ACID-BASE EQUILIBRIA
I.
The Nature of Acids and Bases
A. Acids have traditionally been defined as substances that taste sour and can dissolve active
metals. Bases taste bitter and feel slippery to the touch.
B. Arrhenius defined acids as substances that release H+ ions in aqueous solution and bases as
substances that release OH- ions in aqueous solution.
C. The hydrogen ion is hydrated in water.
The Arrhenius definition of acids is then modified to state that an acidic solution is formed
by a reaction in which an acid transfers a hydrogen ion to water.
D. Bronsted and Lowry independently proposed similar definitions of acids and bases.
1. An acid is a substance capable of donating a proton, and a base is a substance capable of
accepting a proton.
2. Acid-base reactions involve the transfer of a proton from the acid to the base. The
molecule or ion that remains after an acid donates its proton is a base. The acid and the
molecule or ion are referred to as a conjugate acid-base pair.
Discussion Question: Discuss the warning labels placed on household products that are acids or
bases.
II.
The Dissociation of Water and the pH Scale
A. Water can act as a proton donor or a proton acceptor toward itself: H2O(l) <--> H+(aq) +
OH-(aq). The autoionization of water is spontaneous.
B. The autoionization is described by an equilibrium constant: K = [H+][OH-]/[H2O]. Because
[H2O] is a constant, K[H2O] = [H+][OH-] = Kw, where Kw is called the ion-product
constant of water and has a value of 1.0 x 10-14 at 25oC.
C. Acidic, basic, and neutral solutions are defined on the basis of the magnitudes of [H+] and
[OH-].
D. pH = -log [H+]. Acidic solutions are those for which pH < 7.00, basic solutions are those
for which pH > 7.00, and neutral solutions are those for which pH = 7.00 at 25oC.
E. Also, for other quantities, pX = -log (X), where “X” is the quantity of interest.
F. The value of pH can be measured by several means.
1. Acid-base indicators are substances that possess different colors within different ranges
of pH.
2. A pH meter is an instrument that measures the pH of a solution quickly and accurately.
Discussion Question: The autoionization constant of water increases with temperature. Discuss the
effect this phenomenon has on the pH scale.
III.
Strong Acids
A. Strong acids are strong electrolytes. Virtually all the molecules are converted into ions
when the acid is placed in solution. The observed [H+] equals the formal concentration of
the acid. “Formal concentration” in this context means the total concentration of solute in
solution.
B. The common strong acids are HCl, HBr, HI, HNO3,
HClO4, and H2SO4 (first H+ only lost).
IV.
Weak Acids
A. Weak acids are weak electrolytes. All the molecules are not converted into ions when the
acid is placed in water. Weak acids ionize as: HX(aq) <--> H+(aq) + X-(aq). The acid
dissociation constant is given the symbol Ka. The value of Ka can be calculated using the
same methods covered previously for Kc.
B. Calculating pH for solutions of weak acids is an important problem type in working with
acids.
C. Polyprotic acids are those acids that can release more than one proton. The dissociation of
each proton is a separate Bronsted-Lowry acid-base reaction and is described by a separate
value of Ka.
Discussion Question: Qualitatively discuss how the compounds H2CO3/HCO3-/CO32- help the
body maintain a fairly constant pH.
V.
Strong Bases
A. Strong bases are strong electrolytes. Virtually all the molecules are converted into ions
when the base is placed in solution. The observed [OH-] equals the formal concentration of
the base.
B. The common strong bases are the hydroxides of the group 1A and group 2A metals.
C. The pH of a solution of a base is found by determining [OH-], calculating [H+] from the Kw
equation, and then using pH = -log [H+].
VI.
Weak Bases
A. Weak bases are weak electrolytes. All the molecules
are not converted into ions when the base is placed in water. Weak bases ionize as B(aq) +
H2O(l) <--> HB+(aq) + OH-(aq). The base-dissociation constant is given the symbol Kb.
B. Amines are compounds that contain nitrogen bonded to three different atoms and have the
general formula NR3. Amines are bases.
C. Amines and Amine Hydrochlorides describes amines and
drugs and medications.
acid salts and their usage in
D. Anions of weak acids are also bases. Thus, CN-, the anion of HCN, is a base.
VII. Relations Between Ka and Kb
A. If two reactions are combined to yield a third reaction, the equilibrium constant of this third
reaction is given by the product of the equilibrium constant of the two reactions being
combined. Thus, Ka x Kb = Kw. For any base, the Kb value can be found from the Ka of
its conjugate acid. Conversely, the Ka value of any acid can be found from Kb of its
conjugate base.
B. Using the pX notation, pKa + pKb = pKw = 14.00.
VIII. Acid-Base Properties of Salt Solutions
A. Salt solutions are solutions of ionic compounds that contain neither H+ nor OH-. However,
it is common for the ions in salt solutions to react with water to form acidic or basic
solutions. For example, see section VI-D, anions of weak acids.
B. Anions of polyprotic acids that still have at least one ionizable proton can act either acidic
or basic.
C. Metal cations, except those of group 1A and group 2A, act as weak acids in aqueous
solution, bonding with OH- from water and releasing H+. D. The pH of a salt solution can
be predicted.
1. A salt solution derived from a strong acid and a strong base is neutral (pH = 7.00).
2. A salt solution derived from a weak acid and a strong base has a pH value greater than
7.00.
3. A salt solution derived from a strong acid and a weak base has a pH value less than
7.00.
4. The pH value of a salt solution derived from a weak acid and a weak base depends on
the relative values of Ka and Kb for the cation and anion.
IX.
Acid-Base Character and Chemical Structure
A. In order for a hydrogen-containing compound to be an acid, the hydrogen must be bonded
to a more
electronegative element. In general, metal hydrides
are either basic or show no pronounced acid-base character, whereas nonmetal hydrides are
either acidic or show no pronounced acid-base character.
B. Many common acids, called oxyacids, contain oxygen in addition to hydrogen and another
element, usually a nonmetal. Trends as to the strengths of oxyacids are described in detail.
C. Carboxylic acids constitute a large class of oxyacids with the same functional group COOH.
X.
Lewis Acids and Bases
A. G.N. Lewis proposed a theory in which an acid is defined as an electron-pair acceptor and a
base as an electron-pair donor.
B. In the Lewis theory, metal ions that react with water to undergo hydrolysis are acids.
SAMPLE QUIZ QUESTIONS
1. Classify each of the following as being a strong acid, strong base, weak acid, or weak base:
(a) HNO2; (b) NH4+;
© HF; (d) HCl; (e) KOH; (f) F-.
2.
3.
4.
5.
6.
What is [H+] in a 3.00 M solution of HClO4?
The pH of a solution is 3.88. What are [H+] and the pOH?
[H+] is 0.0250 M in aqueous solution. What are the pH, [OH-], and pOH?
[OH-] is 0.500 M in aqueous solution. What is the pH?
Nitrous acid, HNO2, has Ka = 4.5 x 10-4. What is the pH of a solution in which [HNO2] is
0.200 M?
7. Carbonic acid, H2CO3, has Ka = 4.3 x 10-7. If the pH of a solution of carbonic acid is
3.000, what is [H2CO3]?
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8. An aqueous solution of formic acid, HCOOH, has pH = 1.888, and [HCOOH] is 0.987 M.
What is the value of Ka for formic acid?
9. The acid dissociation constants for phosphoric acid vary in the order H3PO4 > H2PO4- >
HPO42-. Explain this order of decreasing values for successive Ka.
10. Show that for an acid, HA, and its conjugate base, A-, the product Ka x Kb = Kw.
11. Describe the relationship between the relative strengths of a series of bases and the relative
strengths of their conjugate acids.
12. Kb for CO32- is 2.1 x 10-4. If [CO32-] is 0.075 M, what is [OH-]?
13. Kb for triethylamine, N(C2H5)3, is 5.2 x 10-4. [OH-] and [HN(C2H5)3+] are each
0.000103 M. What is [N(C2H5)3]?
14. Cyanide ion, CN-, has Kb = 1.6 x 10-5. The pH of a cyanide solution is 12.000. What is
[CN-]?
15. Which of the oxyacids of the halogens would you expect to be the strongest acid? Explain.
16. Indicate whether the following salts will give acidic, basic, or neutral solutions: (a) NH4Cl;
(b) AlCl3; (c) NaCN; (d) KBr.
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