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

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Oxidation State(Oxidation Number)

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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).

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© 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)

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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+))

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© 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)

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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)

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© 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.

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© Copyright 1995- 2001 R.J. Rusay© Copyright 1995- 2001 R.J. Rusay

A Voltaic Cell

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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

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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.