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Oxidation – Reduction Reactions

Oxidation – Reduction Reactions. Review of the Octet Rule. Atoms want 8 electrons in the valence shell Ionic bonds transfer electrons from one atom to another atom Atoms form a charged ion and a complete octet Oxidation numbers show if electrons are gained or lost.

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Oxidation – Reduction Reactions

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  1. Oxidation – Reduction Reactions

  2. Review of the Octet Rule • Atoms want 8 electrons in the valence shell • Ionic bonds transfer electrons from one atom to another atom • Atoms form a charged ion and a complete octet • Oxidation numbers show if electrons are gained or lost

  3. Assigning Oxidation States Basic Rules (MEMORIZE)! • Uncombined elements have an oxidation state of zero • Monatomic ions (one atom ions) have an oxidation state equal to its ionic charge • Group 1 Alkali Metals +1 • Group 2 Alkaline Earth Metals +2 • Group 17 halogens –1 • Hydrogen is +1 when bonding with nonmetals • Sum of oxidation states is equal to zero for a compound • Polyatomic ions retain a charge after the summing of oxidation states

  4. Oxidation Numbers • Used to tell how many electrons have been gained or lost • Can be negative, positive or neutral(zero) • Located on the Periodic Table • Increase – oxidation • Decrease – reduction

  5. Oxidation and Reduction Introduction • Originally: • Used to describe reactions that added and reduced oxygen during reactions • Today: • Still describes adding and removing oxygen • Oxidation also describes atoms that lose electrons (OIL) • Reduction also describes atoms that gain electrons (RIG)

  6. Oxidation When a substance loses one or more electrons • Oxidation Loses electrons (OIL) • Is said to be “oxidized” Ex: Magnesium (s) is burned in oxygen (g) Mg(s) + O2 (g) MgO The Mg is oxidized by losing 2 electrons

  7. Reduction When a substance gains one or more electrons • Reduction Gains electrons (RIG) • Is said to be “reduced” Ex: Magnesium (s) is burned in oxygen (g) Mg(s) + O2 (g) MgO The O is reduced by gaining 2 electrons

  8. Redox Reaction Oxidation – Reduction reaction (REDOX): Oxidized atoms Increase in oxidation state Reduce the other atom Reduced atoms Decrease in oxidation state Oxidize the other atom

  9. Determining Redox Reactions NOT all reactions are redox reactions! • Assign oxidation numbers to each atom • Check if the oxidation states change • Synthesis, single replacement and decomposition reactions can be redox reactions • Double replacement reactions may not be redox reactions

  10. Identifying Redox Reactions 1. Assign oxidation numbers +4 -2 +1 -1 +2 -1 0 +1 -2 MnO2 + 4HCl MnCl2 + Cl2 + 2H2O 2. Look at oxidation states Mg goes from +4 to +2 it gained 2 electrons (REDUCED and is the oxidizing agent) RIG Cl goes from –1 to 0 it lost 1 electron (OXIDIZED and is the reducing agent) OIL

  11. Half Reactions • Can be used to balance redox reactions • Splits up the reaction into the oxidized reaction and the reduced reaction • Reduction Reaction (RIG) Ex: Fe+3(aq) + 3e- Fe(s) • Oxidation (OIL) Ex: Fe(s) Fe+3(aq) + 3e-

  12. How to Write Half Reactions • Assign oxidation numbers • Write partial half reaction to show change in oxidation state • Show the number of electrons needed to change the oxidation state • Check for the conservation of charge

  13. To Balance Redox Reactions • Assign Oxidation Numbers • Identify Oxidized and Reduced elements • Write the half reactions • Multiply each half reaction to balance the number of electrons lost and gained • Add the 2 balanced half reactions eliminating the electrons • Insert the coefficients into the original equation • Balance the rest of the reaction by inspection

  14. Balancing Example 0 +1 +5 -2 +1 +5 -2 +2 -2 +1 -2 • Cu + HNO3 Cu(NO3)2 + NO + H2O • Cu is oxidized 0 to +2 N is reduced +5 to +2 • Cu Cu2+ + 2e- (Oxidation Half reaction) N+5 + 3e- N+2 (Reduction Half Reaction) • 3 (Cu Cu2+ + 2e-) = 3Cu 3Cu2+ + 6e- 2 (N+5 + 3e- N+2) = 2N+5 + 6e- 2N+2

  15. Continued • 3Cu 3Cu2+ + 6e-+ 2N+5 + 6e- 2N+2 3Cu + 2N+5 3Cu2+ + 2N+2 • 3Cu + 2HNO3 3Cu(NO3)2 + 2NO + H2O • 3Cu + 8HNO3 3Cu(NO3)2 + 2NO + 4H2O

  16. Activity Series Table J • Shows reactivity of metals and nonmetals • Elements at the top are most reactive • Decreases in activity as you go down the table • Element above in the table is more reactive than the element below it

  17. Electrochemical Cells • Redox reactions provide useful electrical energy in batteries. • Simple electrochemical cells can be made from Cu and Zn metals with solutions of their sulfates. • Electrons are transferred from the zinc to the copper through an electrically conducting path

  18. Basic Electrochemical Cell http://chemistry.about.com/library/weekly/aa082003a.htm

  19. Electrolytic Cell • Created with metallic electrodes placed in an electrolyte • Uses or generates an electric current. • Forms batteries with one or more cells. • Needs an externally supplied electric current to drive a chemical reaction • Is not spontaneous

  20. Voltaic Cell • Two surfaces called electrodes that are made of metal (Cu and Zn) where oxidation and reduction occur • anode for oxidation • cathode for reduction • Salt Bridge contains an electrolytic solution to allow ions to pass to complete the circuit • Connecting Wire connects the electrodes for the transfer of electrons.

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