OXIDATION-REDUCTION TITRATIONS II: ANALYSIS OF BLEACHExperiment 39

April 6, 2010

Dr. Scott Buzby, Ph.D.

OBJECTIVES

Learn the basics of Oxidation-Reduction (redox) reactions

Show how redox reactions can be used to determine the amount of hypochlorite in household bleach

REDOX REACTIONS Redox (shorthand for reduction-oxidation reaction)

describes all chemical reactions in which atoms have their oxidation number (oxidation state) changed

This can be a simple process such as the oxidation of carbon to yield carbon dioxide or it can be a complex process such as the oxidation of sugar in the human body

The term redox comes from the two concepts of reduction and oxidation Oxidation is the loss of electrons or an increase in

oxidation state by a molecule, atom, or ion Reduction is the gain of electrons or a decrease in

oxidation state by a molecule, atom, or ion Non-redox reactions, which do not involve changes

in formal charge, are known as metathesis reactions

OXIDIZING AND REDUCING AGENTS

The chemical way to look at redox processes is that the reductant transfers electrons to the oxidant. Thus, in the reaction, the reductant or reducing agent loses electrons and is oxidized, and the oxidant or oxidizing agent gains electrons and is reduced

Remember OIL RIG… Oxidation Is Loss and Reduction Is Gain

OXIDIZERS Substances that have the ability to oxidize

other substances are said to be oxidative and are known as oxidizing agents, oxidants, or oxidizers

Put another way, the oxidant removes electrons from another substance, and is thus itself reduced. And, because it "accepts" electrons, it is also called an electron acceptor

Oxidants are usually chemical substances with elements in high oxidation numbers (e.g., H2O2, MnO−4, CrO3, Cr2O2

−7, OsO4) or highly electronegative substances that can gain one or two extra electrons by oxidizing a substance (O, F, Cl, Br)

REDUCERS

Substances that have the ability to reduce other substances are said to be reductive and are known as reducing agents, reductants, or reducers

In other words, the reductant transfers electrons to another substance, and is thus itself oxidized. And, because it "donates" electrons it is also called an electron donor.

PROCEDURE (PAGES 481 – 483) Two redox reactions are involved in the

experiment tonight Initially you will add excess potassium iodide

(KI) to bleach so the iodide ions (I-) are oxidized to iodine (I2) See Eq. 1 Page 479

The iodine is then titrated with a standardized sodium thiosulfate (Na2S2O3) that reduces the iodine (I2) to iodide (I-) See Eq. 2 Page 479

Starch is used for the indicator for this reaction, the endpoint is reached when a drop of thiosulfate turns the solution clear

DUE NEXT WEEK

Report Sheet – Pages 485 – 486 (See Example 39.1 on Page 480)

Questions – Page 487

Pre-Lab Experiment 40 – Pages 492 – 493