Topic 9: Redox Processes

9.1 Oxidation and Reduction

Redox reactions

  • Involve oxidation and reduction

  • Types of redox reaction:

    • Acid-base reactions

    • Precipitation reactions

    • Redox reactions

Oxidation

  • Old definition: Gain of oxygen

  • New definition: Loss of electrons

  • Increase in the oxidation number

Reduction

  • Old definition: Loss of hydrogen

  • New definition: gain of electrons

  • Decrease in the oxidation number

Reducing agent

  • The species that undergoes oxidation

 

Oxidizing agent

  • The species that undergoes reduction 

 

Variable oxidation states

  • Most of the transition metals have more than one oxidation state

  • Most stable oxidation states are:

    • Sc​: +3

    • Ti: +4

    • V: +3, +4, +5

    • Cr: +2, +4, +6

    • Mn: +2, +7

    • Fe: +2, +3

    • Co: +2, +3

    • Ni: +2

    • Cu: +1, +2

Half reactions

  • Oxidation

  • Reduction

​​​Activities series

  • Found in the data booklet

  • Ranks metals according to the ease which they undergo oxidation

  • Top (lithium) is more reactive

Winkler method

  • Technique  to measure the amount of dissolved oxygen in water

  • High oxygen concentration--> low pollution

  • BOD (biochemical Oxygen Demand) in ppm

9.2 Electrochemical Cells

Voltaic (galvanic) cells

  • Convert chemical energy to electric energy

  • Spontaneous, Exothermic process

  • Oxidation happens in the Anode (negative electrode)

  • Reduction happens in the Cathode (positive electrode)

Electrolytic cells

  • Convert electric energy to chemical energy

  • Non-spontaneous

  • Oxidation happens in the Anode (positive electrode)

  • Reduction happens in the Cathode (negative electrode)

TIP: 1) In all electrochemical cells, oxidation happens in the anode and and reduction in the cathode. To remember: vowel with vowel and consonant with consonant. OARC

       2) The positive and negative electrodes are the opposite in the electrolytic cells

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