Chapter 9: Monoprotic Acid-Base Equilibria Chapter 11: Acid-Base Titrations Example: Determination of HCl concentration by titration with NaOH I Solutions and Indicators for Neutralization Titrations A Standard Solutions: The standards solutions used as titrants for unknown w eak acids or bases are alw ays strong bases or acids, respectively Standard titrant acids: dilute solutions of HCl, HClO , or H SO Standard titrant bases: dilute solutions of NaOH, KOH The primary standards may not be strong acids or bases (e.g., potassium acid phthalate, sodium oxalate, sodium bicarbonate) B The Theory of Indicator Behavior pH-sensitive dyes have long been used as indicators Normally, the basic form (In) on the dye has a color different from the acid form, HIn: HIn + H2O H3O+ + InIn + H2O OH- + HIn+ Ka = Kb = [H 3O + ][In − ] [Eq.1] [HIn] [OH − ][HIn + ] [Eq 2] [In] We can see that both equilibrium constant expressions above can be written: [ H 3O + ] = + [ H 3O ] = Ka[HIn] [In − ] Ka[HIn + ] [In] [Eq 1’] {Note: Kw =KaKb = [OH-][H+]} [Eq 2’] Therefore, the [H3O+] determines the ratio of the acid/conjugate base form of the indicator To see the color of a particular form (acid or base) of the indicator, that form must be present at tenfold higher concentration For example, To see the color of acid form, the ratio [HIn]/[In] must be equal to or greater than 10 To see the color of base form, the ratio [HIn]/[In] must be equal to or less than 0.1 This means that: For acid color [H3O+] > Ka (10/1) For base color [H3O+] < Ka (1/10) Hence: indicator range = pKa ± 1, and the pH change in the area of the equivalence point must match this range or at least overlap it significantly to use this indicator for endpoint detection (note: how to select the indicator) 9-1 C Titration Curves – may be linear-segment curve or a sigmoidal curve depending on what is plotted on the y-axis The X-axis units are always reagent or titrant volume The Y-axis may be in increments of analyte reacted or product formed (linear-segment curve) or a p-function such as pH (s-curve) The equivalence point is characterized by large changes in the relative concentrations of the reagent and analyte (See Table 10-2) Titration Curves 9-2 D The Titration of a Strong Acid with a Strong Base Example: Determination of HCl concentration by titration with NaOH NaOH + HCl NaCl + H2O moles = CNaOHVNaOH = CHClVHCl H3O+ in the titration medium has two sources a From the H2O solvent b From the acid solute - usually this is in great excess relative contribution from water because the Kw is so small c The mass balance equation describing this situation is: [ H O + ] = C H C l + [ O H -] = C HCl d T h e s a m e is tr u e fo r a s tr o n g b a s e a n d w e c a n w r ite : [ O H -] = C N a O H + [ H O + ] = C N a O H G Titration Curves for Weak Acids: There are areas to consider 1) before the addition of base 2) before the equivalence point (buffer region 1) 3) at the equivalence point 4) after the equivalence point Before the addition of base: Calculated from the concentration and Ka of the weak acid After the addition of strong base but before the equivalence point: Moles of [OH − ]added Vtotal Moles of [OH − ]added = pK a + log pH = pK a + log Moles of [H 3O + ] Moles of [H 3O + ] Vtotal Note: The half-neutralized point pH = pKa and hence you can measure pKa from titration curve The half-neutralized point means pH at Vb = Ve Note: Vb : volume of titrant Ve: volume of titrant needed to reach the equivalence point Note: You will need these questions and concepts for the calculation in Chem 322 (potentiometric titration experiments) At the equivalence point: The predominant equilibrium is the hydrolysis of H2O by the salt of the weak acid: A- + H2O HA + OH[OH − ][HA] Kb = [A - ] Generally, you can solve the equilibrium constant equation for [OH-] and assume that [A ] = - Moles of [H 3O + ] Vtotal [OH − ] = K b C b Thus, For weak acids that are titrated with strong bases, the pH at the equivalence point will be basic Beyond the equivalence point: Both the anion of the weak acid and the excess base are sources of [OH-] However, due to LeChatelier's Principle the addition of [OH-] in the form of a strong base will suppress the hydrolysis by the weak acid anion so that: Moles of [OH − ]added − Moles of [H 3O + ] [OH ] = Vtotal − 9-5 9-4 HOAC + H2O H3O+ + OAC- [ H 3O + ] = K a C a Moles of [OH − ]added pH = pK a + log Moles of [H 3O + ] − [OH ] = K b C b Moles of [OH − ]added − Moles of [H 3O + ] [OH ] = Vtotal − H Titration Curves for Weak Bases: There are areas to consider 1) before the addition of acid 2) before the equivalence point (buffer region 1) 3) at the equivalence point 4) after the equivalence point 9-6 − [OH ] = K b C b Moles of [OH − ] pH = pK a + log Moles of [H 3O + ] [ H 3O + ] = K a C a + − Moles of [H 3O ]added − Moles of [OH ] [ H 3O ] = Vtotal + I The Effect of Concentration: Again, as with the titration of strong acids with strong bases, as the concentration of the weak acid or base becomes more dilute, the relative change in pH at the equivalence point decreases making the endpoint less sharp A: 0.1000 M acid with 0.1000 M base B: 0.001000 M acid with 0.001000 M base I The Effect of Reaction Completeness: The smaller the Ka, the less sharp the endpoint when a weak acid is titrated with a strong base (Figure 10-11) This depends also on concentration, so that weaker acids can be titrated if concentrated solutions are used Any Questions? Summary How to select standard solutions and indicators for neutralization titrations Theory of Indicator Behavior Titration Curves Theories: Plots and shapes of plots Parameters: pH, Volume of titrants, equivalence point Calculations: Ka, pH, indicator choices, half-neutralized point Effects: Concentration, reaction, composition, temperature, equilibrium constants The titration of a strong acid with a strong base The titration of a week acid with a strong base The titration of a strong base with a strong acid The titration of a week base with a strong acid Buffer solutions: Definition and properties (e.g., buffer capacity) Calculation of pH of the buffer solution Applications Homework 9-B, D, E, 2, 5, 6, 22, 23, 27, 28, 33 11-A, B, F, 3, 6, 14 Before working on Homework, Practice with all examples that we discussed in the class and examples in the textbook!! ... added to the solution to give CHA and CNaA For the species HA and A- we can write: a Pertinent Equilibria HA + H2O A- + H3O+ A-+ H2O HA + OH- 2H2O H3O+ + OHb Equilibrium