oxidation-reduction dr. ron rusay fall 2001 © copyright 2001 r.j. rusay
TRANSCRIPT
Oxidation-Reduction
Dr. Ron Rusay
Fall 2001
© Copyright 2001 R.J. Rusay© Copyright 2001 R.J. Rusay
Oxidation-Reduction
• Oxidation is the loss of electrons.
• Reduction is the gain of electrons.
• The reactions occur together. One does not occur without the other.
• The terms are used relative to the change in the oxidation state of the reactant(s).
Oxidation Reduction Reactions
QuickTime™ and aSorenson Video decompressorare needed to see this picture.
QuickTime™ and aSorenson Video decompressorare needed to see this picture.
Oxidation State(Oxidation Number)
QuickTime™ and aSorenson Video decompressorare needed to see this picture.
Cu 2+ Cu (s)
H2 (g) 2 H +
0
0
+2 e-
- 2 e-
Use oxidation numbers to determine what is oxidized and what is reduced.
Number of electrons gained must equal the number of electrons lost.
- 2 e-
+2 e-
Refer to BalancingOxidation-Reduction Reactions
Balancing Redox Equationsin acidic solutions
1) Determine the oxidation numbers of atoms in both reactants and products.
2) Identify and select out those which change oxidation number (“redox” atoms) into separate “half reactions”.
3) Balance the “redox” atoms and charges (electron gain and loss must equal!).
4) In acidic reactions balance oxygen with water then hydrogen from water with acid proton(s).
1
© Copyright 1995- 2001 R.J. Rusay© Copyright 1995- 2001 R.J. Rusay
Balancing Redox EquationsBalancing Redox Equations
FeFe+2+2(aq)(aq)+ Cr+ Cr22OO77
2-2-(aq) (aq) +H+H++
(aq)(aq)---->---->
FeFe3+3+(aq)(aq) + Cr + Cr3+3+
(aq) (aq) + H+ H22OO(l) (l)
??
Fe Fe 2+2+(aq)(aq)+ Cr+ Cr22OO77
2-2-(aq) (aq) +H+H++
(aq)(aq)---->---->
Fe Fe 3+3+(aq)(aq) + Cr + Cr 3+3+
(aq) (aq) + H+ H22OO(l)(l)
1
© Copyright 1995- 2001 R.J. Rusay© Copyright 1995- 2001 R.J. Rusay
x x == ?? Cr Cr ; ; 2x+7(-2) 2x+7(-2) = = -2-2; ; x x = = +6+6
Balancing Redox EquationsBalancing Redox Equations
Fe Fe 2+2+(aq)(aq) ---> Fe ---> Fe 3+3+
(aq)(aq)
CrCr22OO772-2-
(aq) (aq) + --> Cr + --> Cr 3+3+(aq) (aq)
CrCr = (= (6+6+))
1
© Copyright 1995- 2001 R.J. Rusay© Copyright 1995- 2001 R.J. Rusay
66 (Fe (Fe 2+2+(aq)(aq) -e -e - - ---> Fe---> Fe3+3+
(aq)(aq)))
66 Fe Fe 2+2+(aq)(aq) ---> ---> 6 6 FeFe3+3+
(aq) (aq) + + 6 6 ee - -
CrCr22OO772-2-
(aq) (aq) + 6 e+ 6 e - - --> --> 2 2 CrCr3+3+(aq)(aq)
6 e6 e - -
-e-e - -
22
Balancing Redox EquationsBalancing Redox Equations
66 Fe Fe2+2+(aq)(aq) ---> ---> 6 6 FeFe3+3+
(aq) (aq) + + 6 6 ee - -
CrCr22OO772-2-
(aq) (aq) + 6 e+ 6 e - - --> --> 2 2 CrCr3+3+(aq)(aq)
66 Fe Fe2+2+(aq)(aq)+ Cr+ Cr22OO77
2-2-(aq) (aq) + + ? ? 2nd 2nd HH++
(aq)(aq) ----> ---->
6 6 FeFe3+3+(aq) (aq) + + 2 2 CrCr3+3+
(aq)(aq)+ + ? ? 1stOxygen 1stOxygen HH22OO(l) (l)
1
© Copyright 1995- 2001 R.J. Rusay© Copyright 1995- 2001 R.J. Rusay
OxygenOxygen == 7 7 2nd (Hydrogen)2nd (Hydrogen) == 1414
Balancing Redox EquationsBalancing Redox Equations
Completely Balanced Equation:Completely Balanced Equation:
66 Fe Fe2+2+(aq)(aq)+ Cr+ Cr22OO77
2-2-(aq) (aq) + + 14 14 HH++
(aq)(aq) ----> ---->
6 6 FeFe3+3+(aq) (aq) + + 2 2 CrCr3+3+
(aq)(aq)+ + 77 HH22OO(l) (l)
1
© Copyright 1995- 2001 R.J. Rusay© Copyright 1995- 2001 R.J. Rusay
Balancing Redox EquationsBalancing Redox Equationsin basic solutionsin basic solutions
1) Determine the oxidation numbers of atoms in both reactants and products.
2) Identify and select out those which change oxidation number (“redox” atoms) into separate “half reactions”.
3) Balance the “redox” atoms and charges (electron gain and loss must equal!).
4) In basic reactions balance the oxygen with 4) In basic reactions balance the oxygen with hydroxide then hydrogen from hydroxide with hydroxide then hydrogen from hydroxide with water.water.
1
© Copyright 1995- 2001 R.J. Rusay© Copyright 1995- 2001 R.J. Rusay
A Voltaic Cell
QuickTime™ and aSorenson Video decompressorare needed to see this picture.
Voltaic CellsVoltaic Cells
Voltages of Some Common Voltaic Cells
Voltaic Cell Voltage (V)
Common alkaline battery 1.5
Lead-acid car battery (6 cells = 12 V) 2.0
Calculator battery (mercury) 1.3
Electric eel (~ 5000 cells in 6-ft eel = 750 V) 0.15
Nerve of giant squid (across cell membrane) 0.070
Automotive Application: the Discharge and Recharging of a Lead-Acid Battery
Lithium Solid-State Battery
Nickel-Cadmium(Nicad) Battery
Flow Batteries (Fuel Cells)
The Hydrogen Oxygen Fuel Cell
Anode (oxidation):H2 (g) + CO3
2-(l) H2O(g) + CO2 (g) + 2 e-
Cathode (reduction): 1/2 O2 (g) + CO2 (g) + 2 e- CO3
2-(l)
Overall (cell) reaction:H2 (g) + 1/2 O2 (g) H2O(g) Ecell = 1.2 V
Other Fuel Cells:
2 NH3 (g) + 3/2 O2 (g) N2 (g) + 3 H2O(l)
N2H4 (g) + O2 (g) N2 (g) + 2 H2O (l)
CH4 (g) + 2 O2 (g) CO2 (g) + 2 H2O(l)
Chemistry of Batteries
The signs of Go and Eocell
determine the reaction direction at standard-state conditions.
For a cell to work spontaneously:Go = - (< 0)and Eo
cell = + (> 0)
Comparison of Voltaic and Electrolytic Cells
Electrode
Cell Type G Ecell Name Process Sign
Voltaic < 0 > 0 Anode Oxidation -
Voltaic < 0 > 0 Cathode Reduction +
Electrolytic > 0 < 0 Anode Oxidation +
Electrolytic > 0 < 0 Cathode Reduction -
Electrochemical Reactions
QuickTime™ and aSorenson Video decompressorare needed to see this picture.
Electrolysis: Chrome Plating
Aluminum ProductionUsing an Electrolytic Cell
Do you recycle? It takes only 5% of the energy to produce a can from recycled aluminum vs. aluminum ore.