Acid Dissociation Constant

Size: px

Start display at page:

Download "Acid Dissociation Constant"

Adele Hudson
5 years ago
Views:

1 CE 131 Lecture 37 Lewis Acids and Bases Chapter 16: pp Acid Dissociation Constant C C [ 3 + ][C ] K = [ 2 ][C ] [ 3 + ][C ] K a = K [ 2 ] = [C ]

2 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: F + C 3 C 2 F + C 3 C 2 To address this, we need to know the relative affinity of bases for +. 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. I More acidic Acid pka Conjugate Base Cl 4 10 Cl 4 I 10 I 2 S 4 10 S 4 Br 9 Br Cl 7 Cl N 3 2 N S S 4 3 P P 4 F F C 3 C C 3 C N C C 3 C C +25 CC N N 2 C 2 =C C 2 C C 3 C C 3 C 2 N 4 + More basic

3 pk a and the Direction of an Acid-Base Reaction When 0.1 M solutions of C 3 C 2 and NaF are mixed, which side of the following reaction is preferred? C 3 C 2 + F F + C 3 C 2 Compare the strengths of the bases: F and C 3 C 2 C 3 C 2 is stronger than F C 3 C 2 will have a greater affinity for + than F The reactant side of the reaction is preferred Mixing 0.1 M solutions of F and Na C 3 C 2 will lead to a reaction however, as the proton will associate with the stronger base C 3 C 2 F + NaC 3 C 2 C 3 C 2 + NaF Note that equilibrium lies on the side with the weaker acid (C 3 C 2 ) and the weaker base (NaF). Autoionization of Water K = [ 3 + ][ - ] [ 2 ] 2 K w = K [ 2 ] 2 = [ 3 + ][ - ] =

4 Autoionization of Water K w = [ 3 + ][ - ] = for a neutral solution [ 3 + ] = [ - ] [ 3 + ][ 3 + ] = 1.0 x [ 3 + ] 2 = 1.0 x [ 3 + ] = 1.0 x log([ 3 + ]) = -log( ) p = -log([ 3 + ]) = 7.00 Autoionization of Water a solution is considered acidic when [ 3 + ] > M [ - ] < M a solution is considered basic when [ 3 + ] < M [ - ] > M

5 Concentration Scales a solution is considered acidic when p < 7 and p > 7 a solution is considered bacic when p > 7 and p < 7 pk w = p + p = K w Depends on Temperature T( C) K w / What is the p of pure water at 10 C? As p = p: + = 14 2 = log 0.29 p p p 10 = 14. p = 14.54/2 = 7.27 R as [ 3 + ] = [ ] K w = [ 3 ][ ] = [ 3 ] = [ 3 ] = = p = log( ) = 7.27 ( ) 54

6 Structural factors in acid strength The strength of an acid -A should increase as: -A bond strength decreases Electronegativity of A increases Electrons are withdrawn from the -A bond by substituents Each of these is seen in particular cases Binary Acids A For A in the same period: Electronegativity C 4 N 3 2 F pk a : Acid strength For A in the same column: Bond Strength I Br Cl F pk a : Acid strength

7 xyocids A n For A in the same period and constant number of oxygens: Electronegativity of A I Br Cl pk a : Acid strength For varying number of oxygens: Increasing Cl Cl= Cl(=) 2 Cl(=) 3 pk a : Acid strength Carboxylic Acids with Electronegative Substituents N Substituents Cl C C C C Cl C C Cl C C Cl Cl Cl pk a : Acid strength Note: substituent must have significant electronegativity

8 p.785 Acid strength decreases with decreasing electronegativity of halide. Cl Br I K a : p.786a

9 Acid strength increases with increasing number of oxygen atoms attached to central atom. p.786b More electronegative atoms: stronger acid p.787

p.788 Intramolecular Bronsted-Lowry acid-base reaction Be careful not to over generalize these trends: F is stronger than 2 despite a larger bond energy for F electronegativity effects > bond

10 p.788 Intramolecular Bronsted-Lowry acid-base reaction Be careful not to over generalize these trends: F is stronger than 2 despite a larger bond energy for F electronegativity effects > bond strength effects Br is stronger than Cl despite the lower electronegativity of Br bond strength effects > electronegativity effects In all trends except bond strength, the bond is weakened by increasing withdrawal of electron density from the -A bond

11 Towards a More General Theory of Acids & Bases Consider a typical acid-base reaction: F C N F + C N ow are the following reactions like and unlike this? B N B N C C The interaction between a Brønsted-Lowry base and acid can focus on the lone pair of the acid as well as the proton transfer. Lewis Acids and Bases acid => electron pair acceptor base => electron pair donor the base donates a pair of electrons to the acid forming a coordinate covalent bond

12 Lewis acids include: all Brønsted-Lowry acids molecules with vacant valence orbitals (B 3 ) molecules with polar π bonds (C 2, RC 2 ) metal cations Lewis bases include: all Brønsted-Lowry bases molecules with high energy filled orbitals such as lone pairs (Brønsted-Lowry bases) π bonds Lewis Acid and Base Reactions + + N 3 ΝΗ + 4 acid base Cu N 3 [Cu(N 3 ) 4 +2 ] acid base

13 Acid-Base Theories Arrhenius Acid-Base Theory: Acid: + donor F Base: donor Na Brønsted-Lowry Acid-Base Theory: Acid: + donor F Base: + acceptor F Lewis Acid-Base Theory: Acid: e pair acceptor + Base: e pair donor F Lewis acid-base theory extends the simplicity of acid-base chemistry to many other reactions. Avoid confusion!!! + is the perfect acid in all theories Brønsted-Lowry it gives of itself Lewis it accepts electron pairs donor acceptor acid: + e pair base: e pair +

14 Summary Common acids include binary acids, oxoacids, carboxylic acids, and hydrated metal ions Common bases include hydroxides, salts of weak acids, ammonia and amines Acid strength tends to increase with: Increasing electronegativity for binary acids in the same period Decreasing bond strength for binary acids in the same column Increasing numbers of oxygens for oxoacids Increasing electronegative substituents for carboxylic acids A Lewis acid is a lone pair acceptor; a Lewis base is a lone pair donor

Properties of Acids and Bases

Chapter 15 Aqueous Equilibria: Acids and Bases Properties of Acids and Bases Generally, an acid is a compound that releases hydrogen ions, H +, into water. Blue litmus is used to test for acids. Blue litmus