chemistry oxidation-reduction(redox). i. introduction acids donate _____ and bases accept ____...

ChemistryChemistry

Oxidation-ReductionOxidation-Reduction

(Redox)(Redox)

I. IntroductionI. Introduction

Acids donate _____ and bases accept ____Acids donate _____ and bases accept ____

HH++ HH++

proton(s)proton(s) proton(s)proton(s)

In other reactions, substances donate or In other reactions, substances donate or accept accept electronselectrons

The flow of electrons from one substance to The flow of electrons from one substance to the next is how batteries “make” electricitythe next is how batteries “make” electricity

II. Oxidation NumbersII. Oxidation Numbers

Assigning oxidation numbers to substances Assigning oxidation numbers to substances enable chemists to keep track of the enable chemists to keep track of the “transfer” of electrons“transfer” of electrons

Review:Review:

Ionic compounds: give and take e’sIonic compounds: give and take e’s

Covalent compounds: share e’sCovalent compounds: share e’s

In ionic compounds – e’s are really In ionic compounds – e’s are really transferredtransferred

In covalent compounds – e’s are shared In covalent compounds – e’s are shared unevenlyunevenly but we but we pretend pretend that the most that the most electronegative atom gets electronegative atom gets allall the electrons the electrons

Oxidation NumberOxidation Number: The charge that an atom : The charge that an atom in a molecule would develop if the most in a molecule would develop if the most electronegative atom in the molecule took electronegative atom in the molecule took the shared electrons (if the e transfer was the shared electrons (if the e transfer was complete)complete)

Let’s calculate ox #’s for HCl…Let’s calculate ox #’s for HCl…

Chlorine is more electronegativeChlorine is more electronegative

We pretend it takes all the e’sWe pretend it takes all the e’s

That leaves H’s ox # as +1That leaves H’s ox # as +1

Cl ox # as -1Cl ox # as -1

III. Rules for Assigning Oxidation III. Rules for Assigning Oxidation Numbers Numbers

1.  The 1.  The oxidationoxidation number of any number of any pure elementpure element is is zerozero.  Thus the .  Thus the oxidationoxidation number of H in H number of H in H22 is is zero.zero.

2. The 2. The oxidationoxidation number of a number of a monatomic ionmonatomic ion is is equal to its equal to its chargecharge.  Thus the .  Thus the oxidationoxidation number number of Cl in the Clof Cl in the Cl-- ion is -1, that for Mg in the Mg ion is -1, that for Mg in the Mg 2+2+ ion is +2, and that for oxygen in Oion is +2, and that for oxygen in O2-2- ion is -2. ion is -2.

3. The 3. The sumsum of the of the oxidationoxidation numbersnumbers in a in a compound iscompound is zero if neutral zero if neutral, or , or equal to the equal to the charge if an ioncharge if an ion..

4. The 4. The oxidationoxidation number of alkali metals in number of alkali metals in compounds is +1, and that of alkaline earths in compounds is +1, and that of alkaline earths in compounds is +2.  The compounds is +2.  The oxidationoxidation number of F is number of F is -1 in all its compounds.-1 in all its compounds.

5. The 5. The oxidationoxidation number of number of H is +1 in most H is +1 in most compoundscompounds.  Exceptions are H.  Exceptions are H22 (where H = 0) (where H = 0) and the ionic hydrides, such as NaH (where H = and the ionic hydrides, such as NaH (where H = -1).-1).

6. The 6. The oxidationoxidation number of number of oxygen (O) is -2 in oxygen (O) is -2 in most compoundsmost compounds.  Exceptions are O.  Exceptions are O22 (where O (where O = 0) and peroxides, such as H= 0) and peroxides, such as H22OO22 or Na or Na22OO22, , where O = -1.where O = -1.

• • For other elements, you can usually use If no For other elements, you can usually use If no other rules apply, assume ON is the same as the other rules apply, assume ON is the same as the charge taken on in an ionic compound (“the charge taken on in an ionic compound (“the charge it would like to be) charge it would like to be)

ExamplesExamples

1. What are the following ON’s?1. What are the following ON’s?A. Iron in Fe?A. Iron in Fe?

00B. Iron in FeB. Iron in Fe3+?3+?

+3+3

C. Br in MgBrC. Br in MgBr22??

-1-1

D. Br in HBrOD. Br in HBrO33

+5+5

2. What is the ON of each atom in the 2. What is the ON of each atom in the following?following?

A. HA. H22

00

B. Ca(OH)B. Ca(OH)22

Ca: +2Ca: +2 O: -2O: -2 H: +1 H: +1

C. FC. F22COCO

F: -1F: -1 C: +4 O: -2C: +4 O: -2

D. NaSOD. NaSO44--

Na: +1Na: +1 S: +6 O: -2S: +6 O: -2

E. CE. C22HH44

C: -2 H: +1C: -2 H: +1

F. NClF. NCl33

N: +3 Cl: -1N: +3 Cl: -1

G. MgCoBrG. MgCoBr44

Mg: +2 Co: +2 Br: -1Mg: +2 Co: +2 Br: -1

OYO’sOYO’s

16.1 What is the oxidation number of N in each of 16.1 What is the oxidation number of N in each of the following substances?the following substances?

A. NA. N

B. NB. N3-3-

C. NOC. NO33--

16.2 What are the oxidation numbers of all atoms 16.2 What are the oxidation numbers of all atoms in the following compounds?in the following compounds?

A. LiNHA. LiNH22 B. NB. N22HH22 C. Ca(NOC. Ca(NO22))22 D. COD. CO22

E. BFE. BF44-- F. POF. PO44

3-3- G. ClNO G. ClNO H. SH. S88

III. Oxidation and ReductionIII. Oxidation and Reduction

OxidationOxidation: The process by which an atom loses an electron : The process by which an atom loses an electron or electronsor electrons

ReductionReduction: The process by which an atom gains an : The process by which an atom gains an electron or electronselectron or electrons

LEO goes GERLEO goes GERLose Electrons–Oxidation Gain Electrons- ReductionLose Electrons–Oxidation Gain Electrons- Reduction

OIL RIGOIL RIGOxidation Is Loss (of electrons), Reduction Is Gain (of Oxidation Is Loss (of electrons), Reduction Is Gain (of

electrons)electrons)

ExamplesExamples

1. S atom goes from -2 to +6. Oxidized or 1. S atom goes from -2 to +6. Oxidized or reduced? How many electrons?reduced? How many electrons?

2. C atom goes from -2 to -4. Oxidized or 2. C atom goes from -2 to -4. Oxidized or reduced? How many electrons?reduced? How many electrons?

3. An atom goes from +5 to +3. Oxidized or 3. An atom goes from +5 to +3. Oxidized or reduced? How many electrons?reduced? How many electrons?

4. An atom goes from -6 to -1. Oxidized or 4. An atom goes from -6 to -1. Oxidized or reduced? How many electrons?reduced? How many electrons?

IV. Recognizing Redox ReactionsIV. Recognizing Redox Reactions

Mg(s) + 2HCl(aq)Mg(s) + 2HCl(aq)→→ H H22(g) + MgCl(g) + MgCl22(aq)(aq)

00 +1 -1 +1 -1 0 0 +1 -1 +1 -1Mg is oxidizedMg is oxidizedH is reducedH is reduced

NaOH(aq) + HCl(aq) NaOH(aq) + HCl(aq) →→ H H22O(l) +NaCl(aq)O(l) +NaCl(aq)

+1 -2 +1+1 -2 +1 +1 -1 +1 -1 +1 -2 +1 -1+1 -2 +1 -1NOTNOT a redox reaction! a redox reaction!

2Mg(s) + O2Mg(s) + O22(g)(g)→→ 2MgO(s) 2MgO(s)

00 0 0 +2 -2 +2 -2

Mg is oxidized O is reducedMg is oxidized O is reduced

Examples – Redox, or not?Examples – Redox, or not?

A. Ca(s) + ClA. Ca(s) + Cl22(g) (g) →→ CaCl CaCl22(s)(s)

00 0 0 +2 -1 +2 -1 Ca oxidized Cl reducedCa oxidized Cl reduced

B. AgB. Ag++(aq) + OH(aq) + OH--(aq) (aq) →→ AgOH(s) AgOH(s) +1+1 -2 +1 -2 +1 +1 -2 +1 +1 -2 +1

not a redoxnot a redox

C. 3IFC. 3IF55(aq) + 2Fe(s) (aq) + 2Fe(s) →→2FeF2FeF33(aq) + 3IF(aq) + 3IF33(aq)(aq)

+5 -1 +5 -1 0 0 +3 -1 +3 -1 +3 -1+3 -1Fe oxidized I reduced Fe oxidized I reduced

Thionin – Two-Faced SolutionThionin – Two-Faced Solution

ThioThio++ + 2Fe + 2Fe2+2+ + 2H + 2H++ ↔ ThioH ↔ ThioH22++ + 2Fe + 2Fe3+3+

PurplePurple colorless colorless light activatedlight activated

Methylene BlueMethylene BlueOO22(g)(g) →→ O O22(dissolved)(dissolved)

OO22(dissolved) + methylene blue(dissolved) + methylene blue →→ methylene blue methylene blue colorlesscolorless blueblue

Glucose + OHGlucose + OH-- →→ glucoside glucoside

Glucoside + methylene blueGlucoside + methylene blue →→ methylene blue + OH methylene blue + OH--

blueblue colorlesscolorless

** When a redox reaction occurs, there will ** When a redox reaction occurs, there will alwaysalways be atoms that are oxidized be atoms that are oxidized andand atoms that are reducedatoms that are reduced

The thing that is oxidized is the The thing that is oxidized is the reducing reducing agent agent (provides electrons)(provides electrons)

The thing that is reduced is the The thing that is reduced is the oxidizing oxidizing agent agent (takes electrons)(takes electrons)

OYO’sOYO’s

16.5 For each of the following, determine 16.5 For each of the following, determine whether or not it is a redox reaction. If it is whether or not it is a redox reaction. If it is a redox, identify what was oxidized, what a redox, identify what was oxidized, what was reduced, & the oxidizing and reducing was reduced, & the oxidizing and reducing agents.agents.

A. CuA. Cu2+2+(aq) + Zn(s) (aq) + Zn(s) →→ Zn Zn2+2+(aq) + Cu(s)(aq) + Cu(s)

B. 2HB. 2H22O O →→ 2H 2H22 + O + O22

C. NaCl + AgF C. NaCl + AgF →→ AgCl + NaF AgCl + NaF

D. Ca(s)+ 2HNOD. Ca(s)+ 2HNO33(aq)(aq)→→Ca(NOCa(NO33))22(aq)+ H(aq)+ H22(g)(g)

IV. How Batteries WorkIV. How Batteries Work

The fact that atoms can lose and gain electrons in The fact that atoms can lose and gain electrons in a reaction is the bases for how a battery worksa reaction is the bases for how a battery works

Some reactions (like acid-base) involve the Some reactions (like acid-base) involve the rearrangementrearrangement of electrons of electrons

Others (like formation, decomposition, combustion Others (like formation, decomposition, combustion and and displacementdisplacement reactions) involve the reactions) involve the transfertransfer of electrons (redox)of electrons (redox)

Displacement reactionsDisplacement reactions: an ion (or atom) in a : an ion (or atom) in a compound is replaced by an ion (or atom) of compound is replaced by an ion (or atom) of another elementanother element

CuSOCuSO44(aq) + Zn(s) (aq) + Zn(s) →→ ZnSO ZnSO44(aq) + Cu(s)(aq) + Cu(s)

CuCu2+2+(aq) + Zn(s) (aq) + Zn(s) →→ Zn Zn2+2+(aq) + Cu(s)(aq) + Cu(s)

+2+2 0 0 +2+2 0 0

How could you get the electrons that Zn How could you get the electrons that Zn loses to Cu to create a current?loses to Cu to create a current?

You’d have to connect them!You’d have to connect them!

A simple batteryA simple battery

““salt bridge” salt bridge” – – Permits the migration of Permits the migration of ions.These keep the ions.These keep the

ZnZn2+2+ solution from becoming too positive (as solution from becoming too positive (as negative electrons leave), and the other solution negative electrons leave), and the other solution from becoming too negative (as negative from becoming too negative (as negative electrons arrive)electrons arrive)

-- porous barrier: does -- porous barrier: does notnot allow solutions to mix, allow solutions to mix, only permitsonly permits ions ions to flow to both solutionsto flow to both solutions

Anode and CathodeAnode and Cathode

Anode: where oxidation occursAnode: where oxidation occurs

-- negative in galvanic cells-- negative in galvanic cells

Cathode: where reduction occursCathode: where reduction occurs

-- positive in galvanic cells-- positive in galvanic cells

As electrons travel through the wire, As electrons travel through the wire, electricity is created electricity is created

(This schematic is reversed from the one (This schematic is reversed from the one before.)before.)

ExamplesExamples

Draw a diagram of the Galvanic cell, Draw a diagram of the Galvanic cell, indicating the electron flow, labeling the indicating the electron flow, labeling the anode and cathode, and indicating the anode and cathode, and indicating the positive and negative sides of the Galvanic positive and negative sides of the Galvanic cell for the following reactions:cell for the following reactions:

A. 2AgA. 2Ag++(aq) + Mg(s)(aq) + Mg(s)→→ 2Ag(s) + Mg 2Ag(s) + Mg2+2+(aq)(aq)

B. IB. I22(aq) + Fe(s) (aq) + Fe(s) →→ 2I 2I--(aq) + Fe(aq) + Fe2+2+

OYO’sOYO’s

Draw a diagram of the Galvanic cell, Draw a diagram of the Galvanic cell, indicating the electron flow, labeling the indicating the electron flow, labeling the anode and cathode, and indicating the anode and cathode, and indicating the positive and negative sides of the Galvanic positive and negative sides of the Galvanic cell for the following reactions:cell for the following reactions:

16.6 2H16.6 2H++(aq) + Mn(s) (aq) + Mn(s) →→ H H22(g) + Mn(g) + Mn2+2+(aq)(aq)

16.7 3Cl16.7 3Cl22(aq) + 2Al(s) (aq) + 2Al(s) →→ 2Al 2Al3+3+(aq) + 6Cl(aq) + 6Cl--(aq)(aq)

V. Real BatteriesV. Real Batteries

A. The Dry Cell… (no A. The Dry Cell… (no aqueousaqueous substances) substances)Anode:Anode: Zn(s)Zn(s)→→ Zn Zn2+2+(aq) + 2e(aq) + 2e--

Cathode: Cathode: 0 0 +2 +2

2NH2NH44++

(aq) + 2MnO(aq) + 2MnO22(s) (s) →→ Mn Mn22OO33(s) + 2NH(s) + 2NH33(aq) + H(aq) + H22O(l)O(l)

+4+4 +3 +3

B. The Mercury BatteryB. The Mercury Battery

Anode:Anode: Zn(Hg) + 2OHZn(Hg) + 2OH--(aq)(aq)→→ ZnO(s) + H ZnO(s) + H22O(l) + 2e-O(l) + 2e-

00 +2 +2

Cathode: HgO(s) + HCathode: HgO(s) + H22O(l) + 2eO(l) + 2e- - →→ Hg(l) + 2OH Hg(l) + 2OH--(aq)(aq)

+2+2 0 0

Why do batteries stop working?Why do batteries stop working?

One of the reactants is used up.One of the reactants is used up.

The salt bridge degrades.The salt bridge degrades.

Rechargeable batteriesRechargeable batteries

They “charge” by reversing the reactions to They “charge” by reversing the reactions to re-create the original substancesre-create the original substances

Eventually stop working because the salt Eventually stop working because the salt bridge degradesbridge degrades

B. The Lead Acid BatteryB. The Lead Acid BatteryAnode: Pb(s) + SOAnode: Pb(s) + SO44

2-2-(aq) (aq) →→ PbSO PbSO44(s) + 2e(s) + 2e--

00 +2 +2

Cathode: Cathode:

PbOPbO22(s) + 4H(s) + 4H++(aq) + SO(aq) + SO442-2-(aq) + 2e(aq) + 2e- - →→ PbSO PbSO44(s) + 2H(s) + 2H22O(l)O(l)

+4+4 +2 +2

Lead-acid batteries are charged by your Lead-acid batteries are charged by your car’s alternator.car’s alternator.

Stop working because some of the PbSOStop working because some of the PbSO44(s) (s)

falls to the bottom of the battery and is falls to the bottom of the battery and is removed from the reactionremoved from the reaction

VI. CorrosionVI. Corrosion

All redox reactions are not constructive!!All redox reactions are not constructive!!

2Fe(s) + 2H2Fe(s) + 2H22O(l) + OO(l) + O22(g) (g) →→ 2Fe(OH) 2Fe(OH)22(s)(s)

00 0 0 +2 -2 +2 -2

(Fe(OH)(Fe(OH)22 is further oxidized to Fe is further oxidized to Fe22OO33 (rust) (rust)

Corrosion costs US Govt $276 Billion/yearCorrosion costs US Govt $276 Billion/year

OYOOYO

16.8 Consider the lead acid battery:16.8 Consider the lead acid battery:

Pb(s) + PbOPb(s) + PbO22(s) + 4H(s) + 4H++(aq) + 2SO(aq) + 2SO442-2-(aq) (aq) →→

2PbSO2PbSO44(s) + 2H(s) + 2H22O(l)O(l)

What is being oxidized and what is being What is being oxidized and what is being reduced?reduced?