Hydrolysis, pH, and Buffers

Experiment (8) Hydrolysis, pH, And Buffers Pure water dissociates (ionizes) partially, to a very small extent, to H+ and OH-, eq 1. This process is called autoionlzation of water. At room temperature, only about one out of 10,000,000 (107) molecules of water is ionized. Thus, the ratio of H+ and OH- ions to H2O molecules is about 1:107, i.e the relative concentration of [H+] = [OH-] ~ 10-7[H2O]. The equilibrium expression for the autoionization of water is: The concentration of water in aqueous solution is almost constant, about 55M. Therefore, the above expression can be written as: At 25oC, Kw has the value of 1.0 X 10-14. Equation 2 is valid for aqueous solutions including pure water. In pure water [H+] = [OH-] and the solution is said to be neutral. When [H+] increases in an aqueous solution, [OH-] decreases (as shown by eq 2) and the solution said to be acidic. On the other hand, a solution is said to be basic when [OH-] increases over [H+] in an aqueous solution. We can also express [H+] (or [OH-]) in terms of pH (degree of acidity), which is defined as the negative logarithm of [H+]: At 25oC, an aqueous solution is neutral (pH = 7), acidic (pH < 7) or basic (pH > 7). We can write eq 2 in terms of pH, pOH (-log[OH-] and pKw (-logKw): If a salt is dissolved in water, cations (M+) and anions (X-) are produced, eq 5. When either the cation or the anion reacts with water to form a weak electrolyte, the type of reaction is called hydrolysis. If the cation reacts with water, the equation is: This produces a weak electrolyte (MOH) and H+ ions. Thus, [H+] increase and the solution becomes acidic, i.e. the pH < 7. The hydrolysis constant for the reaction is: If the anion reacts with water, the equation is: This produces a weak electrolyte (HX) and OH- ions. Thus, [OH-] increases (or [H+] decreases) and the solution becomes basic, i.e. the pH > 7. The hydrolysis constant for the reaction is: The value of Kh can be calculated from the experimental pH of a solution containing the ion in question. In this experiment you will determine the hydrolysis constant for a number of salts, and verify the following statements: 1. Salts of strong acids and strong bases (e.g. NaCl) do not hydrolyze. The pH is 7. 2. Salts of weak acids and strong bases (e.g. NaC2H3O2) hydrolyze producing a basic solution. The pH > 7. 3. Salts of strong acids and weak bases (e.g. NH4NO3) hydrolyze producing an acidic solution. The pH < 7. Measuring the pH of solutions. The pH of a solution can be measured accurately by an instrument called a pH meter. A pH meter measures the potential differences between two electrodes of electrochemical cell. One is a reference electrode, which has a fixed potential and the other is a glass electrode, in which the potential depends on the H+ concentration. A less accurate but a more colorfull way to measure pH uses universal indicator, which is a mixture of dyes that shows changes in color at different pH values. A similar principle is used with pH papers. Example: The pH of a 0.100 M NaF solution is 8.1. Calculate the hydrolysis constant. The solution is basic and the hydrolysis equation is: Therefore, we need to calculate [OH-], and note that, according to the hydrolysis equation, Therefore, Procedure: A. The pH Measurements Use the universal indicator or the pH papers to measure the pH of each solution listed in table 1 and record the results in your report sheet. B. Hydrolysis Equations For each salt listed in table 2, write the ionic equation, or, if no reaction occurs write “no reaction”. C. Hydrolysis Constants and Ka/Kb Determination 1. Your instructor will domenstrate the use of the pH meter. 2. Calibrate the pH meter using two pH standards (use 7.00 and 4.00 buffer standards for acidic solution and 7.00 and 10.00 buffer standards for basic solutions). 3. Measure the pH of a 0.100 M of NaCH3COO and a 0.100M of NH4NO3 solutions. 4. Calculate the hydrolysis constants for NH4+ and CH3COO- and enter the results of your calculations in table 3. 5. Calculate Ka or Kb forCH3COOH or NH3, respectively. Data and Results: A. The pH measurements Test Solutions pH 0.10 M HCl ……………. 0.10 M NaOH ……………. 0.10 M ZnCl2 ……………. 0.10 M Cu(NO3)2 ……………. 0.10 M NaCl ……………. 0.10 M NH4Cl ……………. 0.10 M Na3PO4 ……………. 0.10 M Na2CO3 ……………. 0.10 M CaCl2 ……………. 0.10 M KAl(SO4)2. 12H2O B. Hydrolysis equations 1. NaC2H3O2 ………. + H2O = ……… + . 2. NH4NO3 ………. + H2O = ……… . + . 3. NaCl ………. + H2O = ……… ………. + H2O = ……… . ……… . + . 4. CaCl2 ……… ……… . + ……… . 5. Na2CO3 ………. + H2O = ……… + . 6. Cu(NO3)2 ………. + H2O = ……… . + . 7. KAl(SO4)2 . 12H2O ………. + H2O = ……… . ……… ……… . + ……… . C. Hydrolysis constants NaC2H3O2 NH4NO3 1. [H+] ……………. ……………. 2. [OH-] ……………. ……………. 3. [MOH] or [HX] ……………. ……………. 4. [mx]init ……………. ……………. 5. [MX]used ……………. ……………. 6. [MX]eq ……………. ……………. 7. Kh ……………. …………….