EQUILIBRIA IN SOLUTIONS OF
WEAK ACIDS AND BASES
A. Reaction of a Weak Acid with Water
1. All weak acids behave the same way (Bronsted acids)
2. HA + H2O <-- --> H3O+ + A-
3. Acid ionization (dissociation) constant
a. K(a) = [H+][A-]/[HA]
b. pKa = -logKa
1. Larger Ka, stronger acid (more fully ionized)
B. reaction of a Weak Base with Water
1. All weak bases behave the same way (Bronsted bases)
2. B + H2O <-- --> BH+ + OH-
3. Base ionization (dissociation) constant
a. K(b) = [BH+][OH-]/[B]
b. pKb = -logKb
C. Conjugate Acid-Base Pairs and Their Values of Ka and Kb
1. Ka x Kb = [H+][OH-] = Kw
2. pKa + pKb = pKw = 14.00
3. Inverse relationship between the strengths of the acid and base members of a conjugate pair
a. The stronger the acid, the weaker the conjugate base
II. Equilibrium Calculations
A. Calculating Ka and Kb from initial concetrations and Equilibrium Data
1. % ionization = (moles ionized/L)/(moles available/L) X 100%
2. Examples on page 707-709
B. Calculating Equilibrium Concentrations from Ka (or Kb) and Initial concentrations
1. If a solution only contains a weak acid as the solute, the problem must be solved using Ka
2. If the solution only contains a weak base, the problem must be solved using Kb
3. If the solution contains both a weak acid and a weak bae, use either Ka or Kb
C. Simplifications in Acid-Bae equilibrium Calculations
1. If the Ka (or Kb) is small, then subtracting or adding an "x" can be disregarded
a. the initial concentration of the acid is used as if it were the equilibrium concentration
III. Solutions of Salts: Ions as Weak Acids and Bases
A. Salts have both cations and anions
1. pH of solution can be affected by either or both ions
B. Cations as Acids
1. conjugate acids of molecular bases are weak acids
a. salts that contain cations that are the conjugate acid of weak molecular bases can affect the pH of a solution. These cations are weak acids
b. the conjugate acids of molecular bases tend to be acidic
2. Metal Ions with High Charge Densities are Weak Acids
a. When hydrated, release a H+ ion from water
3. Metal Ions with small charges are "nonacids"
a. Singly charged cations do not directly affect pH
b. Doubly charged cations, except Be+2, do not directly affect pH
C. Anions as Bases
1. The anion of a strong acid is too weak a base to influence the pH of solution
2. The anion of a weak acid is a weak base and can influence the pH of a solution. It will tend to make the solution basic.
D. Predicting the Acid-Base Properties of a Salt
1. If neither the cation/anion can affect the pH, the solution is neutral
2. If only the cation of the salt is acidic, the solution is acidic
3. If only the anion of the salt is basic, the solution will be basic
4. If a salt has a cation that is acidic and an anion that is basic, the pH of the solution is determined by the relative strengths of the acid and base
E. Solutions That Contain the Salt of a Weak Acid and a Weak Base
1. if Ka = Kb, no net effect on pH
2. if Ka > Kb, solution is slightly acidic
3. if Kb> Kb, solution is slightly basic
V. Buffers: The control of pH
A. Components of Buffers
1. two solutes, one provides a weak Bronsted acid, the other provides a weak Bronsted base
2. Usually are conjugate pairs
3. Can be used at any pH value, not just pH of 7
4. Buffers only work with relative small amounts of acids/bases being added
B. How a Buffer works
1. a Buffer neutralizes either a strong acid or strong base that is added
a. If an acid is added, the weak acid reacts with the H+ ions to form the conjugate acid
a. If a base is added, the weak base reacts with the OH- ions to form the conjugate base
C. Calculating the pH of a Buffer solution
1. The Acetic Acid - Acetate Ion buffer System
a. Example pg. 727-728
2. Permissible simplificaions in Buffer Calculations
a. use the initial concentrations of both the weak acid and its conjugate base as though they were equilibrium values
b. For Buffer solutions only, we can use either molar concentrations or moles in the Ka (or Kb) expression to express the amounts of the members of the conjugate acid-base pair
3. the Ammonia-ammonium Ion buffer
a. Example pg. 729
D. Preparation of a buffer with a Given pH
1. The Factors That govern the pH of a Buffer Solution
a. Ka of the weak acid
b. ratio of the molarities or the moles
2. Selecting the Weak Acid for the Preparation of a Buffer Solution
a. the [HA]initial/[A-]initial ratio is close to one
b. pKa of the weak acid most determines where on the pH scale the buffer can work best
c. The range of buffers is from 10/1 to 1/10
1. pH = pKa +/-1
E. Buffer Capacity
1. How much strong acid or base the buffer is able to absorb before its buffering ability is destroyed
a. Determined by the sizes of the actual molarities of its components
VI. Ionization of Polyprotic Acids
A. Each ionization makes a contribution to the total molar concentration of H+
1. relate Ka values and the concentration of the acid to this molarity
B. simplifications in Calculations Involving Polyprotic Acids
1. There is a large difference between successive ionization constants
a. [H+]eq = [H+]first step + [H+]second step
b. [H+]first step >> [H+]second step
c. [H+]eq = [H+]first step
2. In a solution that contains a polyprotic acid as the only solute, the molar concentration of the ion formed in the second step of the ionization numerically equals K(a2)
VII. Solutions of Salts of Polyprotic Acids
A. The calculations required are very similar to weak acids
1. the simplifying assumptions in our calculations will be almost identical to those of weak polyprotic acids
a. to calculate the pH of a solution of a basic anion of a polyprotic acid, we may work with k(b1) and ignore further reactions (same reason as diprotic acids)
VIII. Acid-Base Titrations Revisited
A. Titration Curve -- pH of a solution plotted against the volume of titrant added
1. End Point -- when the change in indicator color occurs
2. Equivalence point -- stoichiometrically equivalent amounts of acid and base have combined
B. titration of a strong acid by a strong bae
1. equivalence point occurs at a pH of 7.00
a. the titration of any stron monoprotic acid with a strong base
C. Titration of a weak acid by a strong base
1. equivalence point occurs at a pH above 7.00
2. Before the titration begins -- use Ka to calculate pH
3. During titration but before equivalence point -- use Ka to calculate pH
4. At equivalence point -- use Kb to calculate pOH, then pH
a. Contains salt (anion is weak base)
5. After equivalence point -- concentration of OH- to calculate pOH, then pH
D. Titration of a Weak Base by a Strong Acid
1. equivalence point occurs at a pH below 7.00
2. Before the titration begins -- use Kb to calculate pH
3. During titration but before equivalence point -- use Kb to calculate pH
4. At equivalence point -- use Ka to calculate pH
a. Contains salt (cation is weak acid)
5. After equivalence point -- concentration of H+ to calculate pH
E. Acid-Base Indicators
1. most are weak acids
HIn(aq) <----> H+(aq) + In-(aq)
a. In a strong acid, H+ concentration is high, shifts left, "acid form" of indicator has one color
b. In a strong base, H+ concentration is low, shifts right, "base form" of indicator has a different color
2. How do indicator work
a. The large swing in pH causes a sudden shift in the position of equilibrium for the indicator
3. Selectring the Best Indicator for an Acid-Base Titration
a. we want an indicator whose pK(In) = pH(at equivalence point)
b. Phenolphthalein is an excellent indicator for WA-SB titrations and SA-SB titrations
Outline based upon:
Brady, J. E., Holum, J. R., Russell, J. W. (2000). Chemistry: The Study of Matter and Its Changes. (3rd ed.). New York: John Wiley & Sons, Inc. pp. 701-749.
