Redox Reactions

Or How Batteries Work

REDOX Reactions• The simultaneous transfer of electrons between chemical

species.– Actually 2 different reactions occurring at the same time.

Oxidation: a loss of electronsReduction: a gain of electrons

ReDox

Na +Cl NaCl

Nao +Clo Na+Cl-

Which element was Oxidized?Na

Which element was Reduced?Cl

How do we remember ReDox?

mnemonics

5

LEO growls GER

Losing Electrons Oxidation

Gaining Electrons Reduction

Turn to Page 5

7

OIL RIG

OxidationIsLosing

ReductionIsGaining

How do we know if there is aRedOx Reaction occurring?

We first must find the Oxidation states.

What is the oxidation state of each of the following species?

•Cl in Cl2

• 0•Mg in MgO• +2•Na in NaBr• +1

•F in SrF2

• -1•Sr in SrF2

• +2•O in O2

• 0

10J Deutsch 2003

Regents Question

Which particles are gained and lost during a redox reaction?

(1)Electrons

(2) Protons

(3) Neutrons

(4) Positrons

11J Deutsch 2003

Regents Question

As a Ca atom undergoes oxidation to Ca2+, the number of neutrons in its nucleus

(1)Decreases

(2) Increases

(3) Remains the same

Turn to Page 6

Rules for Determining Oxidation Numbers

1. Free Elements have an oxidation number of zero. Ex: Na, S8, H2

2. All Metals in group 1 have a +1 Oxidation # in compounds. Ex: Na, K, Li

3. All Metals in group 2 have a +2 Oxidation # in compounds. Ex: Mg, Ca, Ba

4. Oxygen has a –2 Oxidation Number1. Unless it is in a Peroxide like H2O2

Continued Rules

5. Hydrogen has a +1 oxidation #.6. Halogens have a -1 oxidation #.7. For any neutral compound, the sum of the

oxidation numbers of the atoms in the compound must equal zero. For ex: H2SO4

8. For a polyatomic ion, the sum of the oxidation #’s must equal the ionic charge of the ion. Ex: SO4 2-

CuF2 PBr3

HNO3 C2O4-2

SO4-2 CO3-2

C12H22O11 H2O

Turn to Page 7

Now we will take a deeper look into Oxidation.

• Can be defined as:– The loss of electrons– The gain of oxygen– The loss of hydrogen

OXIDATION• The loss of electrons

Na(s) Na+ + e-

The sodium ion has been oxidized to the sodium cation.

Na + Cl

Sodium loses electron to chlorine

-ClNa +

+

OXIDATION

• Gain of oxygen– Combustion reactions

are classic examplesC(s) + O2(g) CO2(g)

(burning of coal)

2Fe(s) + 3O2(g) 2Fe2O3(s)

(rusting of iron)

OXIDATION

• Loss of hydrogen– Oxidation can sometimes be best seen as the loss

of hydrogenCH3OH(l) CH2O(l) + H2(g)

Methyl alcohol has been oxidized into formaldehyde

Which one of the following elements were oxidized?

•Cl in Cl2

• 0 → NO•Mg in MgO• +2 → YES•Na in NaBr• +1 → YES

•F in SrF2

• -1 → NO•Sr in SrF2

• +2 → YES•O in O2

• 0 → NO

Turn to Page 8

Now we will take a deeper look into Reduction.

• 3 Definitions

– The gain of electrons– The loss of oxygen– The gain of hydrogen

Gain of electrons– The process of silver

electroplating

Ag+ + e- Ag

Silver cation has gained an electron and has been reduced to silver metal.

REDUCTION

REDUCTION

Loss of oxygen – Reduction can also be seen as the loss

of oxygen in going from reactant to product.

Fe2O2 (s) + 3 CO (g) 2 Fe(s) + 3 CO2(g)

Iron ore is reduced to iron metal in a blast furnace with carbon monoxide

REDUCTION• Gain of hydrogen

A reduction can also be described as the gain of hydrogen atoms going from reactant to product.

CO(g) + 2 H2(g) CH3OH(l)

Carbon monoxide has been reduced to methyl alcohol

One’s loss is another’s gain

• Neither oxidation or reduction can take place without the other.– When those electrons are lost something else has

to gain them.

When trying to remember which is which think of LEO the lion goes GER OIL RIG

Lose Electrons Oxidation Oxidation is LosingGain Electrons Reduction Reduction is Gaining

Turn to Page 9

Use a chart to determineOxidation States.

Elements

Oxidation State

Number of Atoms

Total Charge

First we will do K2PtCl6

Elements K Pt Cl Sum of Oxidation States

Oxidation State

Number of Atoms

Total Charge

Now do the following:

• What is the oxidation state of S in MgSO4?

• What is the oxidation state of N in NaNO3?

• What is the oxidation state of Cl in KClO3?

Now do the following:

• What is the oxidation state of C in CO3-

• What is the oxidation state of Cl in ClO2-

• What is the oxidation state of Cr in CrO42-

• What is the oxidation state of Cr in CrO72-

Packet Page 10-11

Review of Terms

• oxidation-reduction (redox) reaction:–involves a transfer of electrons from the reducing agent to the oxidizing agent.

• oxidation: loss of electrons• reduction: gain of electrons•Reducing Agent get oxidized•Oxidizing agent get reduced

Packet Page 12-13

An oxidation-reduction, (redox), reaction involves the transfer of electrons.

The oxidation numbers of the atoms will change…. one goes up (oxidation) and one goes down (reduction)

Sodium transfers its electrons to chlorine

Reduction is the gain of electrons.

Nonmetals gain electrons to form – ionsThe oxidation number goes down (reduces)

A half-reaction can be written to represent reduction.

Cl0 + 1e- Cl-1

In reduction half reactions, electrons are written on the left because electrons are gained

Oxidation is the loss of electrons.

Metal atoms lose electrons to become + ionsThe oxidation numbers go up (increases)

A half-reaction can be written to represent

oxidation.

Na0 Na1+ + 1e-

In oxidation half reactions, electrons are written on the right because electrons are lost

Half-Reactions

• Show either the oxidation or reduction portion of a redox reaction and the electrons gained or lost.

Steps for Writing Half-Reactions

Cu0 + Ag+1N+5O-23 Cu+2 (N+5O-23)2 + Ag0

1. Assign an oxidation number to each element.2. Write a partial half-reaction to show the

change in oxidation state:Oxidation: Cu0 Cu+2

Reduction: Ag+1 Ag0

Steps Continued…

3. Show the number of electrons needed to explain how the oxidation number changed.

Oxidation: Cu0 Cu+2 + 2e-Reduction: Ag+1 +1e- Ag0

4. Last, but not least, achieve conservation of charge: 2Ag+1 +2e- 2Ag0

Practice Problems

1. Mg + Cl2 MgCl2

2. Cu + 2Ag+ Cu2+ + 2Ag3. Al0 + Cr3+ Al3+ + Cr0

Mg + Cl2 MgCl2

Cu + 2Ag+ Cu2+ + 2Ag

Al0 + Cr3+ Al3+ + Cr0

Regents Question:

Given the reaction:Mg(s) + 2H+(aq) + 2Cl–(aq) Mg2+(aq) + 2Cl–(aq)

+ H2(g)

Which species undergoes oxidation?(1) Mg(s)(2) H+(aq) (3) Cl– (aq)(4) H2 (g)

LEO growls GERLEO

Regents Question:

Given the equation:C(s) + H2O(g) CO(g) + H2 (g)

Which species undergoes reduction?(1) C(s)(2) H +

(3) C2+

(4) H2 (g)LEO growls GERGER

Regents Question:In any redox reaction, the substance that

undergoes reduction will

(1) lose electrons and have a decrease in oxidation number

(2) lose electrons and have an increase in oxidation number

(3) gain electrons and have a decrease in oxidation number

(4) gain electrons and have an increase in oxidation number

Regents Question:

Given the equation:2 Al + 3 Cu2+ 2 Al3+ + 3Cu

The reduction half-reaction is(1) Al Al 3+ + 3e – (2) Cu 2+ + 2e – Cu (3) Al + 3e – Al 3+

(4) Cu 2+ Cu + 2e –

Worksheet Page 16-17

Table J tells us if a redox reaction can occur between an atom and an

ion.• A more active metal will replace an ion

below it on Table J.• A more active nonmetal will replace an ion

below it on Table J.• Any metal above H is more active than H

and will react with an acid to produce H2(g)– The higher up the table, the more readily the

replacement will take place.

Regents Question:

According to Reference Table J, which of these metals will react most readily with 1.0 M HCl to produce H2(g)?

(1) Ca(2) K (3) Mg(4) Zn

A more active metal will replace a less active metal from its compound.

• Zn + CuSO4 Cu + ZnSO4

Zinc replaces copper because zinc is more active than copper.

• Cu + ZnSO4 No ReactionCopper cannot replace zinc

Oxidation is on topReduction is on bottom

Review of Terms

• oxidation-reduction (redox) reaction:– involves a transfer of electrons from the reducing agent to the oxidizing agent.

•oxidation: loss of electrons•reduction: gain of electrons

Half-Reactions

•The overall reaction is split into two half-reactions, one involving oxidation and one reduction.

8H+ + Mn+7O4 + 5Fe2+ Mn2+ + 5Fe3+ + 4H2O

Reduction: Mn+7 + 5e Mn2+

Oxidation: 5Fe2+ 5Fe3+ + 5e

AGENTS

• To recap– Oxidizing agent = the species that’s being reduced

– Reducing agent = the species that’s being oxidized

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