4.3 Redox Reactions

Type 3. Redox Reactions: There are several sub types for Redox Reactions:

1. Single Replacement/Displacement Reactions
2. Decomposition Reactions
3. Combustion Reactions
4. Combination (Synthesis) Reactions

The word “Redox” is made of a combination of two words: Red = Reduction and Ox = Oxidation.

Therefore Redox reactions mean Reduction – Oxidation Reactions.

What is a Reduction? [Valence Electrons gaining]

A reduction is a process by which an element, molecule or compound accepts extra valence electrons from adjust atoms of another element or molecule or compound leading to the “Reduction” of oxidation number of that element, molecule or compound.

Oxidation number is the number of the charge that an atom, or molecule or compound can carry when they are found in solution as ions.

Oxidation Number Rules: [Assigning Oxidation Nubers]

 1. Neutral atoms or molecules or compound have the oxidation number is zero. Examples: Fe, P and Na atoms are neutral and have the oxidation number of zero. Also, molecules such as O₂, N₂, Cl₂, NaCl, and Na₂CO₃ are neutral molecules and compounds have the sum of oxidation numbers of zero.

2. The oxidation number of the ions is equal to the charge that these ions carry. The oxidation number of Ca²⁺ ion is +2, Al³⁺ has the oxidation of +3, and P^3- has the oxidation number of -3, S^2- has the oxidation number of -2. NH₄⁺ ammonium ion has the oxidation number of +1.

3. In general the oxidation number of hydrogen is +1 when hydrogen is forming covalent bonds (combined itself with another none metal(s)). Examples: NH₃, HNO₃, C₂H₆

4. The oxidation number of hydrogen is -1 when hydrogen is forming ionic bonds (combined itself with another metal(s)). Examples:  KH, BaH₂, AlH₃ and LiAlH₄.

5. The metals in Group IA has the oxidation number of +1. Examples Na₃P and K₂S)

6. The metals in Group IA has the oxidation number of +2. Examples Ba₃P₂ and Ba(HCO₃)₂

7. Generally oxygen has the oxidation number of -2. Exceptions include molecules and polyatomic ions that contain O-O bonds, such as O₂, O₃ and H₂O₂, molecules and the O₂^2- ions.

8. Group VIIA elements have in general the oxidation number of -1. Examples FeCl₃, HF, and VI₃)

What is an Oxidation? [Valence Electrons losing]

An oxidation is a process by which an element, molecule or compound gives away some of their valence electrons to adjust atoms of another element or molecule or compound leading to the “Increase” of oxidation number of that element, molecule or compound.

Examples:

Fe⁰     à Fe³⁺   + 3 e^–(oxidation: increase in charge (oxidation number)). This is an oxidation reaction.

The redox reaction is made of 2 halves of reactions: the first half reaction is the oxidation reaction and the second half reaction is the reduction reaction. Both half reactions are interconnected and they occur at the same time.

Example of complete Redox reaction:

Na⁰  →    Na⁺ + e^– [first half reaction: oxidation reaction]

e^–  +  Cl⁰    →  Cl^– [second half reaction: reduction reaction]

Na⁰ + Cl⁰   →   Na⁺  +  Cl^– [Complete Redox Reaction]

The videos illustrate the redox reaction and the oxidation numbers applications:

How to Calculate Oxidation Number Practice Problems

Oxidation and Reduction Reactions – Basic Introduction

Oxidizing and Reducing agents:

K⁰ + Br⁰    →   K⁺  +  Br^–

What does K do on Br?

K is forcing Br to be reduced. K is said to be Reducing Agent. K is providing the valence electrons to Br. K is reducing Br.

What does Br do on K?

Br is forcing K to be oxidized. Br is said to be Oxidizing Agent. Br is removing valence electrons from K. Br is oxidizing K.

Examples:

1. Assign oxidation numbers to each of the atoms in the following compounds:

SO₃^2-               O =      -2         S = +4

SF₄                  F =       -1         S = +4

2. What is the range of oxidation states for carbon? -4 to +4

3. Which compound has C in a +4 state?       CO₂

4. Which compound has C in a -4 state?        CH₄

5. Nitrogen has 5 valence electrons (Group V). It can gain up to 3 electrons (-3), or lose up to 5 (+5) electrons. Fill in the missing names or formulas and assign an oxidation state to each of the following nitrogen containing compounds:

6. During chemical reactions, the oxidation state of atoms can change. This occurs when compounds gain or lose electrons, or when the bonds to an atom change. This is illustrated by the reaction between nitrogen and hydrogen to make ammonia:

N₂(g) + 3 H₂(g) → 2 NH₃(g)

Assign oxidation numbers to each of the atoms in this reaction.

When an oxidation number increases, that species has been oxidized.

Which reactant undergoes an increase in its oxidation number?                          H₂

When an oxidation number decreases, that species has been reduced.

Which reactant undergoes a decrease in its oxidation number?                          N₂

The species that is oxidized is called the reducing agent because it gives up an electron, so that another species can gain an electron (be reduced).

What is the reducing agent in this reaction?                                       H₂

The species that is reduced is called the oxidizing agent because it takes an electron away from another group, raising that group’s oxidation number.

What is the oxidizing agent in this reaction?                                      N₂

7. In each of the following reactions, assign oxidation numbers to all of the elements and identify the oxidizing and reducing agents and the change in oxidation number.

            4 Fe + 3 O₂→2 Fe₂O₃

                     0      0           +3 -2

change in oxidation number oxidizing agent O₂       0 à -2

reducing agent          Fe        0 à +3

              Cr₂O₇^2-+ 2OH^–→2 CrO₄^2-+ H₂O

+6  -2      -2 +1     +6  -2     +1 -2

change in oxidation number oxidizing agent     no change in any oxidation numbers;

reducing agent             not a redox reaction

   NH₄NO₂→N₂+2 H₂O                             *Think of this as NH₄⁺ and NO₂^–

-3 +1 +3 -2      0      +1 -2

change in oxidation number oxidizing agent NH₄NO₂

reducing agent             NH₄NO₂                   *The nitrogens are in different states

P₄+10 Cl₂→4 PCl₅

0          0         +5 -1

change in oxidation number

oxidizing agent          Cl₂       0  à -1

reducing agent          P₄        0  à +5

            2 Cr³⁺+H₂O+ 6ClO₃^–→Cr₂O₇^2-+6ClO₂ + 2H⁺

+3      +1 -2    +5  -2  +6  -2        +4 -2    +1

change in oxidation number oxidizing agent  ClO₃^– +5 à +4

reducing agent             Cr³⁺              +3  →  +6