constant, K a The K a implies the concentrations of the acid and the ions K a > 1 Ionized products greater than intact acid. K a < 1 Ionized products less than intact acid. K a >> 1 Ionization goes to completion (strong acid). (e.g., > 10 3 ) K a << 1 Ionization does not occur to an appreciable amount. (e.g., < 10 3 ) 1

2 The Acidity Constant, K a Since the K a values for various acids have such a wide range, a more manageable way to discuss this measure of acidity is to use pk a = log (K a ) Compare pk a and K a Values strong acids weak acids pk a K a The smaller the value of the pk a the stronger the acid. 2

3 The Henderson-Hasselbalch Equation HA + H 2 O H 3 O + + A - [H 3 O + [HA] ] = K a [A - ] [H 3 O + ][A - ] K a = [HA] ph = -log[h 3 O + ] [HA] [HA] ph = -log K a [A - = -logk a - log ] [A - ] pk a = -logk a ph = pk a + log [A - ] [HA] Henderson-Hasselbalch Equation ph in aqueous solutions is a function of the concentrations of acids and bases within them equation defines ph as a function of the strength of an acid (pk a ) and the concentration of the acid and conjugate base Allows calculation of ph given a certain conc. of acid Allows calculation of amount of acid/base needed to obtain a desired ph when ph = pk a, the [HA] = [A - ] HA Acid Strength HA + H 2 O H 3 O + + A - The difference between a strong acid and a weak acid can be described by the stability of the conjugate base. E N E R G Y ionization easier A - A - WEAK ACID Has a strong conj. base (higher energy) STRONG ACID Has a weak conj. base (lower energy) 3

4 Acid Strength A more stable conjugate base means a stronger acid. A - stabilization E N E R G Y HA A - Acid Strength Factors that influence stability of the conjugate base include: Resonance Electronegativity Atomic Size Hybridization Inductive Effects 4

5 Resonance Effects More or better resonance structures of the conjugate base lead to a stronger acid. increasing quality of resonance Resonance Effects pk a Values

6 Resonance Effects The Acetate Ion acetic acid acetate ion Resonance Stabilized Equivalent structures (charges on oxygens) The Phenolate Ion Resonance Effects - More resonance structures, but not more stable than acetate Nonequivalent structures (note charges on carbon and oxygen) 6

7 Electronegativity Placing the negative charge on a more electronegative element (from the same period) in the conjugate base leads to a stronger acid. increasing electronegativity Electronegativity pk a Values CH 4 >45 RCH NH 3 34 RNH 2 35 H 2 O 16 ROH HF Consider the conjugate bases 7

8 increasing electronegativity Electronegativity pk a Values CH 4 > 45 RCH NH 3 34 RNH 2 35 H 2 O 16 ROH HF Consider the conjugate bases Atomic Size Placing the negative charge on a larger atom (from the same group) in the conjugate base leads to a stronger acid. 8

9 increasing size Atomic Size pk a Values HF 3.5 H 2 O 16 HCl 7 H 2 S 7 HBr 9 H 2 Se H 2 Te 4 3 HI 10 Consider the ionic radii increasing size Electronegativity pk a Values HF 3.5 F 1.36 Å H 2 O 16 HCl 7 Cl 1.81 Å H 2 S 7 HBr 9 Br 1.95 Å H 2 Se H 2 Te 4 3 HI 10 I 2.16 Å Consider the ionic radii 9

10 Hybridization More s character in the orbital bearing the negative charge in the conjugate base leads to a stronger acid. Hybridization sp 3 pk a > 45 pk a : sp : sp 25 As electrons in hybrid orbitals become closer to the nucleus, they are lower in energy : 10

11 Inductive Effects Electron-withdrawing effects due to differences in electronegativity pull electron density away from the negatively charged end of the conjugate base, lowering the energy and stabilizing the conjugate base, making the acid stronger. Inductive Effects Electron-donating effects due to differences in electronegativity push electron density toward the negatively charged end of the conjugate base, increasing the energy and destabilizing the conjugate base, making the acid weaker. 11

12 Electron-withdrawing Groups Inductive Effects Electron-donating Groups F, Cl, Br, O, N R, CH 3, B, Si electronegative elements pull electron density away from carbon alkyl groups and elements less electronegative than carbon push electron density toward carbon Remember, the electron-withdrawing and -donating groups work through the bond system, while resonance groups work through the system. Inductive Effects Chlorine helps to stabilize CO 2 by withdrawing electrons This effect diminishes with distance it extends for about 3 bonds 12

13 increasing electronegativity Inductive Effects 3.13 pk a Values increasing substitution Inductive Effects Increasing substitution pk a :

CHEM 109A Organic Chemistry

CHEM 109A Organic Chemistry https://labs.chem.ucsb.edu/zakarian/armen/courses.html Chapter 2 Acids and Bases Central to Understanding Organic Chemistry Draw the conjugate acid of each of the following: