LinkExchange
LinkExchange Member

Acids and Bases

I. The Nature of Acids
A. Arrhenius postulated that acids produce H+ ions in soln and bases produce OH- ions. The Bronsted-Lowry model, an acid is a proton (H+) donor, and a base is a proton acceptor. Yet another type of definition of acids is the Lewis model. A Lewis acid is a electron-pair acceptor and a Lewis base is a electron-pair donor.
B. When acids dissolve in water, the H+ dissociates from the anion and a water accepts it. Summarized in an equation this means: HA + H2O ó H3O+ + A-. The equilibrium expression for a acid reaction is as follows:
Ka = [H3O+][A-]/[HA]. Ka is called the acid dissociation constant.

II. Acid Strength
A. A strong acid is one for which this equilibrium lies far to the right. A strong acids yields a weak conjugate base. A weak acid is one for which the equilibrium lies far to the left. A weak acid has a strong conjugate base.
B. Most acids are oxyacids, in which the acidic proton is attached to an oxygen atom. Organic acids, those with a carbon-atom backbone, commonly contain the carboxyl group: COOH. Monoprotic acids are those which have one acidic proton.
C. Water is an amphoteric substance, one that has both acid and base properties. The autoionization of water is described as being the combination of two water molecules to form one Hydronium ion and one Hydroxide ion. The equilibrium expression for this reaction is as follows: Kw = [H+][OH-] where Kw equals 1e-14. This constant is always 1e-14 at 25C.

III. The pH Scale
A. pH scale provides a convenient way to represent solution acidity.
pH = -log[H+], pOH = -log[OH-], pK = -logK. Because of the way pH is defined, pH decreases as the concentration of H+ increases. For any aqueous solution at 25C pH + pOH equals 14.

IV. Calculating the pH of Acids
A. The most important step is determining which species in a reaction are important and which can be ignored. The major species are those which are a relatively large amounts. In most strong acids all of the acid dissociates to so that [H+] = [HA].
B. Even in weak acids one source of H+ can be singled our as dominant. As always, the equilibrium expression is used to solve for the [H+]. Although the expression usually turns out to be a quadratic equation, the x in the denominator can usually be neglected because of the small size of Ka. However, if the x is neglected it must be divided by the original HA to check for accuracy. If the x is not less than 5% of the original HA then the quadratic formula must be used to solve the equation.
C. Percent dissociation is defined as the amount dissociated divided by the initial concentration multiplied by 100. Percent dissociation increases in a weak acid as the concentration of HA is diluted and decreases in the opposite system. The big relation with weak acids is as follows: Acid concentration is directly proportional to the H+ concentration and inversely proportional to the percent dissociation.

V. BASES
A. Strong and weak bases are defined in the same way as Strong and weak acids. The equilibrium expression is as follows: Kb = [BH+][OH-]/[B] where Kb is the equilibrium constant for the reaction of a base with water. The same thought process is used to solve basic solution problem as acidic solution problem.

VI. Polyprotic Acids
A. Acids the can produce more than one proton are called polyprotic acids. The first proton that dissociates from a polyprotic acids is usually only the important contributor to the overall [H+]. The acid tends not to "let go" of the second or third protons because of the following principle: as the acid loses more protons its negativity increases; therefore, it tends to "hold on to" the remaining positive protons. As Ka gets smaller less products are produced.

VII. Acid-Base Properties of Salts
A. Salt is simply another name for ionic compound. The dissociated ions of a salt can behave as acids or bases. Salts that consist of the cations of strong bases and the anions of strong acids have no effect on [H+] when dissolved in water.
B. If the anion is from a weak acid the anion will act as a base. If the cation is from a weak base it will act as an acid. If both are a conjugate acid or base of a weak base or acid respectively, the species with a bigger Ka or Kb will determine if the soln is acidic or basic. A highly charged metal ion with act as a acid in the hydrated form.

VIII. The Effect of Structure on Acid-Base Properties
A. The two main factors that effect the strength of the acid are the strength of the bond and the polarity of the bond. The radii of the atoms affect the strength of the bond and the electronegativity affects the polarity. The strength of the bond is inversely related to the strength of the acid while the polarity is directly related.
B. In Oxyacids the amount of oxygen in the molecule varies directly with the strength of the acid. This is because the number of atoms of oxygen effectively changes the polarity. In metal acids, the charge of the metal ion directly affects the potency of the acid.

IX. Acid-Base Properties of Oxides
A. A covalent oxide that is dissolved in water will produce an acid. Ex: CO3 + H2O à H2CO3. A ionic oxide that is dissolved in water will produce a base. Ex: CaO + H2O à Ca(OH)2.