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Week 8 Notes

by: Cassidy Zirko

Week 8 Notes Chem 143

Cassidy Zirko

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About this Document

Covers Buffers and Titrations
General Chemistry 2
Dr. Cracolice
Class Notes
Chemistry, Gen Chem
25 ?




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This 3 page Class Notes was uploaded by Cassidy Zirko on Saturday March 19, 2016. The Class Notes belongs to Chem 143 at University of Montana taught by Dr. Cracolice in Spring 2016. Since its upload, it has received 11 views. For similar materials see General Chemistry 2 in Chemistry at University of Montana.


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Date Created: 03/19/16
Chem 143, Prof. Cracolice, Week 8  Chapter 59: How are Buffer Solutions Prepared? 3/14/16 59.1 How are Buffer Solution Prepared?  Final step is to prepare a buffer  Commercial buffers tend to be expensive   Packaged buffers­ restricted pH designated by manufactures   3 common methods  o 1­ prepping weak acid solution and then adding soluble salt of its conjugate base  o Analyze chemical scenario, sodium salts completely soluble  o 2­ adding limiting quantity of strong acid  source of hydrogen to solution of  weak base  o Strong acid reacts with weak bases  o Some weak base remains­ acid is limiting quantity  o Solution is buffer made from acid and conjugate base  o Essentially acts as limiting reactant problem using a before, change initial table  o 3­ buffer may also be made by partial neutralization of excess weak acid y strong  base  Chapter 60: How are the Concentration of Acid and Base Solution Determined? 3/16/16 60.1 How are Concentrations of Acid and Base Solutions Determined?  Titration­ very careful addition of one solution to another with a buret   Buret­ measures volume of solution which required to react with carefully measured  amount of another dissolved   Indicator­ change color and flow from buret stops   Standardize solution­ a solution of known concentration for use in later titrations   Cannot be weighted accurately because it absorbs water   Primary standard­  standardized against a weighed quantity of something that can be  weighed accurately   Acids are standardized by reaction with bases   What Chemical Changes Occur in a Strong Acid/Base Titration? o Unknown solution is an acid­ titrate with base  o Unkonw base, titrate with acid  o Strong acid­ source of hydronium, strong base­ source of hydroxide  o Solved with stoichiometry  o Monoprotic­ 1 mole hydronium per 1 mole of hydroxide  o Titration curves­ pH (y­axis) vs.  volume of base (x­axis) Chem 143, Prof. Cracolice, Week 8  o pH changes only slightly as base is added  o base neutralizes some acid, strong acid concentration is sufficient to keep pH  o moles of base added equal to moles of acid, solution changes from strong acid to  neutral solution, then to strong base  o All titration curves gradually rise as base added just before the equivalence point  o Strong acid or base, sharper change in pH near equivalence point Chapter 61: How is the Equivalence Point of a Titration Detected? 3/18/15 61.1 How is the Equivalence Point of a Titration Detected?  Equivalence point­ stoichiometrically equivalent quantities exist in solution   Detection of equivalence point is why there is a need for titration   What are the Characteristics of a Titration? o Concentration of acid equilibrium are known in a generic qualitative curve o Base added to acid­ hydroxide ion react with weak acid molecules creating  conjugate base of weak acid and water o Weak acid and conjugate base create buffer system  resists changes in pH  o Buffer system maintained until weak acid is nearly completely reacted  o Equivalence point approached­ buffer is no longer able to resist large changes in  pH­ continued addition of hydroxide Causes rise in pH o At the equivalence point­ reaction of weak acid has gone to completion  o Solution has conjugate base and weak acid­ causing a basic equilibrium  o Equivalence point for weak acid strong base titration generally from pH=8 to  pH=10 o Adding strong base  strong base­weak base system  effect weak base  solution rapidly becomes strong base  o Titration curve between weak acid and strong base, shape varies considerably  depending on acid and base concentration  o Each curve starts with pH appropriate for acid strength  o Titration curves are symmetrical past equivalence point o Equivalence point occurs when moles of base added equal the moles to acid no  matter acid strength  o Weak base to a strong acid  titration curve opposite of weak acid and strong base o Strong acid added to base from conjugate weak acid  o Equivalence point has pH<7 because solution has weak acid of original base   What are the Characteristics of the Titration Curve of Polyprotic Species?  o Before titration weak acid would be slightly ionized  o Creates weakly acidic solution  o Hydroxide ion titrated into solution  reacts with weak acid  o Creates conjugate base of acid  Chem 143, Prof. Cracolice, Week 8  o Creates buffer, pH remains constant  o Hydroxide ion added gets close to stoichiometric needed to completely react with  first proton on weak acid, pH rises sharply through first equivalence point   solution becomes more basic after buffer destroyed  o After acid reacts, a negatively charged remains  titration curve repeats itself at  higher pH o 2  pH goes through buffer  second equivalence point reached  o after 2 , pH rises because strong base added to weak base   How is an Indicator Selected? o Indicators­ allows for visual detection of equivalence point, varies in color  depending on acidity of solution  o Indicators usually weak acids or bases with different color in solution depending  on major species  o Different colors as an acid or a base o Need to make sure there are different colors on either side of equivalence point  o Need to know titration curve for system and equivalence point and range of  vertical part of the curve. 


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