A very, very, very bad joke…

A cation and an anion were walking down the street…

…and the cation says to the anion, “Whoa, I think I lost an electron!”

The anion replies, “Are you sure?”

…to which the cation replies, “Yes…I’m POSITIVE!”

<FORCED LAUGHTER>www.bitterfilms.com

Cations and Anions and Electrons, Oh My!

Al0 Al3+

____ protons ____ protons____ electrons ____ electrons1313

1310

Al0 has lost 3 electrons.

+ 3e-

Al0 has been oxidized:Its oxidation state has increased from 0 3.

Al3+ Al0

____ protons ____ protons____ electrons ____ electrons1310

1313

Al3+ has gained 3 electrons.

+ 3e-

Al3+ has been reduced:Its oxidation state has been reduced from 3 0.

Redox Reactions: A New Type of Reaction All chemical reactions can be categorized, roughly into:

• Acid-base reactions – depending on your definition

• Oxidation-Reduction (redox) reactions – exchange of electrons

Al0 Al3+ + 3e-

I20 + 2e- 2I-

Displacement reactions – exchange of ions

Acid-base reactions – exchange of H+ or OH-

NaBr (aq) + AgNO3 (aq) NaNO3 (aq) + AgBr (s)

HBr (aq) + LiOH (aq) LiBr (aq) + H2O (l)

Redox Rules! Calculating Oxidation States

1. Oxidation states for elements are always zero.e.g. N2(g), Co(s), H2(g), C(gr), P4(s), Na(s), Ne(g)

2. Oxidation states for monatomic ions are the same as their charges.e.g. Al3+, Cd2+, Zn2+, Ag+,and Group IA(1) and IIA(2) ions

3. The oxidation state of oxygen is usually 2-, except for peroxides (-1).e.g. Na2O, Al2O3; compare with H2O2, Na2O2 – remember, O2

2-

4. The oxidation state of hydrogen is usually 1+, except for hydrides (-1).e.g. H2O, OH-; compare with NaH, LiAlH4 – look for H bonded to metal

5. The oxidation state of halides are 1-, except when bonded to oxygen.e.g. HF, NaCl, MgBr2 (F is always 1-); compare with ClO3

-

6. The oxidation numbers MUST add up to the molecule’s total charge.

O

H H

More Practice with Oxidation States

Cr2O72- Cr = ? O = -2

2(?) + 7(-2) = -2Cr = +6 dichromate

TiCl4 4 Cl = 4(-1)

Ti = +4titanium(IV) chloride

H2SO3 S = +4 sulfurous acid

BaO2 2 O = 2(-1) barium peroxide

P2O5 2 P = 2(+5) diphosphorus pentoxide

Back Again?! Aluminum and Iodine

Al (s) + I2 (s) AlI3 (s)

0

2(0) 3(-1)

+3

Al: 0 +3 I: 0 -1

So, now that you know oxidation states…what is oxidized, and what is reduced?

Mnemonic Devices for Remembering Redox Rules

LEO the lion goes GER

O

I

L

GER

LEO

Oxidation

Is

Loss

R

I

G

Reduction

Is

Gain

When an atom/molecule Loses Electrons, it is Oxidized.-its oxidation state becomes more positive-it is the reducing agent, or reductant

When an atom/molecule Gains Electrons, it is Reduced.-its oxidation state becomes more negative-it is the oxidizing agent, or oxidant

http://www.franklinhoward.com/

Al (s) + I2 (s) AlI3 (s)

Back For Good!! Aluminum and Iodine

0

2(0) 3(-1)

+3

Al: 0 +3 I: 0 -1oxidation state increases

loses electronsAl is oxidized

Al is the reducing agent

oxidation state decreasesgains electrons

I2 is reducedI2 is the oxidizing agent

2 Al (s) + 3 I2 (s) 2 AlI3 (s)

Identifying Oxidizing/Reducing Agents Oxidizing/reducing agents are ALWAYS REACTANTS!

Oxidizing agents (oxidants) are reactants that are REDUCED.Reducing agents (reductants) are reactants that are OXIDIZED!

Zn (s) + 2H+ (aq) H2 (g) + Zn2+ (aq)0 2(+1) 2(0) +2

Zn loses/gains electrons

Zn is oxidized/reducedZn is the reductant/oxidant

H+ loses/gains electrons

H+ is oxidized/reducedH+ is the reductant/oxidant

Identifying Oxidizing/Reducing Agents Oxidizing/reducing agents are ALWAYS REACTANTS!

Oxidizing agents (oxidants) are reactants that are REDUCED.Reducing agents (reductants) are reactants that are OXIDIZED!

2 H2O (l) + Al (s) + MnO4- (aq) Al(OH)4

- (aq) + MnO2 (s)

What is oxidized? What is reduced?

What is the oxidizing agent? What is the reducing agent?

How many electrons are transferred?

0 +7 +3 +4

Al (s) MnO4- (aq)

MnO4- (aq) Al (s)

3 e-

Practical Applications: Electroplating

AgNO3 (s) Ag+ (aq) + NO3- (aq)H2O

Ag+ (aq) + e- Ag (s)

Electroplating deposits coats of metal onto objects

Bronze baby shoes

Cu+ (aq) + e- Cu (s)

Recoating Tarnished objects

http://www.castinggifts.com/shoebronzing

http://www.collectorssecret.com/images/sprbeforeafter.jpg

More Practical Applications: Photosynthesis Photosystem II

1. Chlorophyll in P680 molecule absorbs light.2. P680 gives up electrons to a transport chain, becoming positive.3. Electrons from 2 H2O molecules are given to P680 by a Mn4 center.

The Mn4 center works becauseMn2+, Mn3+, Mn4+, and Mn5+ areall accessible oxidation states.

2 H2O O2 + 4 H+ + 4 e-

H2O is a reducing agent!

www.pdphoto.orghttp://www.lbl.gov/Science-Articles/Archive/PBD-water-split.html

More Practical Applications: Thermite Reaction

Fe2O3 (s) + Al (s) Al2O3 (s) + Fe (l)

Railroad trackrepairs

UnderwaterWelding

2 Fe (l) + 3H2O (l) Fe2O3 (s) + 3H2 (g)

http://www.pagesbydave.com/divepics/pages/underwater%20welding.htmlwww.wikipedia.org

More Practical Applications: Thermite Reaction

Fe2O3 (s) + Al (s) Al2O3 (s) + Fe (l)

2(+3)iron(III) oxide

0

0 2(+3)

Iron(III) oxide is the oxidizing agent.

Aluminum oxide is the reducing agent.

Limits to Redox Reactions

What is the oxidation number of S in the following compounds?

H2S

S8

SCl2

Na2SO3

SO42-

-2

0

+2

+4

+6

Which cannot be an oxidizing agent?

Which cannot be a reducing agent?

(reduced)

(oxidized)

H2S – cannot gain more electrons

SO42- – cannot lose more electrons

Disproportionation and ConproportionationThese are special types of redox reactions:

Disproportionation – One compound decomposes into two oxidation states

H2O2 (aq) H2O (l) + O2 (g)

2(-1) -2 2(0)

H2O2 is BOTH oxidized and reduced!

Conproportionation – Two oxidation states become one.

catalyst

Cr1+ (aq) + Cr3+ (aq) 2 Cr2+ (aq)