CHAPTER 20. Redox Reactions. Oxidation The combination of an element or compound with oxygen to give oxides.

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1 CHAPTER 20 Oxidation Reduction Reactions 20.1 The Meaning of Oxidation and Reduction Oxidation reactions are the principal source of energy on earth. Redox Reactions Also known as Oxidation Reduction Reactions: chemical changes that occur when electrons are transferred between reactants. Oxidation originally meant the combination of an element with oxygen to give oxides. Over the years, reduction has meant the loss of oxygen from a compound. May 15 9:45 AM May 15 9:45 AM Oxidation The combination of an element or compound with oxygen to give oxides. 4Fe + 3O 2 2Fe 2 O 3 C + O 2 CO 2 CH 4 + 2O 2 CO 2 + 2H 2 O Reduction The loss of an oxygen from a compound. The amount of solid material has decreased. 2Fe 2 O 3 + 3C 4Fe + 3CO 2 1

2 Examples Iron oxide is reduced to iron by losing an electron. Carbon is oxidized to CO 2 by gaining oxygen. Oxidation and reduction occur simultaneously. No oxidation occurs without reduction and no reduction occurs without oxidation. May 15 9:45 AM May 15 9:45 AM Electron Shift in Redox Reactions Definition has expanded: Oxidation is complete or partial loss of electrons or gain of oxygen Reduction is complete or partial gain of electrons or loss of oxygen LEO the lion goes GER (Lose electrons oxidize) (Gain electrons reduce) Examples page Reducing Agent Oxidizing Agent Table p. 635 LEO the lion goes GER Mg + S MgS (Mg 2+ + S 2 ) Oxidation: Mg Mg e (loss of e ) Reduction: S + 2e S 2 (gain of e ) Reducing Agent: substance in a redox reaction that donates electrons. (Mg) Oxidizing Agent: substance in a redox reaction that accepts electrons. (S) Mg + S MgS May 15 9:46 AM May 15 9:47 AM 2

3 Occurs in Covalent Bonds! 2H 2 + O 2 2H 2 O Hydrogen is oxidized by bonding with oxygen due to partial loss of electrons Oxidation Numbers An oxidation number is a positive or negative number assigned to an atom according to a set of arbitrary rules. May 15 9:47 AM May 15 9:47 AM Ionic Charge 1+ Oxidation Number +1 Rules for assigning oxidation numbers Page 639 MEMORIZE!!! May 15 9:47 AM May 15 9:48 AM Rule # 1 Monoatomic Ion standard Ionic charge There is a difference between Br & Br Br Fe 3+ Mg 2+ Rule # 2 Hydrogen in a compound +1 unless in a metal hydride like NaH, which is 1. Metal hydride is metal bonded directly to hydrogen. H 2 O NaOH KH NH 4 + MgH 2 Hydrogen bonded to a polyatomic ion is not a metal hydride! May 15 9:48 AM May 27 3:34 PM 3

4 Rule # 3 Oxygen is usually 2. (Unless in peroxide: H 2 O 2 ) Rule # 4 4. Uncombined element = 0 Diatomic = 0. (Non polar) Na 2 O CO 2 P 2 O 5 SO 2 N 2 Fe Au O 2 C 4 May 27 3:34 PM May 27 3:35 PM Neutral Compounds In ionic compounds, use the charges as their oxidation numbers. (look for metal) Charged Compounds Polyatomic ions In molecular compounds, the charges are the result of the electrons transferred. May 27 3:35 PM May 29 10:25 AM Rule # 5 In neutral compounds, the sum of oxidation numbers = 0. NaCl Mg 3 N 2 K 2 S S 2 O 3 S 2 O 4 Rule # 6 Polyatomic ions, the sum of the oxidation # equals the charge of the polyatomic ion. (Not Neutral!!!) SO 4 2 MnO 4 NH 4 + NO 3 PO 4 3 May 27 3:35 PM May 27 3:35 PM 4

5 CaO Ca 3(PO 4) 2 H 2SO 4 Oxidation Number Review H 2 O 2 SCl 6 BaCl 2 NaH KOH PbO 2 HSO 4 MgF 2 Al(NO 3 ) 3 CaSO 4 LiNO 3 CO 2 H 2O 2 CH 4 Na 2CO 3 May 30 7:08 AM May 30 8:17 AM OXIDATION NUMBERS Ionic compound: NaCl Na +1 and Cl 1 Molecular Compound: H 2 O H +1, H +1, O 2 Mg + S Oxidation Loss of Electrons Gain of Oxygen MgS Reduction Gain of Electron Loss of Oxygen Increase in Oxidation #. Decrease in Oxidation #. Mg Mg e S + 2 e S 2 Reducing Agent = Mg Oxidizing Agent = S May 22 12:11 PM May 22 12:09 PM Oxidation Number Changes In Chemical Reactions An increase in the oxidation number of an atom signifies oxidation. A decrease in the oxidation number of an atom signifies reduction. It is possible for the same atom to oxidize and reduce May 22 12:09 PM May 31 7:41 AM 5

6 Ca+H 2SO 4 CaSO 4 +H 2 How can the same element oxidize and reduce? It is rare, but does it happen... O: R: OA: RA: 2H 2 O 2 O 2 + 2H 2 O 2C 2H 6+7O 2 4CO 2+6H 2O O: R: OA: RA: O: R: OA: RA: May 30 12:58 PM May 30 12:58 PM 20.3 Balancing Redox Equations flash.htm The total number of electrons gained in reduction must equal the total number of electrons lost in oxidation C 6H 12O 6 + O 2 Mitochondria H 2O + CO 2 May 15 9:57 AM Jun 1 7:55 AM Classes of Reactions All other reactions in which electrons are not transferred. Redox Reactions: where electrons are transferred. Determining Redox Reactions Find oxidation numbers. See if they change? If they change, then it is a redox reaction. May 15 9:58 AM May 15 9:57 AM 6

7 Identifying Redox RXNS Double replacement reactions and acid base reactions are not redox reactions. Most other reactions are redox! Use oxidation numbers to determine if its a redox reaction. Redox: many single replacement, combination, decomposition, and combustion reactions. Double replacement and acid base reactions are not redox reactions. May 15 9:56 AM May 15 9:56 AM Which of these is a Redox reaction? CaSO 4 + Ba(NO 3) 2 Ca(NO 3) 2+BaSO 4 2NaBr + Cl 2 2NaCl + Br 2 H 2 SO 4 + Mg(OH) 2 H 2 O+MgSO 4 Rules for oxidation number change method of balancing redox reactions MEMORIZE!!! Page 648 Jun 1 8:08 AM May 15 9:53 AM Oxidation Number Change Method for Balancing Redox Reactions Step 1: Assign Ox Numbers Step 2: Determine what is oxidized and reduced Step 3: Use bracketed line to connect atoms undergoing oxidation and reduction and write the oxidation change on the line. Determine least common multiple between oxidation number change and reduction number change. May 15 9:53 AM May 15 9:52 AM 7

8 Step 4: Place coefficients into the equation to reach the number multiplied to get the least common multiple. _Mg + _N 2 _Mg 3 N 2 Step 5: Make sure the entire equation is balanced. _Fe + _O 2 _FeO May 15 9:52 AM Jun 1 2:05 PM Identifying Half Reactions _PbS + _O 2 _PbO + _ SO 2 Splitting a Redox reaction in half, focusing on the element that is oxidizing or reducing. _C 2 H 6 + _O 2 _CO 2 + _H 2 O 2Fe + 3Cl 2 2FeCl 3 Assign oxidation numbers and determine what oxidized and what reduced. Jun 1 2:05 PM May 13 12:35 PM Oxidation Half Reaction 2Fe + 3Cl 2 2FeCl 3 Reduction Half Reaction 2Fe + 3Cl 2 2FeCl 3 Fe Fe e As products formed, Fe lost 3 electrons Cl 2 + 2e Cl As products formed, Cl 2 gained 2 electrons May 13 12:35 PM May 13 12:35 PM 8