1 Titrations (Review) In a titration a solution of accurately known concentration is added gradually added to another solution of unknown concentration until the chemical reaction between the two solutions is complete. Equivalence point – the point at which the reaction is complete Indicator – substance that changes color at (or near) the equivalence point Slowly add base to unknown acid UNTIL The indicator changes color (pink)

Buret - long glass tube calibrated in mL; contains standard solution In flask: solution of unknown concentration and an indicator Indicator - a substance that changes color with pH Standard solution is slowly added to the solution in the flask until the mixture changes color The equivalence point is when [H 3 O + ] and [OH - ] are equal M acid x V acid = M base x V base Titration For a 1:1 acid-base reaction: For an a:b acid-base ratio in reaction: M a x V a x b moles base = M b x V b a moles acid

An Acid-Base Titration Point of neutralization (“Equivalence point”) All moles of H + ions present in the original volume of acid have reacted with an equivalent amount of moles of OH - ions form the base added moles of H + (originally in flask) = moles of OH - (added from buret) Volume acid x Molarity acid = Volume base x Molarity base H + (aq) + OH - (aq)  H 2 O(l) Point of neutralization

Exp 16B – An Acid-Base Titration Curve Start of titration Excess of acid Point of neutralization Slight excess of base Addition of base until all acid is neutralized Base Acid Indicator H + (aq) + OH - (aq)  H 2 O(l)

5 Alternative Method of Equivalence Point Detection monitor pH

Introduction Acid-base titration curve shows the change in pH during a titration of an acid with a base –Compare Exp 4C: Titration of acetic acid with NaOH monitor titration with change in color of indicator, phenolphthalein, at the Equivalence Point Exp 16B – An Acid-Base Titration Curve

7 Strong Acid-Strong Base Titrations NaOH (aq) + HCl (aq) H 2 O (l) + NaCl (aq) OH - (aq) + H + (aq) H 2 O (l)

8 Weak Acid-Strong Base Titrations CH 3 COOH (aq) + NaOH (aq) CH 3 COONa (aq) + H 2 O (l) CH 3 COOH (aq) + OH - (aq) CH 3 COO - (aq) + H 2 O (l) CH 3 COO - (aq) + H 2 O (l) OH - (aq) + CH 3 COOH (aq) At equivalence point (pH > 7):

Weak acid=strong base specific titration curves When titrating a weak acid with a strong base (or the other way around) the acid dissociation constant, K a, for the weak acid can be determined from the titration curve It coincides with the middle of the “buffer region” of the titration curve Exp 16B – An Acid-Base Titration Curve

Curve for a weak acid-strong base titration.

Purpose of the experiment Perform an acid-base titration of potassium hydrogen phthalate (“KHP”) with sodium hydroxide, measure and record the changes in pH Plot the change of pH as a function of volume of base added Determine the K a for the weak acid potassium hydrogen phthalate, KHP Determine the equivalence point Exp 16B – An Acid-Base Titration Curve

Titration of potassium hydrogen phthalate with sodium hydroxide Standardization of an NaOH solution NaOH needs to be standardized before it can be used in a titration –solid NaOH is hygroscopic and attracts H 2 O from the air you cannot weigh NaOH accurately you’ll also weigh absorbed water –an NaOH solution absorbs CO 2 from the air, which will react to form carbonic acid, making the solution more acidic than the dissolved amount of NaOH suggests –standardization with potassium hydrogen phthalate (KHP) HC 8 H 4 O 4 - (aq) + OH - (l)  C 8 H 4 O 4 2- (aq) + H 2 O(l) –KHP is a weak acid, molar mass = g/mol HC 8 H 4 O 4 - (aq) + H 2 O(l)  C 8 H 4 O 4 2- (aq) + H 3 O + (aq) Exp 16B – An Acid-Base Titration Curve

Standardization of an NaOH solution Equivalence point [H 3 O + ] = [OH - ] –If [H 3 O + ] is known, [OH - ] can be calculated  Vol acid x Molarity acid = Vol base x Molarity base  V acid (L) x M acid (mol/L) = V base (L) x M base (mol/L)  moles of acid = moles of base Prelab problem #2  25 mL 0.10 M NaOH neutralizes how many grams of KHP? KHP is a solid! Answer  25 mL 0.10 mol/L NaOH = 25 mL x 1L/1000 mL x 0.10 mol/L NaOH = ? mol NaOH  At equivalence point: moles of KHP = moles of NaOH = ? mol  ? mol KHP x g/mol = ?? g KHP Exp 16B – An Acid-Base Titration Curve

Determine pK a Acid dissociation constant K a = [H 3 O + ] [A - ]/[HA] [H 3 O + ] = K a [HA]/[A - ] pH = -log [H 3 O + ] = -log K a + (-log [HA]/[A - ]) = pK a - log [HA]/[A - ] = pK a + log [base]/[acid] (Henderson-Hasselbalch equation) When is pH = pK a ? During a titration of the weak acid HA: HA(aq) + OH - (aq)  A - (aq) + H 2 O(l)  HA(aq)(acid) decreases because it reacts with OH - (aq), forming H 2 O  A - (conjugate base) increases  When [HA] = [A - ], ½ of HA has reacted with OH -  pH = pK a + log [A - ]/[HA] = pK a + log 1 = pK a  This point is halfway between the start of the titration and the equivalence point  Remember: at the equivalence point all acid has reacted with OH - : no HA is left Exp 16B – An Acid-Base Titration Curve

Experimental 1.Calibrate the pH meter. Use buffer pH 4 or pH 7 2.Clean the buret and rinse with water followed by a small amount of the NaOH 3.Weigh between 0.50 – 0.53 g KHP. Record the exact mass to 4 decimal places! 4.Dissolve KHP in 50.0 mL (exact!!) of dH 2 O in a 150 mL beaker*. Dissolve completely (why?) 5.Add 2 drops of phenolpthalein indicator (NOT done) 6.Immerse pH electrode in the KHP solution 7.Set up the buret and fill it with 50 mL 0.1 M NaOH solution. Be careful not to spill NaOH in the KHP solution! (Why does that matter? You are going to mix the solutions anyway)! Record the initial volume reading of the burette in your lab manual. The tip of the buret will be positioned slightly above the surface of the solution 8.Read and record the initial pH of the KHP solution before adding any NaOH 9.Start the titration by adding 1-mL aliquots of NaOH solution to the KHP solution while stirring. 10.Record the burette reading and the pH after each addition 11.When the pH changes fast (>0.3 units/mL of base) reduce the amount of base that you add to ~0.2 mL 12.You will quickly pass the equivalence point. When the pH changes start to lag again, return to the 1-mL additions. Keep titrating until the pH ~  To get a good titration curve you need to go at least 5 mL beyond where the pH >11 13.Repeat the titration with a 2 nd sample of KHP Exp 16B – An Acid-Base Titration Curve

Acid-Base Titration Curve

Exp 16B – An Acid-Base Titration Curve Sample12 Vol (equivalence point) (mL) pH (equivalence point) Vol at half-way point (mL) pH pK a KaKa Calculations

Next Monday: Experiment 18 Spontaneity DUE Monday Nov. 19: Exp 16B: An Acid-Base Titration Curve Datasheet, Calculations, Titration curves Post lab questions 1-2 Postlab question 3 (5 bonus points) Exp 18: Spontaneity Prelab preparations –Goal –Procedure –Physical and Chemical Properties of Ammonium nitrate Prelab Questions