OXIDATION & REDUCTION REACTIONS



OXIDATION & REDUCTION REACTIONS

CHAPTER 18

REDOX REACTIONS

A reaction in which electrons are transferred from one atom to another is called an oxidation–reduction reaction.

For simplicity, chemists often refer to oxidation–reduction reactions as redox reactions.

OXIDATION NUMBERS

Oxidation numbers refer to the charge value of an element in a compound.

They are used to keep track of electrons.

ASSIGNING OXIDATION NUMBERS

1. The oxidation numbers for any element in their standard state

is zero.

Examples:

C = 0

O2 = 0

S = 0

2. The oxidation number for a monatomic ion is its charge.

Examples:

Ca2+ = 2+

Br- = 1-

MgBr2 = Mg = 2+; Br = 1-

3. The oxidation number of hydrogen is usually 1+. The exception is in a metal hydride where the oxidation number will be 1-.

Examples:

HCl H= 1+

H2SO4 H= 1+

NaH H= 1-

4. Oxygen has an oxidation number of 2- except in H2O2 and Na2O2 then its 1-

5. The sum of the oxidation numbers must be zero in compounds or equal to the charge of the ion.

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

Assign the oxidation state to each element in the following:

(a) K2Cr2O7

(b) MnO4-

(c) Cu2SO3

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Terminology for Redox Reactions

Oxidation:

-Loss of electrons

-Increase in oxidation number

Reduction

-Gaining of electrons

-Decrease in oxidation number

Easy way to remember: LEO the lion says GER

Oxidizing Agent:

-Gets reduced

-Gains electrons

Reducing Agent

-Gets oxidized

-Loses electrons

In a redox reaction the substance that is reduced will gain electrons

This means that its number will be reduced because it is gaining electrons (negatives)

The substance that is being oxidized will loose electrons

This means that its number will go up because it is loosing electrons (negatives)

Identifying the components

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Half Reactions

One of the two parts of a redox reaction

Oxidation Half: Produces electrons

Reduction Half: Requires electrons

Balancing Redox Reactions

Use Half Reaction Method

Steps (Acidic)

-Separate equations into half reactions

-Balance elements other than H & O

-Balance O by adding H2O

-Balance H by adding H+

-Balance charge by using electrons

-Multiply equations to make electrons equal

-Add the half reactions

-Simplify

Example

MnO4- + Cl- ( Mn2+ + Cl2

Separate into Half Reactions

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Balance O by adding H2O & H by adding H+

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Balance the electrons

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Balancing Redox Reactions

Multiply oxidation half by 5

Multiply reduction half by 2

Balance the electrons

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Simplify

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Steps (Basic)

-Do everything the same as in acid with two additional steps

-Add the same amount of OH- as H+ on both sides

-If the OH- & H+ are on the same side, combine them to make H2O

Example

MnO4- + H2O2 ( MnO2 + O2

Separate into Half Reactions

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Balance O by adding H2O & H by adding H+

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Add the same number of OH- as H+ to both sides

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Combine OH- & H+ on the same side

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Balance electrons

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Simplify

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