RedOx Chemistry. with. Dr. Nick

Size: px

Start display at page:

Download "RedOx Chemistry. with. Dr. Nick"

Susan Price
6 years ago
Views:

1 RedOx Chemistry with Dr. Nick

2 What is RedOx Chemistry? The defining characteristic of a RedOx reaction is that electron(s) have completely moved from one atom / molecule to another. The molecule receiving the electrons has been Reduced The molecule giving the electrons has been Oxidized

3 What is RedOx Chemistry? OIL RIG Oxidation is Losing Reduction is Gaining (electrons) LEO says GER Losing Electron Oxidation Gaining Electron Reduction

5 Inorganic Chemistry Chapter 1: Table W.H. Freeman

6 The Standard Potential E The standard potential (E ) is a convenient metric for determining the relative strength of RedOx reactions at 1 M concentrations. Think of E as, How easily a reaction will happen. Very positive means very easy, very negative means very difficult. You can change the sign of E by reversing the reaction.

8 Combining Half-Reactions I. Write the half-reactions, including E II. Check if the two are a RedOx pair. If not, reverse the more negative half reaction. III. Balance elements other than oxygen and hydrogen using coefficients. IV. Balance oxygen by adding H2Os to one side. V. Balance hydrogen by adding H + s to one side.

9 Combining Half-Reactions VI. IF THE SOLUTION IS BASIC, neutralize all H + by adding OH - to both sides. VII. Balance charges by adding e -. VIII. Multiply the half reactions to balance the electrons (e - ). IX. Add half reactions and E, cancel terms. X. Final check: make sure all atoms and charges balance!

10 Inorganic Chemistry Chapter 1: Figure W.H. Freeman

11 Inorganic Chemistry Chapter 1: Table W.H. Freeman

Inorganic Chemistry Chapter 5: Figure 5.1 e- X - e- 2H + + 2e - H 2 E = 0.00 Zn +2 + 2e- Zn E = -0.

12 Inorganic Chemistry Chapter 5: Figure 5.1 e- X - e- 2H + + 2e - H 2 E = 0.00 Zn e- Zn E = H + + 2e - H 2 E = 0.00 Zn Zn e - E = Galvanic Cathode The source of the electrons is the Anode 2009 W.H. Freeman

13 Fun with Equations! G = -nfe G: Gibb s free energy. Negative values are more thermodynamically favorable n: Represented in the book as ν, is the coefficient in front of the electrons for the balanced RedOx reaction. F: Faraday s constant, 96,480 J/mol V or C/mol. E : Standard potential of the reaction. What we ve been working with this whole time!

14 Fun with Equations! 2H G = -nfe + + 2e - H 2 E = 0.00 Zn Zn e - E = Zn + 2H + + 2e - Zn +2 + H 2 + 2e - E cell = V G = -(2)(96,480 J/mol V)(0.76 V) G = -150 kj/mol

15 Fun with Equations! E cell = E cell - RT nf lnq NERNST! The NERNST equation allows you to calculate the potential of a cell at nonstandard conditions. R: Ideal Gas Constant, use J/mol K for these calculations. Q: Reaction Quotient, concentrations of products over reactants.

16 Fun with Equations! E cell = E cell - RT nf lnq NERNST! aa + bb cc + dd Q = [C]c [D] d [A] a [B] b

17 Fun with Equations! E cell = E cell - RT nf lnq NERNST! Zn (s) + Sn +2 (aq) + 2e - Zn +2 (aq) + Sn (s) + 2e - E cell = V 0.1 M 0.5 M Q = 0.5 M 0.1 M (8.314 J/mol K)(298 K) 0.5 M E cell = (0.62 V) - ln 2(96,480 J/mol V) 0.1 M E cell = 0.60 V

18 Fun with Equations! E cell = E cell n logq A shorthand of the NERNST equation that condenses the constants into a single value. This assumes a 298 K (25 C) temperature! NERNST LITE! The logarithm is now base 10, not natural log!

19 Fun with Equations! lnk = nfe cell RT If you know the standard potential of the cell, you can calculate the equilibrium constant by rearranging the NERNST equation. Measuring the potential of a battery is one way of determining the ratio of products to reactants.

20 Disproportionation! 2A A + + A - Disproportionation reactions occur when two identical molecules react to donate / accept electron(s). These reactions, like all RedOx reactions, can be broken down into half reactions and examined for spontaneity.

21 Disproportionation! 2Cu + Cu +2 + Cu (s) Cu + + e - Cu (s) E = V Cu + Cu +2 + e - E = V E = V SPONTANEOUS!!!

22 Comproportionation! A + + A - 2A Simply the reverse reaction of disproportionation, these occur when two different molecules donate / accept electron(s) to form two identical molecules. These reactions, like all RedOx reactions, can be broken down into half reactions and examined for spontaneity.

23 Comproportionation! Ag +2 + Ag (s) 2 Ag + Ag +2 + e - Ag + E = V Ag (s) Ag + + e - E = V E = V SPONTANEOUS!!!

24 Latimer Diagrams Latimer Diagrams provide a quick way to assess the successive reductions of a compound. Organized with the most oxidized species on the left, each reduction, represented by an arrow, denotes the standard potential associated with the transformation. Disproportionation reactions are easy to spot in a Latimer diagram.

25 Latimer Diagrams Cu Cu Cu (s) Ag Ag Ag (s) E 1 + E 2 = n 1 E 1 + n 2 E 2 n 1 +n 2

26 Inorganic Chemistry Chapter 1: Figure 5.17 Redox Reactions Redox 2009 W.H. Freeman

28 Inorganic Chemistry Chapter 1: Box W.H. Freeman

29 Hydrogen Fuel Cells Anode 2H 2 4H + + 4e - Cathode 4H + + 4e - + O 2 2H 2 O Overall 2H 2 + O 2 2H 2 O

currently the best catalyst for both the production Coal and 24 oxidation of hydrogen.

30 Alternate Sources of Energy 2H 2 4H + + 4e - 4H + + 4e - + O 2 2H 2 + O 2 Anode Cathode Overall Specific Energy (MJ/Kg) 2H Uranium O 83,140,000 Hydrogen 123 Gasoline 2H 2 O 46 Propane 46 Fat 37 Platinum is currently the best catalyst for both the production Coal and 24 oxidation of hydrogen. Carbohydrates 17 TOF = 6,000 s -1 Platinum: $51, per kg Iron: $0.20 per kg Nickel: $2.94 per kg 30

31 Lead Acid Battery Overall Reaction: a comproportionation reaction

32 Lithium Batteries Disposable Produce voltages from 1.5 V to about 3.7 V. (comparable to a zinc carbon or alkaline battery)

33 Lithium Ion Batteries

34 Lithium Ion Battery

35 Lithium Ion Battery: The Electrochemistry

36 Lithium Ion Batteries: Configurations

37 A zinc/acid battery

38 Daniell Cell. Copper Deposition/Zinc Dissolution

40 Electrolytic Cell

41 Electroplating amp = C/s F = C/mol e - n = mol e-/mol product How many grams of silver will be plated if the cell is run at 5 amps for 1 minute?

42 Electroplating How many grams of silver will be plated if the cell is run at 5 amps for 1 minute? 5 amps x 60 sec = 300 C 300 C 96,500 C/mol e - = mol e mol e - 1 mol e - /mol Ag (s) = mol Ag (s) mol Ag(s) x g/mol = grams

43 Water Reduction 2 H 2 O (l) + 2 e- H 2(g) + 2OH - (aq) If the electron source is a metal, the reaction will yield H2 gas and the metal hydroxide. This is why the first group metals are called alkali metals.

44 Water Oxidation 2 H 2 O (l) O 2(g) + 4 H + (aq) + 4 e -

45 Water Splitting 2 H 2 O (l) O 2(g) + 4 H 2(g)

46 Passivation A metal can react with O2 to form a metal-oxide layer that is impervious to further oxidation.

17.1 Redox Chemistry Revisited

Chapter Outline 17.1 Redox Chemistry Revisited 17.2 Electrochemical Cells 17.3 Standard Potentials 17.4 Chemical Energy and Electrical Work 17.5 A Reference Point: The Standard Hydrogen Electrode 17.6