a.k.a Electrochemistry

a.k.a. Oxidation-Reduction

Redox!

Review of Oxidation numbers

The charge the atom would have in a molecule (or anionic compound) if electrons were completely transferred.

1. Free elements (uncombined state) have an oxidation number of zero.

Na, Be, K, Pb, H2, O2, P4 = 0

2. In monatomic ions, the oxidation number is equal to the charge on the ion.

Li+, Li = +1; Fe3+, Fe = +3; O2-, O = -2

3. The oxidation number of oxygen is usually –2. In H2O2

it is –1. 4.4

Rules for Assigning Oxidation NumbersRules for Assigning Oxidation Numbers

1)The oxidation number of any uncombined element is zero.

2)The oxidation number of a monatomic ion equals its charge.

11

2

00

22

ClNaClNa

Rules for Assigning Oxidation NumbersRules for Assigning Oxidation Numbers

3)The oxidation number of oxygen in compounds is -2, except in peroxides, such as H2O2 where it is -1.

4)The oxidation number of hydrogen in compounds is +1, except in metal hydrides, like NaH, where it is -1.

2

2

1

OH

Rules for Assigning Oxidation NumbersRules for Assigning Oxidation Numbers

5) The sum of the oxidation numbers of the atoms in the compound must equal 0.

2

2

1

OH2(+1) + (-2) = 0 H O

2

122

)(

HOCa(+2) + 2(-2) + 2(+1) = 0 Ca O H

Rules for Assigning Oxidation NumbersRules for Assigning Oxidation Numbers

6) The sum of the oxidation numbers in the formula of a polyatomic ion is equal to its ionic charge.

3

2?

ONX + 3(-2) = -1N O

24

2?

OS

X = +5 X = +6

X + 4(-2) = -2S O

Electron Transfer ReactionsElectron Transfer ReactionsElectron Transfer ReactionsElectron Transfer Reactions

• Electron transfer reactions are oxidation-reduction or

redox reactions.

• Results in the generation of an electric current (electricity)

or be caused by imposing an electric current.

• Therefore, this field of chemistry is often called

ELECTROCHEMISTRY.

2Mg (s) + O2 (g) 2MgO (s)

2Mg 2Mg2+ + 4e-

O2 + 4e- 2O2-

Oxidation half-reaction (lose e-)

Reduction half-reaction (gain e-)

19.1

Electrochemical processes are oxidation-reduction reactions in which:

• the energy released by a spontaneous reaction is converted to electricity or

• electrical energy is used to cause a nonspontaneous reaction to occur

0 0 2+ 2-

Terminology for Redox ReactionsTerminology for Redox ReactionsTerminology for Redox ReactionsTerminology for Redox Reactions

• OXIDATIONOXIDATION—loss of electron(s) by a species; —loss of electron(s) by a species; increase in oxidation number; increase in oxygen.increase in oxidation number; increase in oxygen.

• REDUCTIONREDUCTION—gain of electron(s); decrease in —gain of electron(s); decrease in oxidation number; decrease in oxygen; increase in oxidation number; decrease in oxygen; increase in hydrogen.hydrogen.

• OXIDIZING AGENTOXIDIZING AGENT—electron acceptor; species is —electron acceptor; species is reduced. (an agent facilitates something; ex. Travel reduced. (an agent facilitates something; ex. Travel agents don’t travel, they facilitate travel)agents don’t travel, they facilitate travel)

• REDUCING AGENTREDUCING AGENT—electron donor; species is —electron donor; species is oxidized.oxidized.

• OXIDATIONOXIDATION—loss of electron(s) by a species; —loss of electron(s) by a species; increase in oxidation number; increase in oxygen.increase in oxidation number; increase in oxygen.

• REDUCTIONREDUCTION—gain of electron(s); decrease in —gain of electron(s); decrease in oxidation number; decrease in oxygen; increase in oxidation number; decrease in oxygen; increase in hydrogen.hydrogen.

• OXIDIZING AGENTOXIDIZING AGENT—electron acceptor; species is —electron acceptor; species is reduced. (an agent facilitates something; ex. Travel reduced. (an agent facilitates something; ex. Travel agents don’t travel, they facilitate travel)agents don’t travel, they facilitate travel)

• REDUCING AGENTREDUCING AGENT—electron donor; species is —electron donor; species is oxidized.oxidized.

You can’t have one… without the other!

• Reduction (gaining electrons) can’t happen without an oxidation to provide the electrons.

• You can’t have 2 oxidations or 2 reductions in the same equation. Reduction has to occur at the cost of oxidation

LEOLEO the lion says the lion says GERGER!!oossee

lleeccttrroonnss

xxiiddaattiioonn

aaiinn

lleeccttrroonnss

eedduuccttiioonn

GER!GER!

Another way to remember

•OIL RIGxxiiddaattiioonn

ss oossee

eedduuccttiioonn

ss aaiinn

Oxidation and Reduction (Redox)Oxidation and Reduction (Redox)

• Redox currently says that electrons are transferred between reactants

Mg + S → Mg2+ + S2-

•The magnesium atom changes to a magnesium ion by losing 2 electrons, and is thus oxidized

•The sulfur atom is changed to a sulfide ion by gaining 2 electrons, and is thus reduced.

Oxidation and Reduction (Redox)Oxidation and Reduction (Redox)11

2

00

22

ClNaClNa

Each sodium atom loses one electron:

Each chlorine atom gains one electron:

eNaNa10

10 CleCl

LEO says GER :LEO says GER :

eNaNa10

Lose Electrons = Oxidation

Sodium is oxidized

Gain Electrons = Reduction

10 CleCl Chlorine is reduced

LEO says GER :LEO says GER : - Losing electrons is oxidation, and the substance that loses the electrons is called the reducing agent. - Gaining electrons is reduction, and the substance that gains the electrons is called the oxidizing agent.

Mg(s) + S(s) → MgS(s)

Mg is oxidized – loses e-

S is reduced – gains e-

Mg is the reducing agent

S is the oxidizing agent

Not All Reactions are Redox ReactionsNot All Reactions are Redox Reactions

- Reactions in which there has been no change in oxidation number are not redox reactions.

Examples:

)()()()( 3

2511111

3

251

aqONNasClAgaqClNaaqONAg

)()()()(22

2

1

4

26

2

1

4

26

2

1121

lOHaqOSNaaqOSHaqHONa

Reducing Agents and Oxidizing AgentsReducing Agents and Oxidizing Agents

• An increase in oxidation number = oxidation

• A decrease in oxidation number = reduction

eNaNa10

10 CleCl

Sodium is oxidized – it is the reducing agent

Chlorine is reduced – it is the oxidizing agent

Trends in Oxidation and ReductionTrends in Oxidation and ReductionActive metals:

Lose electrons easily Are easily oxidized Are strong reducing agents

Active nonmetals: Gain electrons easily Are easily reduced Are strong oxidizing agents

Identifying Redox EquationsIdentifying Redox Equations In general, all chemical reactions can be assigned to one of two classes:

1)oxidation-reduction, in which electrons are transferred:• Single-replacement, combination,

decomposition, and combustion

2) this second class has no electron transfer, and includes all others:

• Double-replacement and acid-base reactions

Identifying Redox EquationsIdentifying Redox Equations In an electrical storm, oxygen and

nitrogen react to form nitrogen monoxide:

N2(g) + O2(g) → 2NO(g)

•Is this a redox reaction?

•If the oxidation number of an element in a reacting species changes, then that element has undergone either oxidation or reduction; therefore, the reaction as a whole must be a redox.

YES!

Balancing Redox EquationsBalancing Redox Equations It is essential to write a correctly balanced equation that represents what happens in a chemical reaction

• Fortunately, two systematic methods are available, and are based on the fact that the total electrons gained in reduction equals the total lost in oxidation. The two methods:

1)Use oxidation-number changes

2)Use half-reactions

Using half-reactionsUsing half-reactions A half-reaction is an equation showing

just the oxidation or just the reduction that takes place

they are then balanced separately, and finally combined

Step 1: write unbalanced equation in ionic form

Step 2: write separate half-reaction equations for oxidation and reduction

Step 3: balance the atoms in the half-reactions

Using half-reactionsUsing half-reactions continued

•Step 4: add enough electrons to one side of each half-reaction to balance the charges

•Step 5: multiply each half-reaction by a number to make the electrons equal in both

•Step 6: add the balanced half-reactions to show an overall equation

•Step 7: add the spectator ions and balance the equation

•To obtain a useful To obtain a useful current, we separate the current, we separate the oxidizing and reducing oxidizing and reducing agents so that electron agents so that electron transfer occurs thru an transfer occurs thru an external wire. external wire.

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons

CHEMICAL CHANGE --->CHEMICAL CHANGE --->ELECTRIC CURRENTELECTRIC CURRENT

CHEMICAL CHANGE --->CHEMICAL CHANGE --->ELECTRIC CURRENTELECTRIC CURRENT

This is accomplished in a This is accomplished in a GALVANICGALVANIC or or VOLTAICVOLTAIC cell. cell.

A group of such cells is called a A group of such cells is called a batterybattery..

http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/galvan5.swf

Galvanic Cells

19.2

spontaneousredox reaction

anodeoxidation

cathodereduction

- +

Galvanic Cells

19.2

The difference in electrical potential between the anode and cathode is called:

• cell voltage

• electromotive force (emf)

• cell potential

Cell Diagram

Zn (s) + Cu2+ (aq) Cu (s) + Zn2+ (aq)

[Cu2+] = 1 M & [Zn2+] = 1 M

Zn (s) | Zn2+ (1 M) || Cu2+ (1 M) | Cu (s)

anode cathode

Standard Electrode Potentials

19.3

Zn (s) | Zn2+ (1 M) || H+ (1 M) | H2 (1 atm) | Pt (s)

2e- + 2H+ (1 M) H2 (1 atm)

Zn (s) Zn2+ (1 M) + 2e-Anode (oxidation):

Cathode (reduction):

Zn (s) + 2H+ (1 M) Zn2+ + H2 (1 atm)

Standard Electrode Potentials

19.3

Standard reduction potential (E0) is the voltage associated with a reduction reaction at an electrode when all solutes are 1 M and all gases are at 1 atm.

E0 = 0 V

Standard hydrogen electrode (SHE)

2e- + 2H+ (1 M) H2 (1 atm)

Reduction Reaction

19.3

• E0 is for the reaction as written

• The more positive E0 the greater the tendency for the substance to be reduced

• The half-cell reactions are reversible

• The sign of E0 changes when the reaction is reversed

• Changing the stoichiometric coefficients of a half-cell reaction does not change the value of E0