Acid-Base Titration Curves PDF

Acid- Base Titration Curves 1  Titration In an acid–base titration, a solution of unknown concentration (titrant) is slowly added to a solution of known concentration from a burette until the reaction is complete. When the reaction i...

Acid- Base Titration Curves 1  Titration In an acid–base titration, a solution of unknown concentration (titrant) is slowly added to a solution of known concentration from a burette until the reaction is complete. When the reaction is complete we have reached the endpoint of the titration. An indicator may be added to determine the endpoint. An indicator is a chemical that changes color when the pH changes. When the moles of H3O+ = moles of OH−, the titration has reached its equivalence point. 2 Acid-Base Indicators An acid-base indicator is a weak organic acid/ base (denoted as HIn) whose colour change occurs over a specific, narrow pH range. To select an indicator, the approximate pH of the titration end point should be known. 3 Titration 4  Titration Curve It is a plot of pH versus the amount of added titrant. The inflection point of the curve is the equivalence point of the titration. Prior to the equivalence point, the solution in the flask is in excess, so the pH is closest to its pH. The pH of the equivalence point depends on the pH of the salt solution. Equivalence point of neutral salt, pH = 7 Equivalence point of acidic salt, pH < 7 Equivalence point of basic salt, pH > 7 Beyond the equivalence point, the solution in the burette is in excess, so the pH approaches its pH. 5  Strong acid- Strong Base Titration Curve NaOH(aq) + HCl(aq) NaCl(aq) + H2O(l) Features of the curve 3 distinct regions 1. The pH starts out low, reflecting high [H3O+] of the strong acid, and increases slowly as acid is gradually neutralized by the added base. 2. The pH rises 6 to 8 units very rapidly, this steep increase begins when the amount of OH- added nearly equals the amount of H3O+ originally present in the acid. One or two more drops of base neutralize the remaining tiny excess of acid. 3. The pH increases slowly beyond the steep rise as more base is added. The equivalence point – occurs when the number of moles of added OH- equals the number of moles of H3O+ originally present The end point – occurs when the indicator added, changes colour. An indicator should change its colour close to the pH of the equivalence point 6 Strong acid- Strong Base Titration Curve 7  Weak acid- Strong Base Titration Curve NaOH(aq) + CH3CH2COOH(aq) CH3CH2COO-Na+(aq) + H2O(l) Features of the curve 1. The initial pH is higher, because the weak acid dissociates slightly, much less H3O+ is present than with the strong acid. 2. The curve rises gradually in the so-called buffer region before the steep rise to the equivalence point. As CH3CH2COO- reacts with strong base more CH3CH2COO- forms, which creates CH3CH2COOH/ CH3CH2COO- buffer. At the mid point of the buffer region, half the initial CH3CH2COOH has reacted. (i.e. half of the OH- needed to reach the equivalence point has been added). So, [CH3CH2COOH] = [CH3CH2COO-] or [CH3CH2COO- /CH3CH2COOH] = 1 Middle of the buffer region: pH=pKa + log ([CH3CH2COO- /CH3CH2COOH]) = pKa + log 1 = pKa 8  Weak acid- Strong Base Titration Curve NaOH(aq) + CH3CH2COOH(aq) CH3CH2COONa(aq) + H2O(l) The pH observed at this point is used to estimate the pKa of an unknown acid. 3. The pH at the equivalence point is above 7.00. The solution contains the strong base cation Na+, which does not react with water, and the weak acid anion CH3CH2COO-, acts as a weak base to accept a proton from H2O and yield OH- 4. The pH increases slowly beyond the equivalence point as excess OH- is added The choice of the indicator is more limited than for a strong acid- strong base titration because the steep rise occurs over a smaller pH range. Phenolphthalein will work because it changes color within this range. 9 Weak acid- Strong Base Titration Curve 10  Weak base- Strong acid Titration Curve NH3(aq) + HCl(aq) NH4+(aq) + Cl-(aq) Features of the curve Same features as the strong base weak acid curve, but the pH decreases throughout the process: 1. The initial weak base solution has a pH well above 7.00 2. The pH decreases gradually in the buffer region, where significant amount of NH3 and its conjugate acid, NH4+are present. At the mid point of this region, the pH equals the pKa of NH4+ 3. The curve drops steeply to the equivalence point. All the NH3 has reacted with added HCl, and the solution contains only NH4+ and Cl-. Note that the pH at the equivalence point is below 7.00 because Cl- does not react with water and NH4+ is acidic. NH4+(aq) +H2O (l) NH3(aq) + H3O+ (aq) 4. The pH decreases slowly beyond the equivalence point as excess H3O+ is added 11 Weak base- Strong acid Titration Curve For this titration phenolphthalein changes colour too slowly, but methyl red’s change occurs on the steep of the curve, so it is the perfect indicator 12  Titration Curves for Polyprotic Acids Except sulfuric acid (H2SO4), the common polyprotic acids are all weak acids. Polyprotic acids have different Ka values for each dissociation of H+ If Ka1 >> Ka2, there will be two equivalence points in the titration. The closer the Ka’s are to each other, the less distinguishable the equivalence points are 13  Titration Curves for Polyprotic Acids In a titration of a diprotic acid such as H2SO3, two OH- ions are required to react with the two H+ ions of each acid molecule. Each mole of H+ is titrated separately. Features of the curve 1. The same amount of base (OH-) is required per mole of H+. 2. There are two equivalence points and two buffer regions. The pH at the midpoint of each buffer region is equal to the pKa of that acidic species. 14 Titration Curves for Polyprotic Acids Titration of 25.0 mL of 0.100 M H2SO3 with 0.100 M NaOH 15  Amino Acids as Biological Polyprotic acids Amino acids – the general formula, NH2- CH(R) – COOH, where R can be one of about 20 different groups. Amino acids contain a weak base (-NH2) and a weak acid (-COOH) on the same molecule. Both the amino and carboxylic acid groups are protonated at low pH: +NH – CH(R) – COOH. Thus, in this form amino acids behave like a 3 polyprotic acid. 16