Experiment No. 2Experiment No. 2OXIDATION – OXIDATION – REDUCTION REDUCTION REACTIONSREACTIONS

Report by:Report by:Group #2Group #2

Tricia OjonTricia OjonKatrina Mae Lee Katrina Mae Lee

What is redox?What is redox?

RedoxRedox(shorthand for (shorthand for reduction-reduction-

oxidationoxidation reaction) describes reaction) describes all chemical reactions in which all chemical reactions in which atoms have their oxidation atoms have their oxidation number/oxidation state number/oxidation state changedchanged

Terminology:Terminology:The term redox comes from the two The term redox comes from the two

concepts of concepts of redreduction and uction and oxoxidation. idation. OxidationOxidation

describes the loss of describes the loss of electrons/hydrogen, or gain of oxygen / electrons/hydrogen, or gain of oxygen / increase in oxidation state by a increase in oxidation state by a molecule, atom or ionmolecule, atom or ion

ReductionReductiondescribes the gain of electrons / describes the gain of electrons / hydrogen, or loss of oxygen / decrease hydrogen, or loss of oxygen / decrease in oxidation state by a molecule, atom in oxidation state by a molecule, atom or ionor ion

Substances that have the Substances that have the ability to oxidize other substances ability to oxidize other substances and are said to be oxidativeand are said to be oxidative

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

Oxidizing Oxidizing agentsagents//oxidantsoxidants//oxidizersoxidizers

Reducing Reducing agents/reductants/reduceragents/reductants/reducerss Substances that have the Substances that have the ability to reduce other substances ability to reduce other substances and are said to be reductiveand are said to be reductive

The reductant transfers The reductant transfers electrons to another substance, electrons to another substance, and is, thus, oxidized itself. And, and is, thus, oxidized itself. And, because it "donates" electrons it because it "donates" electrons it is also called an electron donoris also called an electron donor

Redox pairRedox pair

The pair of an oxidizing The pair of an oxidizing and reducing agent that are and reducing agent that are involved in a particular involved in a particular reactionreaction

Oxidation stateOxidation state An indicator of the degree of An indicator of the degree of

oxidation of an atom in a chemical oxidation of an atom in a chemical compoundcompound

The hypothetical charge that an The hypothetical charge that an atom would have if all bonds to atoms atom would have if all bonds to atoms of different elements were 100% ionic of different elements were 100% ionic and is used for purposes such as and is used for purposes such as predicting ratio and balancing reactionspredicting ratio and balancing reactions

Represented by integers, which can Represented by integers, which can be positive, negative, or zero (0).be positive, negative, or zero (0).

Oxidation numberOxidation number

The charge that a central The charge that a central atom in a coordination atom in a coordination compound would have if all compound would have if all the ligands were removed the ligands were removed along with the electron pairs along with the electron pairs that were shared with the that were shared with the central atomcentral atom

Represented using Represented using Roman numeralsRoman numerals

OXIDATION OXIDATION STATESTATE

OXIDATION OXIDATION NUMBERNUMBER

Electrons are Electrons are distributed distributed based on based on electronegativity electronegativity of atomsof atoms

Every electron Every electron belongs to the belongs to the ligand, ligand, regardless of regardless of electronegativity electronegativity

Written using Written using Arabic numerals Arabic numerals

Written using Written using Roman numerals Roman numerals

Ex.: OEx.: O-2-2 Ex.: FeEx.: FeIII III oror

iron(III)iron(III)

Assigning oxidation states:Assigning oxidation states:The oxidation state in the free or uncombined The oxidation state in the free or uncombined

or elemental state such as Fe, Hor elemental state such as Fe, H22, O, O22, is zero , is zero (0).(0).

Fluorine has an oxidation state of −1 in all its Fluorine has an oxidation state of −1 in all its compounds since it is the most compounds since it is the most electronegativeelectronegative

Alkali metals (Group I) have an oxidation Alkali metals (Group I) have an oxidation state of +1 in virtually all of their compoundsstate of +1 in virtually all of their compounds

Hydrogen has an oxidation state of +1 except Hydrogen has an oxidation state of +1 except when bonded to less electronegative when bonded to less electronegative elements such as Na, Al, and B, as in NaH, elements such as Na, Al, and B, as in NaH, NaBH4, LiAlH4, where each H has an NaBH4, LiAlH4, where each H has an oxidation state of -1. oxidation state of -1.

Alkaline earth metals (Group II) have an Alkaline earth metals (Group II) have an oxidation state of +2 in virtually all of their oxidation state of +2 in virtually all of their compounds.compounds.

Halogens, other than fluorine have an Halogens, other than fluorine have an oxidation state of −1 except when they oxidation state of −1 except when they are bonded to oxygen, nitrogen or with are bonded to oxygen, nitrogen or with another halogenanother halogen

Oxygen has an oxidation state of −2 Oxygen has an oxidation state of −2 except where it is −1 in peroxides, +2 except where it is −1 in peroxides, +2 in oxygen difluoride, OF2,+1 in O2F2. in oxygen difluoride, OF2,+1 in O2F2.

For monoatomic ions, the oxidation For monoatomic ions, the oxidation number is the valence of the ion, e.g., number is the valence of the ion, e.g., +2 for Ca+2 for Ca2+2+

The sum of the oxidation numbers in a The sum of the oxidation numbers in a polyatomic ion is equal to the ion chargepolyatomic ion is equal to the ion charge

The sum of the oxidation numbers in a The sum of the oxidation numbers in a compound is zerocompound is zero

Assign oxidation states to Assign oxidation states to reactants and productsreactants and products

KK22CrCr22OO77 + H + H22SOSO44 + H + H22OO22 → K → K22SOSO44 + Cr + Cr22(SO(SO44))33 + + OO22 + H + H22OO

+1 +6 -2 +1 +6 -2 +1 -1 +1 +6-2 +3 +6-+1 +6 -2 +1 +6 -2 +1 -1 +1 +6-2 +3 +6-2 0 +1 -22 0 +1 -2

KK22CrCr22OO77 + H + H22SOSO44 + H + H22OO22 → K → K22SOSO44 + Cr + Cr22(SO(SO44))33 + + OO22 + H + H22OO

Balancing Redox reactions:Balancing Redox reactions:

Determine which elements show Determine which elements show an increase or decrease in an increase or decrease in oxidation stateoxidation state

+1 +6 -2 +1 +6 -2 +1 -1 +1 +6-2 +3 +6-+1 +6 -2 +1 +6 -2 +1 -1 +1 +6-2 +3 +6-2 0 +1 -22 0 +1 -2

KK22CrCr22OO77 + H + H22SOSO44 + H + H22OO22 → K → K22SOSO44 + Cr + Cr22(SO(SO44))33 + + OO22 + H + H22OO

Determine the total change in oxidation Determine the total change in oxidation number of the elements oxidized and number of the elements oxidized and reduced and determine the no. of e- reduced and determine the no. of e- gained/lost per molecule or unit of the gained/lost per molecule or unit of the compoundcompound

Decrease from +6 to +3 ; Gained 6 e- per moleculeDecrease from +6 to +3 ; Gained 6 e- per molecule

+6 -1 +3 0 +6 -1 +3 0

KK22CrCr22OO77 + H + H22SOSO44 + H + H22OO22 → K → K22SOSO44 + Cr + Cr22(SO(SO44))33 + O + O22 + + HH22OO

Increase from -1 to 0 ; Lost 2 e- per moleculeIncrease from -1 to 0 ; Lost 2 e- per molecule

Balance the number of electrons gained Balance the number of electrons gained and lost. The numbers used become the and lost. The numbers used become the coefficients.coefficients.

Gained 6 e- per molecule X 1 = 6 e- gainedGained 6 e- per molecule X 1 = 6 e- gained

+6 -1 +3 0 +6 -1 +3 0

KK22CrCr22OO77 + H + H22SOSO44 + + 33HH22OO22 → K → K22SOSO44 + Cr + Cr22(SO(SO44))33 + + 33OO22 + H+ H22OO

Lost 2 e- per molecule X 3 = 6 e- lostLost 2 e- per molecule X 3 = 6 e- lost

Balance the rest of the equationBalance the rest of the equation

KK22CrCr22OO77 + + 44HH22SOSO44 + + 33HH22OO22 → K → K22SOSO44 + Cr + Cr22(SO(SO44))33 + + 33OO22 + + 77HH22OO

Other way of balancing:Other way of balancing:Separate into two half-reactions: the Separate into two half-reactions: the

oxidation portion and reduction portion.oxidation portion and reduction portion.Each half-reaction is balanced separately Each half-reaction is balanced separately

and then the equations are added together and then the equations are added together to give a balanced overall reaction. We to give a balanced overall reaction. We want the net charge and number of ions to want the net charge and number of ions to be equal on both sides of the final be equal on both sides of the final balanced equation.balanced equation.

Balance the atomsBalance the atomsNext, balance the charges in each half-Next, balance the charges in each half-

reaction so that the reduction half-reaction reaction so that the reduction half-reaction consumes the same number of electrons consumes the same number of electrons as the oxidation half-reaction supplies.as the oxidation half-reaction supplies.

Add the two half-reactionsAdd the two half-reactions

Balancing in Acidic/Basic Balancing in Acidic/Basic Medium:Medium:Divide the equation into an oxidation half-Divide the equation into an oxidation half-

reaction and a reduction half-reaction reaction and a reduction half-reaction Balance these :Balance these :

Balance the elements other than H and O Balance the elements other than H and O Balance the O by adding H2O Balance the O by adding H2O Balance the H by adding H+ Balance the H by adding H+ Balance the charge by adding e- Balance the charge by adding e-

Multiply each half-reaction by an integer Multiply each half-reaction by an integer such that the number of e- lost in one part such that the number of e- lost in one part equals the number gained in the other partequals the number gained in the other part

Combine the half-reactions and cancel the Combine the half-reactions and cancel the e-e-

For Basic:**Add OH- to each side until all For Basic:**Add OH- to each side until all H+ is gone and then cancel again**H+ is gone and then cancel again**

Experimental Results:Experimental Results:In test tube A, the reaction produced In test tube A, the reaction produced

very visible results such as the very visible results such as the change in color of the aqueous change in color of the aqueous solution from aqua blue (Cu(NO3)2) solution from aqua blue (Cu(NO3)2) to colorless (Mg(NO3)2), and to colorless (Mg(NO3)2), and dissolving (Mg) and formation (Cu) of dissolving (Mg) and formation (Cu) of solid substances. The oxidizing agent solid substances. The oxidizing agent in the reaction is Cu(NO3)2 while the in the reaction is Cu(NO3)2 while the reducing agent is Mgreducing agent is MgCu(NO3)2 + Mg → Mg(NO3)2 + CuCu(NO3)2 + Mg → Mg(NO3)2 + Cu

In test tube B, the color of the In test tube B, the color of the solution changed from yellow solution changed from yellow orange to aqua blue. The orange to aqua blue. The oxidizing agent is K2Cr2O7 while oxidizing agent is K2Cr2O7 while the reducing agent is H2O2. the reducing agent is H2O2.

K2Cr2O7 + K2Cr2O7 + 44H2SO4 + H2SO4 + 33H2O2 → H2O2 → K2SO4 + Cr2(SO4)3 + K2SO4 + Cr2(SO4)3 + 33O2 + O2 +

77H2OH2O

In test tube C, the color of In test tube C, the color of the solution changed from the solution changed from reddish brown to being reddish brown to being colorless. The oxidizing agent colorless. The oxidizing agent in this reaction is I2 while the in this reaction is I2 while the reducing agent is Na2S2O3. reducing agent is Na2S2O3.

22Na2S2O3 + I2 → Na2S4O6 Na2S2O3 + I2 → Na2S4O6 + + 22NaINaI

In test tube D, there were several In test tube D, there were several changes in the color of the changes in the color of the substance upon addition of the last substance upon addition of the last reagent. There was a slow change reagent. There was a slow change from deep purple to red, then from deep purple to red, then orange, and lastly to being orange, and lastly to being colorless. The oxidizing agent in this colorless. The oxidizing agent in this reaction is KMnO4 while the reaction is KMnO4 while the reducing agent is Na2C2O4reducing agent is Na2C2O4 22KMnO4 + KMnO4 + 88H2SO4 + H2SO4 + 55Na2C2O4 →Na2C2O4 →

22MnSO4 + MnSO4 + 1010CO2 + CO2 + 88H2O + H2O + 55Na2SO4 + Na2SO4 + K2SO4K2SO4

Visible indications of Visible indications of redox:redox:When transitional metal complexes When transitional metal complexes

are involved, a change in color are involved, a change in color often occursoften occursThe d orbital of a transitional metal The d orbital of a transitional metal

ion are split when it is surrounded by ion are split when it is surrounded by ligands. This is because the ligand ligands. This is because the ligand outer electrons repel some of the d outer electrons repel some of the d electrons more than the others. This electrons more than the others. This allows an energy state transition from allows an energy state transition from one d orbital to another in response one d orbital to another in response to the absorption of a photon of lightto the absorption of a photon of light

As the light is absorbed, the solution takes As the light is absorbed, the solution takes on a color. Changes of oxidation state on a color. Changes of oxidation state therefore change the color of the light therefore change the color of the light absorbed, and so the color of the solution absorbed, and so the color of the solution visiblevisible

So you can observe the change in oxidation So you can observe the change in oxidation state of a transition metal in redox reaction state of a transition metal in redox reaction by observing the change in colorby observing the change in color

Other visible indications that a redox Other visible indications that a redox reaction took place are effervescence reaction took place are effervescence or the evolution of gas and the or the evolution of gas and the conversion of ions into metal (evident conversion of ions into metal (evident in the metal precipitate)in the metal precipitate)

Questions and Answers:Questions and Answers: Do all reactions involve redox? Why?Do all reactions involve redox? Why?

Not all reactions involve oxidation-reduction Not all reactions involve oxidation-reduction because some do not experience any because some do not experience any change in the oxidation states of the change in the oxidation states of the substances. Non-redox reactions, which do substances. Non-redox reactions, which do not involve changes in formal charge, are not involve changes in formal charge, are known as metathesis reactions. known as metathesis reactions.

Why should H2O2 be freshly prepared?Why should H2O2 be freshly prepared? Hydrogen peroxide or H2O2 needs to be Hydrogen peroxide or H2O2 needs to be

freshly prepared because it decomposes freshly prepared because it decomposes exothermically into water and oxygen gas exothermically into water and oxygen gas spontaneously. If not used in the freshly spontaneously. If not used in the freshly prepared state, it would not be useful for prepared state, it would not be useful for the characteristic reactions of H2O2.the characteristic reactions of H2O2.

Applications/ ImportanceApplications/ Importance Redox reactions have a large number of Redox reactions have a large number of

practical applications, includingpractical applications, including the storage and later use of electric powerthe storage and later use of electric power the collection of hydrogen and oxygen from waterthe collection of hydrogen and oxygen from water the deposition of a thin layer of one metal on top of the deposition of a thin layer of one metal on top of

anotheranother an indicator not only of a system's capacity for cycling an indicator not only of a system's capacity for cycling

waste, but indeed of chemically supporting fish, plant, waste, but indeed of chemically supporting fish, plant, and invertebrate lifeand invertebrate life

oxidation of cyanide wastes and reduction of oxidation of cyanide wastes and reduction of chromate wastechromate waste

There are both oxidation (e.g. biological There are both oxidation (e.g. biological conversion of ammonia to nitrites to nitrates) conversion of ammonia to nitrites to nitrates) and reduction (ridding systems of nitrate aka and reduction (ridding systems of nitrate aka denitrification, bio-phosphate PO3) that must denitrification, bio-phosphate PO3) that must occur readily in a truly closed system to support occur readily in a truly closed system to support (macro-) life(macro-) life

APPENDIXAPPENDIX

ELEMENTELEMENT COMPOUND / COMPOUND / IONION

COLOR (Aq. COLOR (Aq. Sol’n)Sol’n)

OXIDATION OXIDATION STATESTATE

CuCu Cu(NOCu(NO33))22 Aqua blueAqua blue +2+2

CuCu Brick redBrick red 00

MgMg Mg(NOMg(NO33))22 ColorlessColorless +2+2

MgMg Silvery whiteSilvery white 00

CrCr KK22CrCr22OO77 Yellow orangeYellow orange +6+6

Chromic ionChromic ion GreenGreen +3+3

II II22Reddish Reddish brownbrown 00

KIKI ColorlessColorless -1-1

OO HH22OO ColorlessColorless -2-2

HH22OO22 ColorlessColorless -1-1

SS NaNa22SS22OO33 ColorlessColorless +2+2

NaNa22SS44OO66 ColorlessColorless +2.5+2.5

MnMn KMnOKMnO44 Deep purpleDeep purple +7+7

MnSOMnSO44 ColorlessColorless +2+2

MnOMnO22Brown Brown

precipitateprecipitate +4+4

CC COCO22Colorless Colorless

gasgas +4+4

NaNa22CC22OO44 ColorlessColorless +3+3

ENDEND