# General Chemistry 2

Topics: Entropy, Thermodynamics, Energy Pages: 15 (3750 words) Published: January 21, 2013
Chemistry Book Notes:

Chapter 21: Buffers and the Titration of Acids and Bases

21-1 Henderson-Hasselbalch Equation

THE HH EQUATION OFTEN CAN BE USED TO CALCULATE THE pH OF A BUFFER SOLUTION

-buffer
>a solution containing both a weak acid and its conjugate base can resist a change in pH by neutralizing either an added acid or an added base.
Ex. acetic acid-acetate soln (acid with conj. Base)
> Kc for a buffer reaction can = 1/Ka or 1/Kb if you add an acid or base because it is the reverse of an acid or base equlibrium reaction
**the larger the Kc value indicates that the reaction goes essentially to completion and that all the acid/base added converts to the acid/base in the soln
ex. in this example H3O+ ( CH3COOH; OH- ( CH3COO-
-HH equation
> pH= pKa + log (base/acid) (initial concentrations)
*Conditions for HH eqn:
1) value of Ka for the conjugate acid-base pair should be in the range of 10^-4 -10^-11
2) ratio of base and acid must be between .1 and 10
3) values of acid and base should be between 10^-3 and 1 M

21-2 Buffers

BUFFER SOLUNTIONS SUPRESS A CHANGE IN pH WHEN A SMALL AMT OF EITHER AN ACID OR BASE IS ADDED

-when working with dilute solutions we work in mmol
-calculating mmols of base and acid when an more acid is added (continue with CH3COO example)
** + for CH3COOH because you are adding an acid
-buffer capacity: the capacity of a buffer to resist changes in pH is not unlimited
>if you add enough acid to neutralize the conjugate base then the pH will drop more
>if you add enough base to neutralize the conjugate acid then the pH will rise more -buffer dilution
>buffers resist change even when they are diluted
>if you dilute a buffer by a factor of two then the concentrations of the acid and base are decreased by a factor of two but their ratio is still constant
**therefore pH does not change

21-3 Indicators

AN INDICATOR IS USED TO SIGNAL THE END POINT OF A TITRATION

-titrations
>adding a precisely measured volume of a solution of known concentration to a solution with an unknown concentration measured at a precise volume
>solutions are chosen so that they react completely and the completion is signaled usually by a color change given by an indicator sensitive to pH change -titration curve
>when titrating an acid and a base, a plot of the pH of the resulting solution as a function of the volume is a titration curve -equivalence point
>stoichiometrically equivalent amounts of acid and base have reacted and the reaction is complete
** # of moles A=# of moles B
➢ pH at equivalence point= 7 when neutralizing an acid or base with a salt -end point
> the point at which the indicator changes color in an acid base titration
>end point is the experimental estimate of the equivalence point

21-4 Strong Acid-Strong Base Titration

THE pH CHANGES ABRUPTLY AT THE EQUIVALENCE POINT OF THE TITRATION OF A STRONG ACID WITH A STRONG BASE

21-5 Weak Acid-Strong Base Titration ???

WEAK ACIDS CAN BE TITRATED WITH STRONG BASES

-titration curves of strong bases with weak acids looks different than titration curves of strong acids an bases

21-6 pH=pKa at Midpoint

pH = pKa at the midpoint in the titration of a weak acid with a strong base

- we calculate pH at two points for titrations of weak acid and strong base
>equivalence and initial points
- midpoint
>can use the HH eqn to find the pH at the midpoint and the points around the midpoint of the titration
>the point at which on-half of the acid has reacted with strong base
>acid and base are equal because the amount added is the amount reacted of both acid and base so the pH =pKa in the HH eqn -buffer region
>points along a titration curve near the midpoint

21-7 Weak Base-Strong Acid Titration

Weak bases can be titrated with strong acids

Chapter 23:...