““Oxidation-Reduction Reactions”Oxidation-Reduction Reactions”

LEO SAYS GER

Pre-AP Chemistry

Charles Page High School

Stephen L. Cotton

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

A process called “reduction” is the opposite of oxidation, and originally meant the loss of oxygen from a compound Oxidation and reduction always occur

simultaneously The substance gaining oxygen (or

losing electrons) is oxidized, while the substance losing oxygen (or gaining electrons) is reduced.

Oxidation and Reduction (Redox)Oxidation and Reduction (Redox) Today, many of these reactions may

not even involve oxygen Redox currently says that electrons

are transferred between reactants

Mg + S → Mg2+ + S2-

•The magnesium atom (which has zero charge) changes to a magnesium ion by losing 2 electrons, and is oxidized to Mg2+

•The sulfur atom (which has no charge) is changed to a sulfide ion by gaining 2 electrons, and is reduced to S2-

(MgS)

Leo the Lion!

• LEO the lion says GER– Loss of electrons is oxidation, gain of electrons

is reduction

LET'S LOOK AT

HALF REACTIONS

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

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc.

7

Identify each of the following half reactions as:

1) oxidation or 2) reduction

__A. Sn(s) Sn4+(aq) + 4e−

__B. Fe3+(aq) + 1e− Fe2+(aq)

__C. Cl2(g) + 2e− 2Cl−(aq)

Learning Check

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc.

8

Identify each of the following as:

1) oxidation or 2) reduction

1 A. Sn(s) Sn4+(aq) + 4e−

2 B. Fe3+(aq) + 1e− Fe2+(aq)

2 C. Cl2(g) + 2e− 2Cl−(aq)

Solution

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-, becomes a Mg2+ ion

S is reduced: gains e- = S2- ion

Mg is the reducing

agent

S is the oxidizing agent

Oxidation and Reduction (Redox)Oxidation and Reduction (Redox) It is easy to see the loss and gain of electrons in ionic compounds, but what about covalent compounds?

Oxygen is highly electronegative, so, in water:

the oxygen gains electrons (is reduced and is the oxidizing agent), and the hydrogen loses electrons (is oxidized and is the reducing agent)

Other rules to remember:

• Flourine is ALWAYS -1.• Other halogens are going to be -1, EXC

EPT when bonding with oxygen.• Group I elements have an oxidation st

ate +1• Group II elements have an oxidation st

ate of +2

Water as a product

• If the H[+1] was taken from a reactant compound, it already had a +1 charge.

• If O[-2] was taken from a reactant compound, it already had -2 charge.

• Therefore, there will be no change in the oxidation number of the elements in H and O when water is made and the H2O will have a total oxidation number of zero.

Oxidation Reduction

Gains oxygen Lose Oxygen

Lose Hydrogen Gains hydrogen

Lose Electrons Gains electrons

Oxygen

Hydrogen

e-

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