Review of Redox Chemistry

​16.1 Spontaneity

16.2 Entropy

16.3 The Second and Third Laws of Thermodynamics

16.4 Free Energy

​Learning Objectives

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By the end of this section, you will be able to:

• Describe defining traits of redox chemistry

• Identify the oxidant and reductant of a redox reaction

• Balance chemical equations for redox reactions using the half-reaction method

​By definition, a redox reaction is one that entails changes in oxidation number (or oxidation state) for one or more of the elements involved.

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​1- Oxidation numbers

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​2- Half-reaction method

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1. Write skeletal equations for the oxidation and reduction half-reactions.

2. Balance each half-reaction for all elements except H and O.

3. Balance each half-reaction for O by adding H₂O.

4. Balance each half-reaction for H by adding H⁺.

5. Balance each half-reaction for charge by adding electrons.

6. If necessary, multiply one or both half-reactions so that the number of electrons consumed in one is equal to the number produced in the other.

7. Add the two half-reactions and simplify.

8. If the reaction takes place in a basic medium, add OH⁻ ions the equation obtained in step 7 to neutralize the H⁺ ions (add in equal numbers to both sides of the equation) and simplify.

Balancing Equations for Redox Reactions in Acidic Solutions

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Write the balanced equation representing reaction between solid copper and nitric acid to yield aqueous copper(II) ions and nitrogen monoxide gas.

Write the balanced equation representing reaction between aqueous permanganate ion,MnO₄⁻, and solid chromium(III) hydroxide, Cr(OH)₃, to yield solid manganese(IV) oxide, MnO₂, and aqueous chromate ion,CrO₄²⁻CrO₄²⁻The reaction takes place in a basic solution.