Redox Reaction

REDOX REACTIONS

Introduction to Redox Reactions

• Chemistry involves the transformation of one type of matter into another, categorized into various types of reactions.

• Redox Reactions: A major category where oxidation and reduction occur simultaneously.

• Extensive applications:   - Pharmaceutical   - Biological   - Industrial   - Metallurgical   - Agricultural

• Environmental relevance: Includes topics like the Hydrogen Economy and the development of the Ozone Hole.

7.1 Classical Idea of Redox Reactions – Oxidation and Reduction Reactions

Definition

• Oxidation: Initially defined as the addition of oxygen to a substance.

• Common examples:   - $ext{2 Mg (s) + O}_2 (g) ightarrow ext{2 MgO (s)}$   - $ext{S (s) + O}_2 (g) ightarrow ext{SO}_2 (g)$

• Extension of Oxidation Paradigm: The term also includes:   - Removal of hydrogen from a substance.   - Oxidation reactions may involve other electronegative elements (e.g., fluorine, chlorine):     - $ext{Mg (s) + F}_2 (g) ightarrow ext{MgF}_2 (s)$     - $ext{Mg (s) + Cl}_2 (g) ightarrow ext{MgCl}_2 (s)$     - $ext{Mg (s) + S (s) ightarrow MgS (s)}$

Summary of Oxidation

• Defined as addition of:   - Oxygen/electronegative element OR removal of hydrogen/electropositive element.

Definition of Reduction

• Definition evolved from removal of oxygen:

• Currently, reduction includes:   - Removal of oxygen/electronegative element.   - Addition of hydrogen/electropositive element.

Examples of Reduction Processes

• $ext{2 HgO (s) ightarrow 2 Hg (l) + O}_2 (g)$ (removal of oxygen)

• $ext{2 FeCl}_3 (aq) + ext{H}_2 (g) ightarrow 2 ext{FeCl}_2 (aq) + 2 ext{HCl}(aq)$ (removal of chlorine)

• $ext{CH}_2 = ext{CH}_2 (g) + ext{H}_2 (g) ightarrow ext{H}_3 ext{C – CH}_3 (g)$ (addition of hydrogen)

• $ext{2HgCl}_2 (aq) + ext{SnCl}_2 (aq) ightarrow ext{Hg}_2 ext{Cl}_2 (s) + ext{SnCl}_4 (aq)$ (addition of mercury, simultaneous oxidation of Sn).

Definition of Redox Reactions

• Redox Reaction: Redox indicates that oxidation and reduction happen simultaneously.   - Key to examining common equations and understanding electron transfer.

Problems and Solutions

Problem 7.1: Identify Oxidation and Reduction

1. Reaction: $ext{H}_2 ext{S (g) + Cl}_2(g) ightarrow 2 ext{HCl (g) + S (s)}$    - Oxidation: $ext{H}_2 ext{S}$ is oxidized (loses hydrogen).    - Reduction: $ext{Cl}_2$ is reduced (gains hydrogen).

2. Reaction: $3 ext{Fe}<em>3 ext{O}_4 (s) + 8 ext{Al} (s) ightarrow 9 ext{Fe} (s) + 4 ext{Al}_2 ext{O}_3 (s)$    - Oxidation: Aluminum becomes Al${2}$O$_{3}$.    - Reduction: Iron(III) is reduced to Fe.

3. Reaction: $2 ext{Na} (s) + ext{H}_2 (g) ightarrow 2 ext{NaH} (s)$    - Oxidation: Na is oxidized.    - Reduction: Hydrogen is reduced.

7.2 Redox Reactions in Terms of Electron Transfer

• Redox reactions can be expressed as half-reactions, indicating gain or loss of electrons.

Half-Reaction Example

• Formation of Sodium Chloride:

1. Oxidation half-reaction:    - $2 ext{Na} (s) ightarrow 2 ext{Na}^+ (g) + 2 ext{e}^-$

2. Reduction half-reaction:    - $ext{Cl}_2 (g) + 2 ext{e}^- ightarrow 2 ext{Cl}^- (g)$

• Full reaction:   - $2 ext{Na (s) + Cl}_2(g) ightarrow 2 ext{NaCl (s)}$

Summary of Basic Definitions

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

• Oxidizing agent: Electron acceptor.

• Reducing agent: Electron donor.

Problem 7.2: Justification of Redox Change

• Reaction: $2 ext{Na} (s) + ext{H}_2(g) ightarrow 2 ext{NaH} (s)$

• Oxidation half-reaction: $2 ext{Na} (s) ightarrow 2 ext{Na}^+ (g) + 2 ext{e}^-$

• Reduction half-reaction: $ext{H}_2 (g) + 2 ext{e}^- ightarrow 2 ext{H}^- (g)$

• Conclusion: Sodium oxidized, hydrogen reduced - Therefore, it is a redox change.

7.2.1 Competitive Electron Transfer Reactions

• Example: Zinc with Copper Nitrate:   - Reaction: $ext{Zn (s) + Cu}^{2+} (aq) ightarrow ext{Zn}^{2+} (aq) + ext{Cu (s)}$   - Zinc oxidized; copper reduced.

Observations for Zinc Experiment

• Zinc rod coated with copper; observed color change transitioning from blue to colorless due to Cu$^{2+}$ ions disappearing.

Laboratory Applications of Electron Transfer

• State of equilibrium can be tested by placing metallic copper in zinc sulfate solution - displays minimal reaction.

Construction of Electrode Processes

• Redox Couples: An oxidized and reduced form together in electrode reactions (e.g., $ext{Zn}^{2+}/ ext{Zn, Cu}^{2+}/ ext{Cu}$).

7.3 Oxidation Number

Concept of Oxidation Number

• A method to track electron transfer in covalent bond reactions.

• Rules for Assigning Oxidation Number:

1. Free elements have oxidation number of zero.

2. Ion oxidation number equals their ionic charge.

3. Oxygen typically has an oxidation number of -2.

4. Hydrogen usually +1; can be -1 when bonded to metals.

5. Fluorine is -1 in compounds; other halogens have +1 when with oxygen.

6. The overall oxidation number must equal the charge of the molecule/ion.

Stock Notation for Oxidation Number

• Utilizes Roman numerals after metals in compounds to depict oxidation states (e.g., $ext{Au(I)Cl, Sn(II)Cl_2}$).

7.3.1 Types of Redox Reactions

1. Combination Reactions

• Formed when two or more reactants produce a single product.   - Example: $C (s) + O_2(g) ightarrow CO_2(g)$

2. Decomposition Reactions

• Breakdown of a compound into simpler substances.   - Example: $2 ext{H}_2 ext{O} ightarrow 2 ext{H}_2(g) + O_2(g)$

3. Displacement Reactions

• Involves an element replacing another in a reaction.    - Example: $ext{CuSO}_4(aq) + ext{Zn}(s) ightarrow ext{Cu}(s) + ext{ZnSO}_4(aq)$

4. Disproportionation Reactions

• An element undergoes both oxidation and reduction.   - Example: $2 ext{H}_2 ext{O}_2 ightarrow ext{2H}_2 ext{O} + ext{O}_2$

Summary of Redox Reactions

• Final Note: Processes are tied to energy exchanges, balancing equations can use oxidation states or half-reactions. Understanding and predicting reactions involves recognizing electron transfer and determining oxidation states or applying oxidation number concepts. Redox principles underpin biochemical processes, environmental changes, and industrial applications, demonstrating their comprehensive and vital role in chemistry.