1. Chapter 16 Oxidation and Reduction

2. Tro - Chapter 162 Oxidation-Reduction Reactions oxidation-reduction reactions are also called redox reactions all redox reactions involve the transfer of electrons from one atom to another spontaneous redox reactions are generally exothermic, and we can use their released energy as a source of energy for other applications convert the heat of combustion into mechanical energy to move our cars use electrical energy in a car battery to start our car engine

3. Tro - Chapter 163 Combustion Reactions combustion reactions are always exothermic in combustion reactions, O 2 combines with all the elements in another reactant to make the products 4 Fe(s) + 3 O 2 (g) → 2 Fe 2 O 3 (s) + energy CH 4 (g) + 2 O 2 (g) → CO 2 (g) + 2 H 2 O(g) + energy

4. Tro - Chapter 164 Reverse of Combustion Reactions since combustion reactions are exothermic, their reverse reactions are endothermic the reverse of a combustion reaction involves the production of O 2 energy + 2 Fe 2 O 3 (s) → 4 Fe(s) + 3 O 2 (g) energy + CO 2 (g) + 2 H 2 O(g) → CH 4 (g) + 2 O 2 (g) reactions in which O 2 is gained or lost are redox reactions

5. Tro - Chapter 165 Oxidation and Reduction Oxidation and Reduction One Definition when an element attaches to an oxygen during the course of a reaction it is generally being oxidized in CH 4 (g) + 2 O 2 (g) → CO 2 (g) + 2 H 2 O(g), C is being oxidized in this reaction, but H is not when an element loses an attachment to oxygen during the course of a reaction it is generally being reduced in 2 Fe 2 O 3 (s) → 4 Fe(s) + 3 O 2 (g) the Fe is being reduced one definition of redox is the gain or loss of O, but it is not the bestone definition of redox is the gain or loss of O, but it is not the best

6. Tro - Chapter 166 Another Oxidation–Reduction consider the following reactions: 4 Na(s) + O 2 (g) → 2 Na 2 O(s) 2 Na(s) + Cl 2 (g) → 2 NaCl(s) the reaction involves a metal reacting with a nonmetal in addition, both reactions involve the conversion of free elements into ions 4 Na(s) + O 2 (g) → 2 Na + 2 O – (s) 2 Na(s) + Cl 2 (g) → 2 Na + Cl – (s)

7. Tro - Chapter 167 Oxidation and Reduction Another Definition in order to convert a free element into an ion, the atoms must gain or lose electrons of course, if one atom loses electrons, another must accept them reactions where electrons are transferred from one atom to another are redox reactions atoms that lose electrons are being oxidized, atoms that gain electrons are being reduced 2 Na(s) + Cl 2 (g) → 2 Na + Cl – (s) Na → Na + + 1 e – oxidation Cl 2 + 2 e – → 2 Cl – reduction Leo Ger

8. Tro - Chapter 168 Oxidation–Reduction oxidation and reduction must occur simultaneously if an atom loses electrons another atom must take them the reactant that reduces an element in another reactant is called the reducing agent the reducing agent contains the element that is oxidized the reactant that oxidizes an element in another reactant is called the oxidizing agent the oxidizing agent contains the element that is reduced 2 Na(s) + Cl 2 (g) → 2 Na + Cl – (s) Na is oxidized, Cl is reduced Na is the reducing agent, Cl 2 is the oxidizing agent

9. Tro - Chapter 169 Practice – Identify the Element being Oxidized and Element being Reduced; and the Oxidizing and Reducing Agents 2 Mg(s) + O 2 (g) → 2 MgO(s) Fe(s) + Cl 2 (g) → FeCl 2 (s) Zn(s) + Fe 2+ (aq) → Zn 2+ (aq) + Fe(s)

10. Tro - Chapter 1610 Practice – Identify the Element being Oxidized and Element being Reduced; and the Oxidizing and Reducing Agents 2 Mg(s) + O 2 (g) → 2 MgO(s) Fe(s) + Cl 2 (g) → FeCl 2 (s) Zn(s) + Fe 2+ (aq) → Zn 2+ (aq) + Fe(s) Mg is oxidized, O is reduced Mg is the reducing agent, O 2 is the oxidizing agent Fe is oxidized, Cl is reduced Fe is the reducing agent, Cl 2 is the oxidizing agent Zn is oxidized, Fe is reduced Zn is the reducing agent, Fe 2+ is the oxidizing agent

11. Tro - Chapter 1611 Electron Bookkeeping for reactions that are not metal + nonmetal, or do not involve O 2, we need a method for determining how the electrons are transferred chemists assign a number to each element in a reaction called an oxidation state that allows them to determine the electron flow in the reaction even though they look like them, oxidation states are not ion charges!  oxidation states are imaginary charges assigned based on a set of rules  ion charges are real, measurable charges

12. Tro - Chapter 1612 Rules for Assigning Oxidation States rules are in order of priority 1.free elements have an oxidation state = 0 Na = 0 and Cl 2 = 0 in 2 Na(s) + Cl 2 (g) 2.monatomic ions have an oxidation state equal to their charge Na = +1 and Cl = -1 in NaCl 3.(a) the sum of the oxidation states of all the atoms in a compound is 0 Na = +1 and Cl = -1 in NaCl, (+1) + (-1) = 0

13. Tro - Chapter 1613 Rules for Assigning Oxidation States 3.(b) the sum of the oxidation states of all the atoms in a polyatomic ion equals the charge on the ion N = +5 and O = -2 in NO 3 –, (+5) + 3(-2) = -1 4.(a) Group I metals have an oxidation state of +1 in all their compounds Na = +1 in NaCl 4.(b) Group II metals have an oxidation state of +2 in all their compounds Mg = +2 in MgCl 2

14. Tro - Chapter 1614 Rules for Assigning Oxidation States 5.in their compounds, nonmetals have oxidation states according to the table below nonmetals higher on the table take priority NonmetalOxidation StateExample FCF 4 H+1CH 4 O-2CO 2 Group 7ACCl 4 Group 6A-2CS 2 Group 5A-3NH 3

15. Tro - Chapter 1615 Practice – Assign an Oxidation State to each Element in the following Br 2 K + LiF CO 2 SO 4 2- Na 2 O 2

16. Tro - Chapter 1616 Practice – Assign an Oxidation State to each Element in the following Br 2 Br = 0, (Rule 1) K + K = +1, (Rule 2) LiFLi = +1, (Rule 4a) & F = -1, (Rule 5) CO 2 O = -2, (Rule 5) & C = +4, (Rule 3a) SO 4 2- O = -2, (Rule 5) & S = +6, (Rule 3b) Na 2 O 2 Na = +1, (Rule 4a) & O = -1, (Rule 3a)

17. Tro - Chapter 1617 Oxidation and Reduction Oxidation and Reduction A Better Definition oxidation occurs when an atom’s oxidation state increases during a reaction reduction occurs when an atom’s oxidation state decreases during a reaction CH 4 + 2 O 2 → CO 2 + 2 H 2 O -4 +1 0 +4 –2 +1 -2 oxidation reduction

18. Tro - Chapter 1618 Identify the Oxidizing and Reducing Agents in Each of the Following 3 H 2 S + 2 NO 3 – + 2 H +  S + 2 NO + 4 H 2 O MnO 2 + 4 HBr  MnBr 2 + Br 2 + 2 H 2 O

19. Tro - Chapter 1619 Identify the Oxidizing and Reducing Agents in Each of the Following 3 H 2 S + 2 NO 3 – + 2 H +  S + 2 NO + 4 H 2 O MnO 2 + 4 HBr  MnBr 2 + Br 2 + 2 H 2 O +1 -2 +5 -2 +1 0 +2 -2 +1 -2 ox agred ag +4 -2 +1 -1 +2 -1 0 +1 -2 oxidation reduction oxidation reduction red agox ag

20. Tro - Chapter 1620 Balancing Redox Reactions 1)assign oxidation states and determine element oxidized and element reduced 2)separate into oxidation & reduction half-reactions 3)balance half-reactions by mass a)first balance atoms other than O and H b)then balance O by adding H 2 O to side that lacks O c)finally balance H by adding H + to side that lacks H Fe 2+ + MnO 4 – → Fe 3+ + Mn 2+ +2+7-2+3+2 oxid red Fe 2+ → Fe 3+ MnO 4 – → Mn 2+ Fe 2+ → Fe 3+ MnO 4 – → Mn 2+ MnO 4 – → Mn 2+ + 4H 2 O MnO 4 – + 8H + → Mn 2+ + 4H 2 O

21. Tro - Chapter 1621 Balancing Redox Reactions 4)balance each half-reaction with respect to charge by adjusting the numbers of electrons a)electrons on product side for oxid. b)electrons on reactant side for red. 5)balance electrons between half-reactions 6)add half-reactions, canceling electrons and common species 7)Check Fe 2+ → Fe 3+ + 1 e - MnO 4 – + 8H + + 5 e - → Mn 2+ + 4H 2 O MnO 4 – + 8H + → Mn 2+ + 4H 2 O +7+2 Fe 2+ → Fe 3+ + 1 e - MnO 4 – + 8H + + 5 e - → Mn 2+ + 4H 2 O } x 5 5 Fe 2+ → 5 Fe 3+ + 5 e - MnO 4 – + 8H + + 5 e - → Mn 2+ + 4H 2 O 5 Fe 2+ + MnO 4 – + 8H + → Mn 2+ + 4H 2 O + 5 Fe 3+

22. Tro - Chapter 1622 Practice – Balance the Following Equation Cu + + I 2 → Cu 2+ + I –

23. Tro - Chapter 1623 Practice – Balance the Following Equation Cu + + I 2 → Cu 2+ + I – +10+2 oxid red ox: Cu + → Cu 2+ red:I 2 → I – ox: Cu + → Cu 2+ red:I 2 → 2 I – ox: Cu + → Cu 2+ + 1 e - red:I 2 + 2 e - → 2 I – ox: Cu + → Cu 2+ + 1 e - } x 2red:I 2 + 2 e - → 2 I – 2 Cu + + I 2 → 2 Cu 2+ + I 2

24. Tro - Chapter 1624 Practice – Balance the Following Equation I – + Cr 2 O 7 2- → Cr 3+ + I 2

25. Tro - Chapter 1625 Practice – Balance the Following Equation I – + Cr 2 O 7 2- → Cr 3+ + I 2 +60+3 oxid red ox: I – → I 2 red:Cr 2 O 7 2– → Cr 3+ -2 ox: 2 I – → I 2 red:Cr 2 O 7 2– → 2 Cr 3+ red:Cr 2 O 7 2– → 2 Cr 3+ + 7 H 2 O red:Cr 2 O 7 2– + 14H + → 2Cr 3+ + 7H 2 O ox: 2 I – → I 2 + 2e - red:Cr 2 O 7 2– + 14H + + 6e - → 2Cr 3+ + 7H 2 O ox: 2 I – → I 2 + 2e - }x3red:Cr 2 O 7 2– + 14H + + 6e - → 2Cr 3+ + 7H 2 O Cr 2 O 7 2– + 14 H + + 6 I – → 2 Cr 3+ + 7 H 2 O + 3 I 2

26. Tro - Chapter 1626 Will a Reaction Take Place? reactions that are energetically favorable are said to be spontaneous they can happen, but the activation energy may be so large that the rate is very slow the relative reactivity of metals can be used to determine if some redox reactions are spontaneous

27. Tro - Chapter 1627 Single Displacement Reactions also known as single replacement reactions a more active free element displaces a less active element in a compound metals displace metals or H Cu + 2 AgNO 3  Cu(NO 3 ) 2 + 2 Ag Mg + 2 HCl  MgCl 2 + H 2 nonmetals displace nonmetals 2 KI + Br 2  2 KBr + I 2 carbon displaces metals from oxides 3 C + Fe 2 O 3  3 CO + 2 Fe always redox

28. Tro - Chapter 1628 K Ba Sr Ca Na Mg Al Mn Zn Cr Fe Cd Co Ni Sn Pb H Sb As Bi Cu Hg Ag Pd Pt Au displace H 2 from cold H 2 O from steam from acids react with O 2 in the air to make oxides Fe is above Cu, so Cu metal will not displace Fe 2+ K Ba Sr Ca Na Mg Al Mn Zn Cr Fe Cd Co Ni Sn Pb H Sb As Bi Cu Hg Ag Pd Pt Au displace H 2 from cold H 2 O from steam from acids react with O 2 in the air to make oxides Gold is at the bottom, so it is very unreactive. K Ba Sr Ca Na Mg Al Mn Zn Cr Fe Cd Co Ni Sn Pb H Sb As Bi Cu Hg Ag Pd Pt Au displace H 2 from cold H 2 O from steam from acids react with O 2 in the air to make oxides Zn is above H, so Zn will react with acids Zn + Fe 2+  Fe + Zn 2+ Activity Series of Metals listing of metals by reactivity free metal higher on the list displaces metal cation lower on the list metals above H will dissolve in acid Cu + Fe 2+  no reaction Zn + 2 H +  H 2 + Zn 2+ K Ba Sr Ca Na Mg Al Mn Zn Cr Fe Cd Co Ni Sn Pb H Sb As Bi Cu Hg Ag Pd Pt Au displace H 2 from cold H 2 O from steam from acids react with O 2 in the air to make oxides Fe is below Zn, so Zn metal will displace Fe 2+

29. Tro - Chapter 1629 Mg is above Cu on the Activity Series Mg will react with Cu 2+ to form Mg 2+ and Cu metal but Cu will not react with Mg 2+

30. Tro - Chapter 1630 Predict the Products & Balance the Equation Mg + H 3 PO 4  Cu + H 2 SO 4  Al + Fe 2+ 

31. Tro - Chapter 1631 3 Mg + 2 H 3 PO 4  Mg 3 (PO 4 ) 2 + 3 H 2 Cu + H 2 SO 4  no reaction 2 Al + 3 Fe 2+  Al 3+ + 3 Fe Predict the Products & Balance the Equation

32. Tro - Chapter 1632 Electrochemical Cells electrochemistry is the study of redox reactions that produce or require an electric current the conversion between chemical energy and electrical energy is carried out in an electrochemical cell spontaneous redox reactions take place in a voltaic cell also known as galvanic cells batteries are voltaic cells nonspontaneous redox reactions can be made to occur in an electrolytic cell by the addition of electrical energy

33. Tro - Chapter 1633 Electrochemical Cells oxidation and reduction reactions kept separate half-cells electron flow through a wire along with ion flow through a solution constitutes an electric circuit requires a conductive solid (metal or graphite) electrode to allow the transfer of electrons through external circuit ion exchange between the two halves of the system electrolyte

34. Tro - Chapter 1634 Electrodes Anode electrode where oxidation occurs anions attracted to it connected to positive end of battery in electrolytic cell loses weight in electrolytic cell Cathode electrode where reduction occurs cations attracted to it connected to negative end of battery in electrolytic cell gains weight in electrolytic cell  electrode where plating takes place in electroplating

35. Tro - Chapter 1635 Voltaic Cell

36. Tro - Chapter 1636 Current and Voltage the number of electrons that flow through the system per second is the current Electrode surface area dictates the number of electrons that can flow the amount of force pushing the electrons through the wire is the voltage the farther the metals are separated on the activity series, the larger the voltage will be

37. Tro - Chapter 1637 Current The amount of water that passes a point each second is called the current of the river. The number of electrons that pass a point each second is called the current of the electricity.

38. Tro - Chapter 1638 Voltage Gravity is the force pulling the water down the river. Voltage is the force pushing the electrons down the wire.

39. Tro - Chapter 1639 Dead Battery As the reaction proceeds, the reactants get consumed and the voltaic cell “dies”. The current decreases until electrons can no longer flow through the wire.

40. Tro - Chapter 1640 LeClanche’ Acidic Dry CellDry Cell electrolyte in paste form ZnCl 2 + NH 4 Cl  or MgBr 2 anode = Zn (or Mg) Zn(s)  Zn 2+ (aq) + 2 e - cathode = graphite rod MnO 2 is reduced 2 MnO 2 (s) + 2 NH 4 + (aq) + 2 H 2 O(l) + 2 e -  2 NH 4 OH(aq) + 2 Mn(O)OH(s) cell voltage = 1.5 v expensive, nonrechargeable, heavy, easily corroded

41. Tro - Chapter 1641 Alkaline Dry Cell same basic cell as acidic dry cell, except electrolyte is alkaline KOH paste anode = Zn (or Mg) Zn(s)  Zn 2+ (aq) + 2 e - cathode = brass rod MnO 2 is reduced 2 MnO 2 (s) + 2 NH 4 + (aq) + 2 H 2 O(l) + 2 e -  2 NH 4 OH(aq) + 2 Mn(O)OH(s) cell voltage = 1.54 v longer shelf life than acidic dry cells and rechargeable, little corrosion of zinc

42. Tro - Chapter 1642 Lead Storage Battery 6 cells in series electrolyte = 6 M H 2 SO 4 anode = Pb Pb(s) + SO 4 2- (aq)  PbSO 4 (s) + 2 e - cathode = Pb coated with PbO 2 PbO 2 is reduced PbO 2 (s) + 4 H + (aq) + SO 4 2- (aq) + 2 e -  PbSO 4 (s) + 2 H 2 O(l) cell voltage = 2.09 v rechargeable, heavy

43. Tro - Chapter 1643 Fuel Cells like batteries in which reactants are constantly being added so it never runs down! Anode and Cathode both Pt coated metal Electrolyte is OH – solution Anode Reaction: 2 H 2 + 4 OH – → 4 H 2 O(l) + 4 e - Cathode Reaction: O 2 + 4 H 2 O + 4 e - → 4 OH –

44. Tro - Chapter 1644 Corrosion corrosion is the spontaneous oxidation of a metal by chemicals in the environment since many materials we use are active metals, corrosion can be a very big problem

45. Tro - Chapter 1645 Preventing Corrosion one way to reduce or slow corrosion is to coat the metal surface to keep it from contacting corrosive chemicals in the environment paint some metals, like Al, form an oxide that strongly attaches to the metal surface, preventing the rest from corroding another method to protect one metal is to attach it to a more reactive metal that is cheap sacrificial electrode

46. Tro - Chapter 1646 Nonspontaneous Redox Reaction the reverse of a spontaneous reaction is nonspontaneous to get it to run, an outside energy source must be supplied nonspontaneous redox reactions can be made to work by using a battery to force the electrons to flow in the nonspontaneous direction

47. Tro - Chapter 1647 Electrolysis electrolysis is the process of using electricity to break a compound apartelectrolysis electrolysis is done in an electrolytic cell electrolytic cells can be used to separate elements from their compounds generate H 2 from water for fuel cells recover metals from their ores

48. Tro - Chapter 1648 Electrolytic Cell the + terminal of the battery = anode the - terminal of the battery = cathode cations attracted to the cathode, anions to the anode cations pick up electrons from the cathode and are reduced, anions release electrons to the anode and are oxidized in electroplating the workpiece is the cathode cations are reduced at cation and plate to the surface the anode is made of the plate metal, the anode oxidizes and replaces the metal cations in the solution

49. Tro - Chapter 1649 Electrolytic Cell - ElectroplatingElectroplating