Today’s Objectives

Define a buffer as relatively large amounts of aweak acid or base and its conjugate in equilibriumthat maintain a relatively constant pH when smallamounts of acid or base are added.

Section 16.4 (pp. 751-767)

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Chemistry 20 Review pH curve graph showing the continuous change of pH

during an acid-base titration, which continues until the titrant is in great excess

Endpoint point in a titration analysis where the addition of titrant is stopped.

defined empirically by the observed color change of an indicator.

Equivalence Point point in any chemical reaction where chemically equivalent amounts of the reactants have combined.

determine using stoichiometry2

Chemistry 20 Review

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Chemistry 20 Review selecting proper indicators for titration analysis

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Chemistry 20 Review selecting proper indicators for titration analysis

Alizarin yellow is not a suitable indicator

change color long before the equivalence point of this strong acid-strong base reaction, which theoretically has a pH of exactly 7.

Orange IV is also unsuitable

color change would occur too late.

The pH at the middle of the colour change range for bromothymol blue is 6.8 very closely matches the equivalence point pH

bromothymol blue should give accurate results

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Interpreting pH Curves Buffering property of some solutions to resist any

significant change in pH when an acid or base is added

Buffering region (nearly level portions of the graph)

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Interpreting pH Curves Buffering b/c initially, the solution is mainly all water

and OH- ions.

Any additional acid added (H3O+) immediately reacts

with OH- to become water – which does not change the pH significantly.

This “leveling effect” finally fails near the equivalence point, when the OH- is almost completely consumed.

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Interpreting pH Curves Once excess acid has been added, the solution consists

of water and H3O+ ions, so the pH has dropped to the

acid range.

Then any additional acid that is added, simple increase the H3O

+ concentration slightly, but does not change the pH much.

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Acid-Base Indicators any indicator is really two entities for which we use the

same name, a Brønsted-Lowry acid-base conjugate pair

WA & CB pair

At least one of the entities is visibly colored, so qualitative observations can be made when it forms or is consumed.

Examples:

Phenolphthalein – conjugate acid is colorless, conjugate base is bright pink

Bromothymol Blue – conjugate acid is yellow, conjugate base is blue, and when they are in equal quantities (appear green to the human eye)

Litmus Paper – red (HIn) to blue (In-)

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Acid-Base Indicators use the designation HIn for the conjugate acid and

In- for the conjugate base as their actual formulas can be very complex.

Summary:

An indicator is a conjugate weak acid-weak base pair formed when an indicator dye dissolves in water.

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Polyprotic Entities Chem 20 Review:

Polyprotic acids – can lose more than one proton

Polyprotic bases – can gain more than one proton

If more than one proton transfer occurs in a titration, chemists believe the process occurs as a series of single proton transfer reactions. On a graph, this means there will be more than one equivalence point

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Polyprotic Entities

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Polyprotic Entities carbonate ion is a diprotic base

First proton transfer is 100%

Second proton transfer is also 100%

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Polyprotic Entities A pH curve for the addition of NaOH(aq) to a sample of

H3PO4(aq) displays only two rapid changes in pH, even though H3PO4(aq) is triprotic.

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Polyprotic Entities

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Polyprotic Entities This is because only two of the transfers are quantitative.

The third reaction never goes to completion, but instead establishes an equilibrium.

General Rule:

Only quantitative reactions produce detectable equivalence points in an acid-base titration.

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Homework

Practice Qs – p. 754 #1-4; p. 759 #5-10

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General Rule

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General Rule Titrations of Monoprotic Acids and Bases of equal concentrations

Strong Acid and Strong Base: pH at equivalence point is always equal to 7

Strong Acid to Weak Base: pH at equivalence point is always lower than 7

Strong Base to Weak Acid: pH at equivalence point is always higher than 7

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General Rule

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pH Curve Shape SA-SB: water is the only acid or base present ⇒ neutral solution

SA-WB: a weak acid (like NH4+) is present along with water, at the

equivalence point, so the solution is acidic (pH < 7)

WA-SB: a weak base (like CH3COO-) is present along with water, at the equivalence point, so the solution is basic (pH > 7)

WA-WB: do not have detectable equivalence points b/c the rxns are usually not quantitative

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Buffers Buffer relatively large amount of any weak acid

and its conjugate base, in the same solution

in equilibrium, they maintain a relatively constant pH when small amounts of acid or base are added

i.e. buffers resist ΔpH

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Buffers

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Buffers The addition of a small amount of base produces more acetate ions.

The very small change in the acid-base conjugate pair ration and the complete consumption of the OH- explains why the pH change is very slight

The addition of a small amount of acid produces more acetic acid.

The very small change in the acid-base conjugate pair ratio and the complete consumption of the H3O

+ explains why the pH change is very slight

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Buffers Example: Blood Plasma

remarkable buffering ability, as shown by the following table

very useful b/c a change of more than 0.4 pH units can be lethal

If the blood were not buffered, the acid absorbed from a glass of orange juice would likely be fatal.

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Buffering Capacity

limit of the ability of a buffer to maintain a pH level

When one of the entities of the acid-base conjugate pair reacts with an added reagent and is completely consumed, the buffering fails and the pH changes dramatically.

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Buffering Capacity

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All of the CH3COOH(aq) is used up, OH- additions will

now cause the pH to drastically increase

All of the CH3COO-(aq) is

used up, H3O+ additions

will now cause the pH to drastically decrease

Homework

Practice Qs – p. 762 #11-15; p. 766 #16-21

Section 16.4 Review – p. 767 #1-12

Section 16.4 Extra Exercises handout

Chapter 16 Review – p. 772 #1-25

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