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Chapter 20

Chapter 20. Oxidation-Reduction Reactions. Oxidation and Reduction. Oxidation and reduction always occur simultaneously Oxidation  the complete or partial loss of electrons or gain of oxygen Reduction  the complete or partial gain of electrons or loss of oxygen

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Chapter 20

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  1. Chapter 20 Oxidation-Reduction Reactions

  2. Oxidation and Reduction • Oxidation and reduction always occur simultaneously • Oxidation  the complete or partial loss of electrons or gain of oxygen • Reduction  the complete or partial gain of electrons or loss of oxygen LEO the lion goes GER OIL RIG

  3. Oxidation and Reduction • Oxidation-reduction reaction  a reaction that involves the transfer of electrons between reactants • Also referred to as a redox reaction Mg + S  Mg+2 + S-2 **For now, we do not know how to predict whether it is a redox reaction or not…that’s coming soon! ☺

  4. Oxidation and Reduction • The substance losing electrons is said to be oxidized • The substance gaining electrons is said to be reduced Mg  Mg+2 + 2e- Mg is being oxidized S + 2e-  S-2 S is being reduced

  5. Oxidation and Reduction • Where did the electrons lost by magnesium go? • They were gained by sulfur Mg + S  Mg+2 + S-2

  6. Oxidation and Reduction • Reducing agent  the substance being oxidized • Oxidizing agent  the substance being reduced ►Magnesium reduces sulfur by losing it’s electrons ►Sulfur oxidizes magnesium by gaining magnesium’s electrons

  7. Oxidation and Reduction • Determine what is oxidized and what is reduced in the following reaction. Identify the oxidizing agent and the reducing agent. 2Na + S  2Na+1+ S-2

  8. Oxidation and Reduction • Oxidation is losing electrons, reduction is gaining electrons • When a substance loses electrons, it’s charge becomes more positive • When a substance gains electrons, it’s charge becomes more negative Oxidation: 2Na  2Na+1 Reduction: S  S-2 • Na is the reducing agent, S is the oxidizing agent + 2e- + 2e-

  9. Try it on your own! • Determine what is oxidized and what is reduced in the following reaction. Identify the oxidizing agent and the reducing agent. 4Al + 3O2 4Al+3 + 6O-2

  10. Oxidation and Reduction • We have been looking at ionic compounds, so the transfer of electrons makes sense. But, what if we are dealing with a covalent compound? • How can oxidation and reduction occur if the substances can not transfer electrons? • Look back at the definition of oxidation and reduction.

  11. Oxidation and Reduction • Oxidation  the complete or partial loss of electrons or gain of oxygen • Reduction  the complete or partial gain of electrons or loss of oxygen 2H2 + O2  2H2O H – H O – O H – O Equal sharing of electrons Oxygen is more electronegative, therefore the electrons are shifted away from hydrogen toward oxygen Equal sharing of electrons H

  12. Oxidation and Reduction • Isn’t there an easier way to do this for covalent compounds, Mrs. Harnew?!? Yes! There is…but you have to memorize a few rules

  13. Oxidation Numbers • Oxidation number  a positive or negative number assigned to an atom to indicate its degree of oxidation or reduction • It shows the electron distribution • From the reactant to the product, if the oxidation number… • Increases = oxidation • Decreases = reduction

  14. Assigning Oxidation Numbers • Determine the oxidation numbers of each element in UF6 UF6 Oxidation state: +6 -1 Sum: +6 -6 = 0

  15. Assigning Oxidation Numbers • Determine the oxidation numbers of each element in SO4-2 SO4-2 Oxidation state: +6 -2 Sum: +6 -8 = - 2

  16. Assigning Oxidation Numbers • Determine the oxidation numbers of each element in KClO3 KClO3 ClO3-1 Oxidation state: +1 +5 -2 Sum: +1 +5 -6 = 0 +5 -2 Oxidation state: Sum: +5 -6 = -1

  17. Oxidation Numbers • Use the changes in oxidation numbers to identify which atoms are oxidized and which are reduced in the following reaction. 2KNO3 2KNO2 + O2

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