WOLPA/AP CHEMISTRY/CDO Chapter 18 Acid-Base Equilibria

WOLPA/AP CHEMISTRY/CDO Acid – Base Reactions Strong acid + strong base: HNO 3 + Ca(OH) 2 H + (aq) + OH - (aq) > H 2 O(l)

WOLPA/AP CHEMISTRY/CDO Acid-Base Reactions Weak acid + strong base: HF + KOH HF(aq) + OH - (aq) F - (aq) + H 2 O(l)

WOLPA/AP CHEMISTRY/CDO Acid-Base Reactions Strong acid + weak base: HClO 4 + NH 3 H + (aq) + NH 3 (aq) NH 4 + (aq) H 2 SO 4 + Na 2 CO 3 H + (aq) + CO 3 2- (aq) HCO 3 - (aq)

WOLPA/AP CHEMISTRY/CDO The Common Ion Effect The common ion effect is when an ion common to the ionization of an acid is present in the solution in an amount greater than that produced by the acid ionization. The presence of this ion, according to LeChatelier's principle, limits the extent to which the acid will ionize and thus affects the pH of the solution.

WOLPA/AP CHEMISTRY/CDO Example Determine the pH of a solution of 0.25 M acetic acid. Ka = 1.8 x 10 -5

WOLPA/AP CHEMISTRY/CDO Example Determine the pH of a solution of 0.25 M acetic acid in a solution of 0.10 M sodium acetate.

WOLPA/AP CHEMISTRY/CDO Example Determine the pH of a solution prepared by mixing 50.0 mL of M HOCl with 50.0 mL of M NaOCl (K a = 3.5x10  8 ).

WOLPA/AP CHEMISTRY/CDO Buffers Acid-base buffers confer resistance to a change in the pH of a solution when hydrogen ions (protons) or hydroxide ions are added or removed. An acid-base buffer typically consists of a weak acid, and its conjugate base. Prepared by adding both the weak acid HB and the salt of its conjugate base B- to water.

WOLPA/AP CHEMISTRY/CDO General Expressions for Buffer Solutions Assume equilibrium is established, therefore Ka = [H 3 O + ] [B - ] / [HB] and [H 3 O + ] = Ka [HB] /[B - ] or Kb = [HB] [OH - ] / [B - ] and [OH - ] = Kb [B - ] /[HB]

WOLPA/AP CHEMISTRY/CDO Example Determine the pH of a solution of 0.10 M acetic acid in a solution of 0.10 M sodium acetate.

WOLPA/AP CHEMISTRY/CDO Effect of H 3 O + or OH - on Buffer System A buffer system contains one species (HB) that will react with added hydroxide ions and another species (B - ) that will react with added hydronium ions. Both reactions will go virtually to completion hence, the added hydronium or hydroxide ions are consumed and the effect on the overall pH is negligible.

WOLPA/AP CHEMISTRY/CDO Buffer Calculations Determine the concentrations of HB and B- after the addition of H3O + or OH -

WOLPA/AP CHEMISTRY/CDO Addition of an Acid 0.10 M HC 2 H 3 O 2 with 0.10 M NaC 2 H 3 O 2 add 50.0 mL 0.10 M HCl

WOLPA/AP CHEMISTRY/CDO Addition of a Base 0.10 M HC 2 H 3 O 2 with 0.10 M NaC 2 H 3 O 2 add 50.0 ml 0.10 M NaOH

WOLPA/AP CHEMISTRY/CDO The Henderson-Hasselbalch Equation pH = pKa + log [conjugate base]/[acid] This equation clearly shows that the pH of the solution of a weak acid and its conjugate base is controlled primarily by the strength of the acid. Can also be written pOH = pKb + log [conjugate acid]/[base]

WOLPA/AP CHEMISTRY/CDO Henderson-Hasselbalch Equation Allows us to predict pH when HB/B  mixed. When [B  ] /[HB] = 1 (i.e. [HB]=[B  ]), pH = pK a

WOLPA/AP CHEMISTRY/CDO Example Calculate pH of solution containing 0.040M Na 2 HPO 4 and 0.080M KH 2 PO 4. pK a2 =7.20.

WOLPA/AP CHEMISTRY/CDO Example Determine the ratio of the concentration of the conjugate acid to concentration of the conjugate base for a weak acid in which the pH was 5.45 and pK a was 5.75.

WOLPA/AP CHEMISTRY/CDO Example Determine the pH of a solution consisting of M NH 3 and M NH 4 Cl.

WOLPA/AP CHEMISTRY/CDO Preparation of a Buffer Solution Selection of weak acid or weak base (pKa ~ pH)

WOLPA/AP CHEMISTRY/CDO Titrations of Weak Acids and Strong Bases Typical net ionic equation HC 2 H 3 O 2 + OH > C 2 H 3 O H 2 O K = 1/Kb(C 2 H 3 O 2 - ) = 1/5.6 x = 1.8 x 10 9 The reaction goes essentially to completion

WOLPA/AP CHEMISTRY/CDO pH Changes -pH starts off at about 2.4, the pH of 1 M HC 2 H 3 O 2 -Region, centered around the half-way point of the titration where pH changes very slowly. In this region there are appreciable amounts of two different species: unreacted HC 2 H 3 O 2 and C 2 H 3 O 2 - ions. Hence, we have a buffer system -Equivalence point - we have a solution of sodium acetate. This solution is basic because C 2 H 3 O 2 - is a weak base. pH at equivalence is > 7.00

WOLPA/AP CHEMISTRY/CDO pH Calculations Write a net ionic equation to determine the extent of the reaction. Calculate the initial pH from the Ka or Kb. Calculate pH at midpoint using the buffer relation. Calculate pH at equivalence point using Ka of conjugate acid or Kb of conjugate base.

WOLPA/AP CHEMISTRY/CDO pH Titration Curves Titration curve: plot of pH of the solution as a function of the volume of base (acid) added to an acid (base).

WOLPA/AP CHEMISTRY/CDO Sharp rise in curve is equivalence point. pH at equivalence point is 7.0 for SA but higher for WA. Equivalence point can be used to determine the concentration of the titrant.

WOLPA/AP CHEMISTRY/CDO Example The equivalence point for mL of an acid occurred when mL of M NaOH was added. What was the molarity of the acid?

WOLPA/AP CHEMISTRY/CDO SA–SB Titrations Base removes some acid and pH increases. Letn b = moles of base added n a,r = moles of acid remaining n a,r = n a  n b = M a V a  M b V b Moles of hydronium ion same as moles of acid remaining. n H3O+ = n a,r ; Valid until very close to equivalence point.

WOLPA/AP CHEMISTRY/CDO Equivalence point(EP): pH = 7.00 Beyond EP: pH due only to base added (i.e. excess base). Use total volume.

WOLPA/AP CHEMISTRY/CDO Example Determine pH of 10.0 mL of 0.100M HCl after addition of 5.00, 10.0 and 15.0mL of 0.100M NaOH.

WOLPA/AP CHEMISTRY/CDO Titration of SB with SA Acid removes some of the base and pH is changed by amount of base removed. Letn a = moles of acid added n b,r = moles of base remaining n b,r = M b V b  M a V a Moles of hydroxide ion same as moles of base remaining. n OH  = n b,r ; –Valid until EP. EP: pH = 7.00 Beyond EP: pH due only to excess acid. Use total volume.

WOLPA/AP CHEMISTRY/CDO Example Determine pH of 10.0 mL of 0.100M NaOH after addition of 5.00, 10.0 and 15.0mL of 0.100M HCl.

WOLPA/AP CHEMISTRY/CDO WA with SB Titration As above base removes some of the acid and pH is changed by amount of acid removed. Letn b = moles of base added n HA = moles of acid remaining n HA = M HA V HA  M b V b n A  = n b = M b V b Up to equivalence point moles of hydronium ions must be determined from equilibrium expression. Equivalence point: pH = pH of salt of WA Beyond Equivalence point: Use amount of excess base to determine pH.

WOLPA/AP CHEMISTRY/CDO Example Determine pH of 10.0 mL of 0.100M HA after addition of 5.00, 10.0 and 15.0mL of 0.100M NaOH. K a = 1.75x10  5.

WOLPA/AP CHEMISTRY/CDO WB–SA Titrations Acid removes some of the base and decreases the pH. Letn a = moles of acid added n b,r = moles of base remaining n b,r = C b V b  C a V a n BH+ = n a = C a V a Moles of hydroxide ions must be determined from equilibrium expression. Valid until EP. EP: pH = pH of salt of weak base. Beyond EP: pH due only to presence of acid added after endpoint (i.e. excess acid) as seen for strong base. Volume correction needed as above (total volume).

WOLPA/AP CHEMISTRY/CDO Example Determine pH of 10.0 mL of 0.100M B after addition of 5.00, 10.0 and 15.0mL of 0.100M HCl. K b = 1.75x10  5.

WOLPA/AP CHEMISTRY/CDO Acid-Base Indicators An acid-base indicator (HIn) is usually an organic dye that is itself a weak acid governed by an equilibrium constant. The acid form has one color and the base form has another. In principle, the color of the indicator changes when [H3O+] = Ka of the indicator.(because HIn = In- at this point)

WOLPA/AP CHEMISTRY/CDO Choice of Indicator 1. Strong acid-Weak base Solution at equivalence point is weakly acidic. Choose indicator which turns color below pH 7. ex methyl red (pH 5)

WOLPA/AP CHEMISTRY/CDO Choice of Indicator 2. Weak acid - Strong base Solution at equivalence point is weakly basic, Choose indicator which turns color above pH 7. ex. phenolphthalein (pH 9)

WOLPA/AP CHEMISTRY/CDO Choice of Indicator Strong acid - Strong base Solution at equivalence point is neutral. However, pH changes so rapidly near the end point that any indicator that changes color between 5 and 9 will work.