Unit 8: Redox and Electrochemistry

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1 May 20, 2014 Unit 8: Redox and Electrochemistry

2 Oxidation Number numbers assigned to atoms that allow us to keep track of electrons. Rule #1: Oxidation number of any uncombined atom is zero. Example: C, H2, Al, Cl2...etc.

3 Rule #2: The oxidation number of a monatomic ion is equal to its charge. Example: Na +, Mg 2+, Cl -, S 2-

4 Rule #3: The oxidation number of the more electronegative atom in a molecule or complex ion is the same as the charge it would have if it were an ion. Example: NH3 of -3. Nitrogen has oxidation number Rule #4: The oxidation number of fluorine in a compound always -1.

5 Rule #5: Oxygen has an oxidation number of -2 in most compounds. Exception 1: In peroxides (like H2O2), oxidation number = -1. Exception 2: When bonded to fluorine, oxidation number = +2

6 Rule #6: The oxidation number of hydrogen in most of its compounds is +1 except when bonded to metals, where it is -1. Example: H2O, MgH2 Rule #7: In compounds, the elements of groups 1 and 2, and aluminum have oxidation numbers +1, +2, and +3 respectively.

7 Rule #8: The sum of the oxidation numbers in a neutral compound is zero. Example: NaCl, CaBr2, CCl4 Rule #9: The sum of the oxidation numbers in a polyatomic ion is equal to the charge of the ion. Example: SO3 2-, OH -

8 *side note: Charges are written with signs after the number: 2-, 3-, 2+, 3+ Oxidation numbers are written with signs before the number -3, -2, -1, +1, +2, +3,

9 Example 1: Assign oxidation numbers a) F2 b) Na2O c) F - d) BH3 e) NaOH f) PO4 3-

10 Oxidation-Reduction Reactions also called Redox reactions reaction in which one or more electrons are transferred from one atom to another Oxidation and reduction always happens together Oxidation = losing electrons (oxidation number increases) Reduction = gaining electrons (oxidation number decreases)

11 LEO the lion says GER reduction electrons gaining oxidation electrons losing

12 Why does redox happen? Atoms transfer electrons to another atom. The more electronegative atom attracts electrons more strongly, resulting in a transfer of electrons. May 20, 2014

13 Example 2: Identify the following as oxidation or reduction a) I2 + 2e - 2I - b) K K + + e - c) Fe 2+ Fe 3+ + e -

14 Oxidation numbers in Redox Reaction 1. Assign oxidation numbers to all elements 2. When an atom is oxidized, its oxidation # increases 3. When an atom is reduced, its oxidation # decreases 2K + Br2 2KBr

15 Example 3: Cu + AgNO3 Ag + CuNO3 Assign oxidation numbers. Which atom is oxidized? Which atom is reduced?

16 Example 4: 2KBr + Cl2 2KCl + Br2 Assign oxidation numbers. Which atom is oxidized? Which atom is reduced?

17 Example 5: CH4 + 2O2 CO2 + 2H2O Assign oxidation numbers. Which atom is oxidized? Which atom is reduced?

18 Balancing Redox Reactions oxidation = reduction # of electrons lost = # electrons gained We will learn to balance redox reactions using the half-reaction method.

19 Half-reactions Equations that have electrons as reactants or products One half reaction represents oxidation One half reaction represents reduction Example 6: SnCl4 + Fe SnCl2 + FeCl3 Example 7: Fe + CuSO4 Cu + Fe2(SO4)3

20 Using Half-Reactions to Balance Redox Equations 1. Identify the species oxidized and the species reduced 2. Write the half-reaction 3. Multiply the half-reaction by the smallest coefficient possible so that the # of e- is the same. 4. Rewrite as a complete balanced equation (add 2 half-reactions together). May 20, 2014

21 Example 8: Rewrite half reactions from example 6.

22 Example 9: Rewrite half reactions from example 7.

23 Example 10: Balance the following using the halfreaction method. H2S + Cl2 HCl + S a. write half reactions b. balance oxidation/reduction c. rewrite balanced equation

24 Electrochemistry Study of how chemical energy is converted to electrical energy or vice versa. Electrochemistry Redox Electrochemical Cell Voltaic Cell Electrolytic Cell

25 May 20, 2014 Voltaic Cells converts chemical energy to electrical energy spontaneous redox reaction generates a current

26 Voltaic Cells consists of two half-cells Separate oxidation and reduction reaction Each half-cell contains: > electrode > solution Anode: oxidation Cathode: reduction May 20, 2014

27 Voltaic Cells Half-cells are connected by a salt bridge > allows ions to pass from one side to another > prevents build up of ions that prevent redox reactions May 20, 2014

28 Example 11: Sketch the diagram from the animation and identify anode and half-reaction cathode and half-reaction write the overall balanced cell reaction May 20, 2014

29 Cell Notation shows you the oxidation and reduction half-cells in a voltaic cell Anode Cathode Zn Zn 2+ Cu 2+ Cu Oxidation Reduction Salt Bridge Example 12: Write the cell notation for the voltaic cell in example 11.

30 Electrochemical Cell Potential reduction potential: tendency of a substance to gain electrons > reduction potential of an electrode is measured in volts > standard reduction potential (E 0 )

31 Electrochemical cell potential: difference in potential between the half-reactions > potential must be > 0 in order for the redox reaction to be spontaneous > In a voltaic cell, the half-reaction with the lower reduction potential will be the oxidation reaction (opposite reaction given on chart) E 0 cell = E 0 reduction - E 0 oxidation

32 Example 12: Write the cell notation and calculate the cell potential for the following redox reaction. Sn(s) + 2Cu + (aq) Sn Cu(s)

33 Example 13: Write the cell notation and calculate the cell potential for the following redox reaction. Mg(s) + Pb 2+ (aq) Pb(s) + Mg 2+ (aq)

34 Example 14: Given the pair of half-reactions, write the balanced equation for the overall cell reaction calculate the cell potential write the cell notation Co 2+ (aq) + 2e- Co(s) Cr 3+ (aq) + 3e- Cr(s)

35 Example 15: Given the pair of half-reactions, write the balanced equation for the overall cell reaction calculate the cell potential write the cell notation Fe 2+ (aq) + 2e- Fe(s) I2(s) + 2e- 2I - (s)

An oxidation-reduction (redox) reaction involves the transfer of electrons (e - ). Sodium transfers its electrons to chlorine

Oxidation-Reduction An oxidation-reduction (redox) reaction involves the transfer of electrons (e - ). Sodium transfers its electrons to chlorine 2 Chemists need a way to keep track of what happens in