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Chemistry

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Published in: Chemistry
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This PDF File explains about the acid and base nature with the equation derivation.

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  1. Acid-Base Concepts Ionization of Water Water dissociates into hydronium (H30+) and hydroxyl (OH-) ions. For simplicity, we refer to the hydronium ion as a hydrogen ion (H+) and write the equilibrium as The equilibrium constant K eq of this dissociation is given by (1) in which the terms in brackets denote molar concentrations. Because the concentration of water (55.5 M) is changed little by ionization, expression 1 can be simplified to give (2) in which K w is the ion product of water. At 250C, K w is 1.0 x 10-14. Note that the concentrations of H+ and OH- are reciprocally related. If the concentration of H+ is high, then the concentration of OH- must be low, and vice versa. For example, if [H+] = 10-2 M, then [OH-I = 10-12 M. Definition of Acid and Base An acid is a proton donor. A base is a proton acceptor. Acid + base CH3COOH + CH3CO()- A«tic acid Ammonium ion Acetate —H++ NHS Ammonia The species formed by the ionization of an acid is its conjugate base. Conversely, protonation of a base yields its conjugate acid. Acetic acid and acetate ion are a conjugate acid-base pair. Definition of pH and PK The pH of a solution is a measure of its concentration of H+. The pH is defined as pH log10(1/lH+l)— -log10[H+] The ionization equilibrium of a weak acid is given by The apparent equilibrium constant K a for this ionization is (3) (4)
  2. The PK a of an acid is defined as pKa — —log Ka — log(1/Ka) (5) Inspection of equation 4 shows that the PK a of an acid is the pH at which it is half dissociated, when [A- Henderson-Hasselbalch Equation What is the relation between pH and the ratio of acid to base? A useful expression can be derived from equation 4. Rearrangement of that equation gives Taking the logarithm of both sides of equation 6 gives Substituting pH for log I/[H+] and PK a for log I/K a in equation 7 yields pH — + which is commonly known as the Henderson-Hasselbalch equation. (6) (7) (8) The pH of a solution can be calculated from equation 8 if the molar proportion of A- to HA and the PK a of HA are known. Consider a solution of 0.1 M acetic acid and 0.2 M acetate ion. The PK a of acetic acid is 4.8. Hence, the pH of the solution is given by pH = 4.8 + log(O.2/CL1) — 4.8 + log 2.0 4.8 + 0.3 — Conversely, the PK a of an acid can be calculated if the molar proportion of A- to HA and the pH of the solution are known. Buffers An acid-base conjugate pair (such as acetic acid and acetate ion) has an important property: it resists changes in the pH of a solution. In other words, it acts as a buffer. Consider the addition of OH- to a solution of acetic acid (HA): HA + OLI- A- + H20 A plot of the dependence of the pH of this solution on the amount of OH- added is called a titration curve Note that there is an inflection point in the curve at pH 4.8, which is the PK a of acetic acid. In the vicinity of this pH, a relatively large amount of OH- produces little change in pH. In other words, the buffer maintains the value of pH near a given value, despite the addition of other either protons or hydroxide ions. In general, a weak acid is most effective in buffering against pH changes in the vicinity of its PK a value. pKa Values of Amino Acids An amino acid such as glycine contains two ionizable groups: an a-carboxyl group and a protonated u- amino group. As base is added, these two groups are titrated (Figure 3.62). The PK a of the u-COOH group is 2.4, whereas that of the u- NH3 + group is 9.8. The PK a values of these groups in other amino acids are similar (Table 3.4). Some amino acids, such as aspartic acid, also contain an ionizable side
  3. chain. The PK a values of ionizable side chains in amino acids range from 3.9 (aspartic acid) to 12.5 (arginine). Titration Curve of Acetic Acid 0.5 pH Titration of the a-Carboxyl and u-Amino Groups of an Amino Acid HOH HOH COOH coo- coo-

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