Intersection 13:Acid/Base Titration &

Electrochemistry Introduction

11/28/06

Reading: 5.4 p 185-188

19.1-19.2 p 909-917

A Gateway to Scientific EthicsScientists Employ the Scientific Method to Explore the Physical World.

• What are the ethical considerations associated with being a scientist?• How do they relate to the use of the Scientific Method?• Are there ethical considerations beyond the Scientific Method?

We will employ a case-study approach to examining these questions including a prominent case of fraud from Lucent Technologies as wellas the classic Millikan-Ehrenhaft debate.

6 – 8 PM Thursday Nov. 30th

Please RSVP by email to Prof Banaszak Holl(mbanasza@umich.edu)

Gateway evenings are optionaland will not affect your course grade

Gateway Chemistry 130/125/126Section 600

Dinner Provided

Outline

• Acid/Base Titration

• Electrochemistry

– Oxidation numbers

Titration

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Titrations: A Closer Look

A titration is a method of determining the concentration of a dissolved substance by addition of a reagent of known concentration until a stoichiometric amount has been added indicated by a known effect (precipitation, color change, change in heat, etc.)

Titrations are popular because: Absolute content of a sample can be determined. Speed. Versatility Sample size: micrograms up to several grams Accuracy and reproducibility Price

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Common Uses of Titrations

• TAN(Total Acid Number) and TBN(Total Base Number) determinations of motor oil

• Various weak base drug assays by titration with perchloric acid in a glacial acetic acid media

• Differentiation of nitric and sulfuric acids in nitrating acid used in the explosives industry.

• Determination of salt in various foods and snacks.• Determination of chloride in drinking water• Simultaneous determination of chloride, bromide, and iodide in

seawater.• Determination of 925 silver in the jewelry industry• Determination of silver in photographic developing solutions.

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Titration

• Precipitation: Cl-(aq) + AgNO3(aq) →AgCl(s)

– What was the purpose?– How did you find the end-point?

• Acid-base (Oxiclean)

• Redox (coming up…)

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Acid/Base Reactions

Need to determine the concentration of a solution of HCl. How?

20 mL sample of HCl diluted to 50 mL

1.0 M NaOH used for titration

Reaction:

How will you know when you have reached the end-point?

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Determining End-point?M

Finding the Concentration

Suppose it takes 15 mL of NaOH, what was the concentration of HCl?

20 mL sample of HCl diluted to 50 mL

1.0 M NaOH used for titration

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Problem 1

Strong Acid/Strong Base

HCl(aq) + NaOH(aq) H2O(l) + NaCl(aq)

Weak Acid/Strong Base

CH3COOH (aq) + NaOH (aq)

25 mL 20 mL of 0.1 M

What is the concentration CH3COOH?

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Question 1Each of the solutions in the table has equal volume

and the same concentration of 0.1 M

Acid pH

HCl 1.1

HCOOH 2.3

CH3COOH 2.9

HCN 5.1

Which solution requires the greatest volume of 0.1 M NaOH to reach the titration end-point (stoichiometric point)? Explain

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What happens when you add less than a stoichiometric amount of a strong base to

a weak acid?

2 mols CH3COOH + 1 mol NaOH in 1 L of water?

(Ka CH3COOH = 1.8 x10-5)

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Question 2

Each of the solutions in the table has equal volume and the same concentration of 0.1 M?

Which will have the highest pH at the stoichiometric point?

Acid pH

HCl 1.1

HCOOH 2.3

CH3COOH 2.9

HCN 5.1

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Problem 2

When 40.00 mL of a weak monoprotic acid solution is titrated with 0.100 M NaOH, the equivalence point is reached when 35.00 mL of base have been added. After 20.00 mL of NaOH solution has been added, the titration mixture has a pH of 5.75. Calculate Ka.

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Solution: Initial Concentration HA

HA + NaOH H2O + NaA

40 mL 35 mL

0.10 M

0.1 mol NaOH * (0.035L) * 1 mol HAL 1 mol NaOH

= 0.0035 mol HA in 0.040 L

= 0.0875 M HA

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Solution: Finding Initial Values for ICE Table

HA + NaOH -> H2O + NaA

40 mL 20 mL

0.0875 M 0.1 M

0.0035 mol 0.002 mol limiting reagent ?

-0.002 mol -0.002 mol + 0.002 mol + 0.002 mol

0.0015 mol 0 0.002 mol 0.002 mol

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Solution: ICE table to find Ka

HA H2O <-> A- H3O+

I 0.0015 mol

40 +20 mL

0.002 mol

40 + 20 mL

0

C -x + x +x

E 0.025 – x 0.033+x x

pH = 5.75 = -log[H3O+] 1.78x10-6 = [H3O+] = x

Ka = [H3O+][A-] = 1.78x10-6*0.033 [HA] 0.025

= 2.35 x10-6

0.025 0.033 1.78 x10-6

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Exam 3

• Tuesday, December 5th 8-10 pm CHEM 1400

• Focus on material through this slide

• Please remember your significant figures

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http://genchem.chem.wisc.edu/demonstrations/Gen_Chem_Pages/17electropage/potato_clock.htm

ElectrochemistryA

Redox Reactions

• Chemical reactions: acid/base or reduction/oxidation ("redox")

• Redox reactions involve the transfer of electrons from one reactant to another.

• The reaction energy can also be harnessed in the form of electricity.  

• The branch of chemistry that deals with the relationship between electricity and chemical reactions is called electrochemistry.

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Oxidation States

The concept of oxidation state (or oxidation number) was developed to help scientists keep track of the movement of electrons in reactions.

2 Fe(s) + 3 Cl2(g) 2 FeCl3(s)

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Oxidation Number “Rules”

1. For an atom in its elemental form, the oxidation state is always 0.

2. For any monatomic ion, the oxidation state equals the charge on the ion.

2 Fe(s) + 3 Cl2(g) 2 FeCl3(s)

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3. Specific elements a) The oxidation state of oxygen is almost always -2.  The

only major exception is in peroxides (H-O-O-H), where oxygen has an oxidation state of -1.

b) The oxidation state of hydrogen is +1 when bonded to non-metals and -1 when bonded to metals.

c) The oxidation state of fluorine is always -1.  The other halogens will usually have an oxidation state of -1 in binary compounds.  In some instances the halogens other than fluorine may have different oxidation states.  For example, in oxyanions the halogens have positive oxidation states.

H2O

HNO3

ZnH2

CF4

NaClO4

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4. The sum of the oxidation states of all atoms in a neutral compound must equal 0.  The sum of the oxidation states of all atoms in a polyatomic ion equals the charge of the ion.

Usually, the most electronegative atom in the molecule will have a negative oxidation state.

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2 Fe(s) + 3 Cl2(g) 2 FeCl3(s)