Chem. 31 – 5/6 Lecture

Announcements Lab Stuff –Reminder that resubmissions must have original report attached –IC and Formal Lab Reports due 5/11 Today’s Lecture –Chapter 10: Acid – Base Titrations Strong Acid – Strong Base Titrations Qualitative Aspects of Acid Base Titrations Weak Acid – Strong Base Titrations

Chapter 10 – Acid Base Titrations Strong Acid – Strong Base Titrations Strong Acid – Strong Base Titration –How does pH Change as NaOH is added? –3 regions to titrations (different calculations in each region): before equivalence point at equivalence point after equivalence point –Go to Board to show pH at 5 mL, 12.5 mL, and 15 mL M NaOH M HCl, 25 mL on this one

Chapter 10 – Acid Base Titrations Strong Acid – Strong Base Titrations M NaOH M HCl, 25 mL Titration Plot

Chapter 10 – Acid Base Titrations Strong Acid – Strong Base Titrations What affects shape of curve? –Concentration of acid and base –Curves are normally sharp. Only exception occurs when intial concentrations are very low (e.g. [H + ] o, [OH - ] o < M) –Note: At low concentrations, one also needs to ensure CO 2 doesn’t affect NaOH solutions Note: Base conc. changed by same factor

Chapter 10 – Acid Base Titrations General Acid – Base Titrations What affects accuracy and precision? –Depends on method for determining equivalence point –End point = “measured” volume of titrant –Titration error (systematic) = difference between end point and equivalence point –With indicators, systematic errors occur when pH(end point) ≠ pK a (indicator) – This causes the titration error –With indicators, random errors occur from difficulty in determining correct color shade which depends on [In - ]/[HIn] –A factor of 2 uncertainty in [In - ]/[HIn] (e.g. 0.5 to 2) translates to pH meas = pH true + 0.3

Chapter 10 – Acid Base Titrations General Acid – Base Titrations What affects accuracy and precision? Example 1 – precise + accurate titration Uncertainty from judging indicator color (e.g. Methyl orange)  V is small pH Vol. Base

Chapter 10 – Acid Base Titrations General Acid – Base Titrations What affects accuracy and precision? Example 2 – mainly imprecise – due to unsharp titration (e.g. low concentrations) Uncertainty from judging indicator color (e.g. Methyl orange)  V is large pH Vol. Base

Chapter 10 – Acid Base Titrations General Acid – Base Titrations What affects accuracy and precision? Example 3 – mainly inaccurate titration (due to selection of wrong indicator small unc. in V but significant error pH Vol. Base equiv. pt. pH indicator color change region (pH < pK a )

Chapter 10 – Acid Base Titrations General Acid – Base Titrations Overview: –What affects accuracy and precision? Best titrations are sharp Best titrations measure pH change near actual equivalence point pH value

Chapter 10 – Acid Base Titrations Some Questions mL of M NaOH is being titrated with M HNO 3. The student wants to estimate the pH after adding 45 mL of HNO 3. To do the calculation, should the student calculate [H + ] or [OH - ] first? 2.For a titration in which HCl is added to a weak base, the pH at the equivalence point is calculated to be A student chooses to use Congo Red as an indicator. This indicator has a pK a of 4.0. a)Will selection of this indicator result in a systematic error? b)Besides, the pK a, what other factor will affect errors in estimating the equivalence point volume?

Weak Acid – Strong Base Titration –How does pH Change as NaOH is added? –Reaction: HA + OH - ↔ A - + H 2 O K = 1/K b Example – acetic acid: K a = 1.74 x –4 regions to titrations (different calculations in each region): initial pH before equivalence point at equivalence point after equivalence point –Go to Board to show pH at 5 mL, 12.5 mL, and 15 mL M NaOH M HA, 25 mL

Chapter 11 – Acid Base Titrations Weak Acid – Strong Base Titrations What affects shape of curve? –pK a values (low pK a or stronger weak acid gives sharper titration) –pK a affects position of curve before and at equivalence point –Concentration of acid and base (higher concentrations give sharper titrations) –Concentration affects curve shape at and after equivalence point –50  M solution will also have greater calculation errors due to poor assumptions (e.g. buffer calculations) 50 mM example pK a = 4.76 example