Unformatted text preview: Chapter 20: ELECTROCHEMISTRY Electrochemistry involves the relationship between electrical energy and chemical energy. Essentially dealing with OXIDATION-REDUCTION REACTIONS Electron transfer reaction SPONTANEOUS REACTIONS: Examples: voltaic cells, batteries. NON-SPONTANEOUS REACTIONS: Examples: electrolysis, electrolytic cells. QUANTITATE REACTIONS How much current flows
MJ Bojan Electrochemistry 1 1 Electrochemical ReacBons Zn(s) + CuSO4(aq) ZnSO4(aq) + Cu(s) In electrochemical reacBons: Something loses electrons or is Something gains electrons or is The oxidizing agent is The reducing agent is Electrochemistry 1 MJ Bojan 2 Things to Remember: L An Ox G Red Cat MJ Bojan Electrochemistry 1 3 Oxidation reduction reactions
Zn(s) + CuSO4(aq) ZnSO4(aq) + Cu(s)
Ionic equation: Net ionic equation: What is oxidized? What is reduced? What is the oxidizing agent? What is the reducing agent? MJ Bojan Electrochemistry 1 4 Rules for determining OxidaBon States 1. Oxidation state of atom in elemental form is zero. e.g. Cl2 O2 P4 C(s) S8 2. The oxidation number of a monatomic ion equals its charge. 3. Some elements have "common" oxidation numbers that can be used as reference in determining the oxidation numbers of other atoms in the compound. Alkali metals +1 Alkaline earth metals +2 Fluorine 1 O usually 2 (peroxides (-1) & superoxides possible) H usually +1 (Hydrides: metal-H compounds (1)) Cl, Br, I almost always 1 4. Sum of oxidation numbers is equal to overall charge of molecule or ion: For a neutral compound the sum of oxidation numbers equals zero. For a polyatomic ion, the sum of the oxidation numbers is equal to the charge on the ion. 5. Shared electrons are assigned to the more electronegative atom of the pair: more electronegative atom will have a negative oxidation number.
MJ Bojan Electrochemistry 1 5 Electrochemical ReacBons In order to keep track of what loses electrons and what gains them, we assign oxidaBon numbers. MJ Bojan Electrochemistry 1 6 Periodic trends in oxidation states
Driving force: atoms tend to lose or gain electrons to achieve: For main group elements (s and p block)
The highest possible positive oxidation state is equal to the group number! (We won't worry about the transition metals right now)
MJ Bojan Electrochemistry 1 7 Periodic trends in oxidation states
H -1 +1 Li +1 Na +1 K +1 Rb +1 Be +2 Mg +2 Ca +2 Sr +2 Ba +2 Al +3 Ga +3 In +1 +3 Tl +1 +3 Ge +2 +4 Sn +2 +4 Pb +2 +4 P +3 +5 As +3 +5 Sb +3 +5 Bi +3 +5 Can we rationalize these common oxidation states?
O -2,-1 F -1 S Cl -2 -1 +2,4,6 +1,3,5,7 Se Br -2 -1 +2,4,6 +1,3,5,7 Te I -2 -1 +2,4,6 +1,3,5,7 Blue: most common oxidation states in Group 5
MJ Bojan Electrochemistry 1 8 BALANCING REDOX REACTIONS
1. Assign Oxidation Numbers. 2. Write incomplete half-reactions. 3. Balance each half-reaction separately. a. Balance atoms undergoing redox. b. Balance remaining atoms. i. Add H2O to balance oxygens. ii. Add H+ to balance hydrogens. 4. Balance charges by adding electrons. 5. Multiply each half-reaction so that the same number of electrons are involved in the reduction and the oxidation. 6. Add the half-reactions. 7. In basic solutions, add OH- to neutralize H+
MJ Bojan Electrochemistry 1 9 Half ReacBons The following reacBon is not balanced. Write the balanced half- reacBons and overall reacBon for: Sn2+(aq) + 2Fe3+(aq) Sn4+(aq) + 2Fe2+(aq) oxidaBon reducBon Balanced ReacBon MJ Bojan Electrochemistry 1 10 Balancing Redox Reactions Balance the following reacBon in acid soluBon: C + HNO3 NO2 + CO2 + H2O MJ Bojan Electrochemistry 1 11 Balancing Redox Reactions Balance the following reacBon in basic soluBon: PbO2(s) + Cl-(aq) + OH-(aq) Pb(OH)3-(aq) + ClO-(aq) MJ Bojan Electrochemistry 1 12 Balancing Redox Reactions
When the following reacBon is balanced (in acid) what is the coefficient in front of water? NO2(g) + H2O(l) NO3-(aq) + NO(g) MJ Bojan Electrochemistry 1 13 SPONTANEOUS ELECTROCHEMICAL REACTIONS
Is this reacBon SPONTANEOUS? Zn0(s) + Cu2+(aq) Zn2+(aq) + Cu0(s) If the reacBon is spontaneous: MJ Bojan Electrochemistry 1 14 Voltaic Cell MJ Bojan Electrochemistry 1 15 Voltaic Cell What happens at the cathode? What happens at the anode? Role of SALT BRIDGE: Which electrode will increase in mass? Which will decrease? Which direction do the electrons flow? MJ Bojan Electrochemistry 1 16 Voltaic Cells Consist of:
A. Electrodes: The two solid metals (cathode and anode) Anode: what process?_________________ Cathode: what process?_________________ B. Electrolyte soluBons C. What does the salt bridge or porous divider do? "Rules" of voltaic cells: 1. At the anode electrons are products. 2. At the cathode electrons are reagents. 3. Electrons cannot swim! MJ Bojan Electrochemistry 1 17 ElectromoBve Force (emf) Water only flows one way spontaneously in a waterfall. Likewise, electrons only flow spontaneously one way in a redox reacBon --from higher to lower potenBal energy. MJ Bojan 18 Electrochemistry 1 Cell PotenBal The potenBal difference between the anode and cathode in a cell is called the electromoBve force (emf). It is also called: Cell potenBal is measured in volts (V). Ecell : Intensive property, energy per electron MJ Bojan Electrochemistry 1 19 VOLTAIC CELL VOLTAGE Cell voltage (EMF or Ecell) is the measure of reaction spontaneity The more spontaneous a reaction, Cell voltage depends on: 1) 2) 3)
MJ Bojan Electrochemistry 1 20 STANDARD POTENTIAL FOR AN ELECTROCHEMICAL CELL The standard potential for an electrochemical cell is the potential (voltage) generated when reactants and products of a redox reaction are in their standard states. Standard State defined so potentials can be tabulated: T = 25C. Gases, p = 1 atm. [Solutions] = 1M It is convenient to tabulate redox reactions as half reactions. Standard half-cell potentials When all substances are in standard state: MJ Bojan Electrochemistry 1 21 MJ Bojan Electrochemistry 1 22 HALF-CELL POTENTIAL The half-cell potential is the potential associated with the half-reaction. Rules for half-cell potentials: 1. 2. 3. The sum of two half-cells potentials in a cell equals the overall cell potential: Ecell = E1/2(oxid) + E1/2(reduc) For any half-reaction: E1/2(oxid) = - E1/2(reduc) Standard half-cell is a hydrogen electrode: H2(g,1atm) 2H+ (aq, 1M) + 2eE1/2(oxid) = E1/2(reduc) = 0 V MJ Bojan Electrochemistry 1 23 E0cell for Cu/Zn cell What is E1/2(reduc) ? What is E1/2(oxid)? E0cell = E1/2(reduc) + E1/2(oxid) =
MJ Bojan Electrochemistry 1 24 How do we get half cell potentials? What is E1/2(reduc) ? What is E1/2 (oxid)? E0cell = E1/2(reduc) + E1/2(oxid) =
MJ Bojan Electrochemistry 1 25 CELL POTENTIAL
What is Ecell for the following reaction? Al(s) + Cu+2(aq) Al+3(aq) + Cu(s) MJ Bojan Electrochemistry 1 26 Oxidizing and Reducing Agents The strongest oxidizers: The strongest reducers: MJ Bojan Electrochemistry 1 27 Oxidizing and Reducing Agents The greater the difference between the two, the greater the voltage of the cell. MJ Bojan Electrochemistry 1 28 Oxidizing and Reducing Agents
The more posiBve Ered the stronger the oxidizing agent on the leo. The more negaBve Ered the stronger the reducing agent on the right. A species which is higher and to the leo on the table of standard reducBon potenBals will spontaneously oxidize a species that is lower to the right in the table. Example: F2 will oxidize H2 or Li Ni2+ will oxidize Al(s) MJ Bojan Electrochemistry 1 29 Oxidizing and Reducing Agents Br2 O2 Fe2+ Na+ Which one of these is the best oxidizing agent? Put these in order of increasing ability to oxidize. Strategy: write out half cell potentials for REDUCTION MJ Bojan Electrochemistry 1 30 Examples: Which is the best reducing agent of the following? Cl-(aq) Fe(s) Fe+3(aq) Fe+2(aq) Cl2(g)
MJ Bojan Electrochemistry 1 31 Examples: Which is the best oxidizing agent of the following? Cl-(aq) Fe(s) Fe+3(aq) Fe+2(aq) Cl2(g)
MJ Bojan Electrochemistry 1 32 Free Energy and E G for a redox reacBon : G = -nF E
n is the number of moles of electrons transferred in a balanced reacBon F is a constant, the Faraday: 1 F = 96,485 C/mol = 96,485 J/V-mol Under standard condiBons: MJ Bojan Electrochemistry 1 33 Example: Is this reacBon spontaneous? Cd(s) + 2H+ Cd2+ + H2 (g) For spontaneous electrochemical reactions: Gocell must be negative, therefore, Eocell must be positive Gocell= -nF Eocell Cd2+ + 2e- Cd (s) Eored = -0.403V 2H+ + 2e- H2 (g) Eored = 0 The more positive Eored will be the electrode where reduction takes place. H+ cathode: Cd anode: 2H+ + 2e- H2 Cd Cd2+ + 2e- Eored = 0 Eooxid =+0.403V Eocell = Eored (cathode) + Eooxid (anode) = 0 + 0.403V = +0.403V Go = -nF Eo = - 2 x 96,500 J/(V-mol) x 0.403V = -8.3 x 104 J = -83 kJ/mol Yes: the reaction is spontaneous under standard conditions!
MJ Bojan Electrochemistry 1 34 Is it Spontaneous? The standard cell potential (Ecell) for the reaction below is 0.89V. What is the value of G for the reaction? 2Cr(s) + 3 Sn4+ (aq) 2 Cr3+ (aq) + 3 Sn2+ (aq) MJ Bojan Electrochemistry 1 35 Putting things together
G = -2.303 RT log Keq and G = -n F E Combining these two equaBons: E = 2.303 RT log Keq n F R = 8.314 J/K-mole F = 96,500 J/V-mole e- At 25C = 298K: E = (2.303)(8.314)(298)log Keq n(96,500) E = (0.0592) log Keq n MJ Bojan Electrochemistry 1 36 Effect of Concentration Standard concentration 1M solution, 1 atm gas pressure What if concentrations are different? G = G + RT ln Q G = -nF E -nF E = -nF E + RT ln Q For electrochemical cell at equilibrium: G = 0, E cell = 0
MJ Bojan Electrochemistry 1 37 Effect of Concentration RT E =E " lnQ n#
o So for a half reaction: aA + bB + n e- cC + dD E 1 = E0 1
2 2 !
E 1 = Eo 1 2 2 ! RT [C] [D] " ln a b n# [ A] [B]
c d c d 2.303 RT [C] [D] " log a b n# [ A] [B]
0.059 [C] [D] " log a b at 298K n [ A] [B]
Electrochemistry 1 38 c d !
MJ Bojan E1 = E
2 o 1 2 Examples What is the half cell potential of the Ag/Ag+ redox couple (E0 = +0.799 V) in a 1 M NaCl solution that contains solid AgCl (Ksp = 1.1 x 10-10)? MJ Bojan Electrochemistry 1 39 Effect of ConcentraBon For an overall cell reaction: lL + mM pP + qQ
E cell = E
0 cell RT [P] [Q] " ln l m n# [L ] [M] p q ! E cell = E 0 cell 2.303 RT [P] [Q] " log l m n# [L ] [M]
0.0592 [P] [Q] " log l m n [L ] [M]
Electrochemistry 1 p q p q !
MJ Bojan E cell = E 0 cell 40 Sample Problem
A voltaic cell has the following reaction Al(s) + 3Ag+(aq) Al3+ (aq) + Ag(s) 1. What will happen to the cell emf (Ecell) if water is added to the anode compartment? a) Ecell will increase b) Ecell will decrease c) This will have no effect on Ecell 2. What will happen to the cell emf (Ecell) if the size of the Al electrode is increased? MJ Bojan Electrochemistry 1 41 A voltaic cell has the following reaction Al(s) + 3Ag+(aq) Al3+ (aq) + Ag(s) 3. What will happen to the cell emf (Ecell) if AgNO3 is added to the cathode compartment increasing the quantity of Ag+(aq), but not changing its concentration? a) Ecell will increase b) Ecell will decrease c) This will have no effect on Ecell 4. What will happen to the cell emf (Ecell) if HCl is added to the AgNO3 solution? MJ Bojan Electrochemistry 1 42 ...
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