Acids Chapter14: & Bases
Chapter 14. Acids & Bases
Arrhenius Definition of Acid & Base
Arrhenius Acid: Substance that dissociates to form hydrogen ions (H+) in solution:
HA(aq) H+(aq) + A(aq)
Arrhenius Base: Substance that dissociates in, or reacts with water to form hydroxide ions (OH):
MOH(aq) M+(aq) + OH(aq)
Bronsted-Lowry Definition of Acid & Base
Conjugate Acid-Base Pairs for Acid
BrnstedLowry Acid: Substance that can donate H+ BrnstedLowry Base: Substance that can accept H+
Chemical species with formulas that differ by one proton are known as conjugate acidbase pairs.
Strong versus Weak Acid
Relative Strengths of Acids & Conjugate Bases
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Chapter14: Acids & Bases
Conjugate Acid-Base Pairs for Base
Identifying the Conjugate Acid
In the reaction: NH4+(aq) + NO2-(aq) HNO2(aq) + NH3(aq) A. NO2-(aq) B. HNO2(aq) C. NH3(aq)
What is the conjugate acid of NO2-(aq)?
Water as Acid or Base
Equilibrium Constant for H2O Dissociation
Water can act as an acid or as a base. H2O(l) H+(aq) + OH(aq)
Equilibrium constant for H2O dissociation is known as the ion product constant for water.
This dissociation is the autoionization of water.
Kw = Kc = [H+][OH] = 1.0 x 1014
If we know either [H+] or [OH], then we can determine the other quantity.
H2O(l) + H2O(l)
H3O+(aq) + OH(aq)
Using Equilibrium Constant for H2O to Determine H+ Concentration
Definition of pH
The concentration of OH ions in a household ammonia cleaning solution is 0.0025 M. What is the concentration of H+ ions? A. 2.5 x 10-3 M B. 4.0 x 10-10 M C. 4.0 x 10-12 M D. 2.5 x 10-12 M
pH of a solution is the negative logarithm of the hydrogen ion concentration (in mol/L). pH = log [H+] pH + pOH = 14
Acidic solutions: Basic solutions: [H+] > 1.0 x 107 M, [H+] < 1.0 x 107 M, pH < 7.00 pH > 7.00 pH = 7.00
Neutral solutions: [H+] = 1.0 x 107 M,
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Chapter14: Acids & Bases
Demonstration: pH of Common Household Items
Many Household Items Span Entire pH Range
Calculating the H+ Concentration of Orange Juice
pH = log pH + pOH = 14 The pH of a certain orange juice is 3.33. Calculate the H+ ion concentration. Is orange juice acidic? [H+]
Determining the pH of Blood
The OH ion concentration of a blood sample is 2.5 x 107 M. What is the pH of the blood? pH = log [H+]; [H+][OH] = 1.0 x 1014 A. B. C. D. 6.6 7.4 8.1 8.7
Strong vs. Weak Acids & Bases
Strong acids and bases: are strong electrolytes that are assumed to ionize completely in water. Weak acids and bases: are weak electrolytes that ionize only to a limited extent in water.
Proton is Always Transferred to the Stronger Base
Direction of reaction to reach equilibrium is proton transfer from the stronger acid to the stronger base to give the weaker acid and the weaker base.
Stronger acid + stronger base weaker acid + weaker base
Example: HNO2(aq) + CN(aq) HCN(aq) + NO2(aq)
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Chapter14: Acids & Bases
Determining the Direction of Reaction
Which direction will this reaction go if proton transfer is from the stronger acid to the stronger base? HF(aq) + NH3(aq) F(aq) + NH4+(aq) A. Left to Right B. Right to Left C. Equally Balanced
Acid Ionization Constant
Acid Ionization Constant: the equilibrium constant for the ionization of an acid.
HA(aq) + H2O(l) H3O+(aq) + A(aq)
Or simply:
HA(aq) H+(aq) + A(aq)
Ka = [H+ ][A - ] [HA]
Wide Range of Acid Ionization Constants
ACID HF HNO2 C9H8O4 (aspirin) HCO2H (formic) C6H8O6 (ascorbic) C6H5CO2H (benzoic) CH3CO2H (acetic) HCN C6H5OH (phenol) Ka 7.1 x 10 4 4.5 x 10 4 3.0 x 10 4 1.7 x 10 4 8.0 x 10 5 6.5 x 10 5 1.8 x 10 5 4.9 x 10 10 1.3 x 10 10 CONJ. BASE F NO2 C9H7O4 HCO2 C6H7O6 C6H5CO2 CH3CO2 CN C6H5O Kb 1.4 x 10 11 2.2 x 10 11 3.3 x 10 11 5.9 x 10 11 1.3 x 10 10 1.5 x 10 10 5.6 x 10 10 2.0 x 10 5 7.7 x 10 5
Diprotic & Polyprotic Acids
Diprotic and polyprotic acids yield more than one hydrogen ion per molecule. One proton is lost at a time. Conjugate base of first step is acid of second step. Ionization constants decrease as protons are removed.
Dissociation Constants for Polyprotic Acids
Dissociation Constants for Polyprotic Acids
ACID Ka CONJ. BASE Kb
H2SO4 HSO4 C2 H2 O 4 C2HO4 H2SO3 HSO3 H2CO3 HCO3 H2 S HS H3PO4 H2PO4 HPO42
Very Large 1.3 x 10 2 6.5 x 10 2 6.1 x 10 5 1.3 x 10 2 6.3 x 10 8 4.2 x 10 7 4.8 x 10 11 9.5 x 10 8 1 x 10 19 7.5 x 10 3 6.2 x 10 8 4.8 x 10 13
HSO4 SO4 2 C2HO4 C2O42 HSO3 SO3 2 HCO3 CO3 2 HS S 2 H2PO4 HPO42 PO43
Very Small 7.7 x 10 13 1.5 x 10 13 1.6 x 10 10 7.7 x 10 13 1.6 x 10 7 2.4 x 10 8 2.1 x 10 4 1.1 x 10 7 1 x 10 5 1.3 x 10 12 1.6 x 10 7 2.1 x 10 2
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Chapter14: Acids & Bases
Demonstration: Acidity of Dry Ice
When dry ice is added to a graduated cylinder containing blue liquid (basic water with universal indicator), the dry ice produces bubbles and fog and turns the solution yellow. CO2 produces carbonic acid (diprotic acid) in solution. Dissociation of carbonic acid produces H+ in solution and neutralizes OH-. CO2(aq) + H2O(l) --> H2CO3(aq) <--> 2H+(aq) + CO32-(aq) H+(aq) + OH-(aq) --> H2O(l)
pH and Equilibria for Weak Acids
Determining pH and Concentrations in Solutions of Weak Acids:
1. Write a balanced and equation equilibrium expression. 2. Identify all species present that may affect pH. 3. Major species will be HA, H+, A. Ignore H2O & OH. 4. Summarize the changes in concentration of HA, H+, A using an I.C.E. table.
Equilibrium Calculations
Solving the Quadratic Equation
Initial Change Equilibrium Table: Determine the pH of 0.50 M HA solution at 25C. Ka = 7.1 x 104.
The Quadratic Equation:
ax 2 + bx + c = 0
HA (aq) H + + A-(aq) (aq) 0.50 Initial (M): Change (M): x Equilib (M): 0.50 x 0.00 +x x 0.00 +x x
Values are substituted into the below expression and solved for a positive solution to x.
x=
- b b2 - 4ac 2a
Simplifying Assumptions
For weak acid problems, Ka is very small. Consequently, the amount of dissociated acid, x, is negligible relative to the initial concentration. Under these conditions, can ignore x in terms involving the initial concentrations. After solving for x, can go back and verify assumption.
Another Equilibrium Calculation
Calculate the pH and the concentrations of all species present in 0.050 M HF. Ka for HF is 3.5 x 10-4.
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Chapter14: Acids & Bases
Percent Dissociation
Percent Dissociation Decreases as Concentration Increases
Percent Dissociation is a measure of the strength of an acid.
% Dissociation = [H+ ] 100% [HA]
Stronger acids have higher percent dissociation. Percent dissociation of a weak acid decreases as its concentration increases.
Determining the Percent Dissociation
What is the percent dissociation for a 1.0 M weak acid solution with Ka= 1.0 x 10-4?
% Dissociation = [H+ ] 100% [HA]
Base Ionization Constant
Base Ionization Constant is the equilibrium constant for the ionization of a base. Ionization of weak bases is treated in the same way as the ionization of weak acids. B(aq) + H2O(l) BH+(aq) + OH(aq) Calculations follow the same procedure as used for weak acids but [OH] is calculated instead of [H+].
A. 3.2% B. 0.32% C. 1.0% D. 0.10%
Wide Range of Base Ionization Constants
BASE Kb CONJ. ACID Ka
Product of Ka and Kb
Multiplying out the expressions for Ka and Kb equals Kw. Ka Kb = Kw = 1.0 x 10-14 Use this expression to solve for acid ionization constant if know base ionization constant or visa versa.
C2H5NH2 (ethylamine) C8H10N4O2 (caffeine) NH3 (ammonia) C5H5N (pyridine) C6H5NH2 (aniline) NH2CONH2 (urea)
5.6 x 10 4 4.1 x 10 4 1.8 x 10 5 1.7 x 10 9 3.8 x 10 10 1.5 x 10 14
C2H5NH3+ C8H11N4O2+ NH4+ C5 H6 N+ C6H5NH3+ NH2CONH3+
1.8 x 10 11 2.4 x 10 11 5.6 x 10 10 5.9 x 10 6 2.6 x 10 5 0.67
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Chapter14: Acids & Bases
Determining Ka from Kb
CH3NH2(aq) + H2O(l) CH3NH3+(aq) + OH-(aq) Kb CH3NH3
+(aq)
Acid-Base Properties of Salts
Salts that produce neutral solutions are those formed from strong acids and strong bases. Salts that produce basic solutions are those formed from weak acids and strong bases. Salts that produce acidic solutions are those formed from strong acids and weak bases.
+ H2O(l) H3
O+(aq)
+ CH3NH2(aq) Ka
Kb for CH3NH2 is 4.4 x 10-4. Calculate Ka for the methylamine ion, CH3NH3+. A. 7.82 x 10-8 B. 3.9 x 10-9 C. 2.3 x 10-11
Table of Different Salt Behavior
Neutral Solutions from Salts Formed from Strong Acids and Strong Bases
Example: NaCl Derived from NaOH (strong base) and HCl (strong acid). Neither Na+ or Cl- will react with water and change [H+] or [OH-]
Basic Solutions from Salts Formed from Weak Acids and Strong Bases
Example: NaCN Derived from NaOH (strong base) and HCN (weak acid). Na+ will not react with water and change [H+] of [OH-] CN- will react with H2O and produce OH- and HCN. Basic Solution
Acidic Solutions from Salts Formed from Strong Acids and Weak Bases
Example: NH4Cl Derived from NH3 (weak base) and HCl (strong acid). Cl- will not react with water and change [H+] NH4+ will react with H2O and produce H3O+ and NH3. Acidic Solution
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Chapter14: Acids & Bases
Demonstration: Acidic or Basic Solutions from Salts
The pH of acidic, basic, and neutral aqueous solutions of salts is shown with ammonium chloride, sodium acetate, and sodium chloride.
Determining the pH of a Salt Solution
What is the pH of a 0.15 M solution of sodium acetate (CH3COONa)? Ka for CH3COOH is 1.8 x 10-5. A. 7.81 B. 8.32 C. 8.96 D. 9.31
Lewis Definition of Acid & Base
Lewis Base Molecules
Electron Pair Donors
Lewis Acid is an electron-pair acceptor. These are generally cations or neutral molecules with vacant valence orbitals, such as Al3+, Cu2+, H+, BF3. Lewis Base is an electron-pair donor. These are generally anions or neutral molecules with available pairs of electrons, such as H2O, NH3, O2.
Lewis Acid + Lewis Base Interaction
Factors Affect Acid Strength
Strength of acid depends on its tendency to ionize. For general acids of the type HX: The stronger the bond, the weaker the acid. The more polar the bond, the stronger the acid.
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Chapter14: Acids & Bases
Stronger Bond Strength Weaker Acid
Larger Electronegativity Stronger Acid
For binary acids in the same group, HA bond strength decreases with increasing size of A and acidity increases.
For binary acids in the same row, HA polarity increases with increasing electronegativity of A and acidity increases.
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