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Electrochemistry

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**Electrochemistry Terminology #1**

Oxidation – A process in which an element attains a more positive oxidation state Na(s) Na+ + e- Reduction – A process in which an element attains a more negative oxidation state Cl2 + 2e- 2Cl-

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**Electrochemistry Terminology #2**

An old memory device for oxidation and reduction goes like this… LEO says GER Lose Electrons = Oxidation Gain Electrons = Reduction

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**Electrochemistry Terminology #3**

Oxidizing agent The substance that is reduced is the oxidizing agent Reducing agent The substance that is oxidized is the reducing agent

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**Electrochemistry Terminology #4**

Anode The electrode where oxidation occurs Cathode The electrode where reduction occurs Memory device: Reduction at the Cathode

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**Table of Reduction Potentials**

Measured against the Standard Hydrogen Electrode

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**Measuring Standard Electrode Potential**

Potentials are measured against a hydrogen ion reduction reaction, which is arbitrarily assigned a potential of zero volts.

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**Galvanic (Electrochemical) Cells**

Spontaneous redox processes have: A positive cell potential, E0 A negative free energy change, (-G)

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**Zn - Cu Galvanic Cell From a table of reduction potentials:**

Zn2+ + 2e- Zn E = -0.76V Cu2+ + 2e- Cu E = +0.34V

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**Zn - Cu Galvanic Cell Zn Zn2+ + 2e- E = +0.76V**

The less positive, or more negative reduction potential becomes the oxidation… Cu2+ + 2e- Cu E = +0.34V Zn Zn2+ + 2e E = +0.76V Zn + Cu2+ Zn2+ + Cu E0 = V

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**Line Notation Zn(s) | Zn2+(aq) || Cu2+(aq) | Cu(s) | || |**

An abbreviated representation of an electrochemical cell Zn(s) | Zn2+(aq) || Cu2+(aq) | Cu(s) Anode material Anode solution Cathode solution Cathode material | || |

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**Calculating G0 for a Cell**

G0 = -nFE0 n = moles of electrons in balanced redox equation F = Faraday constant = 96,485 coulombs/mol e- Zn + Cu2+ Zn2+ + Cu E0 = V

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**The Nernst Equation R = 8.31 J/(molK) T = Temperature in K**

Standard potentials assume a concentration of 1 M. The Nernst equation allows us to calculate potential when the two cells are not 1.0 M. R = 8.31 J/(molK) T = Temperature in K n = moles of electrons in balanced redox equation F = Faraday constant = 96,485 coulombs/mol e-

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**Nernst Equation Simplified**

At 25 C (298 K) the Nernst Equation is simplified this way:

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**Equilibrium Constants and Cell Potential**

At equilibrium, forward and reverse reactions occur at equal rates, therefore: The battery is “dead” The cell potential, E, is zero volts Modifying the Nernst Equation (at 25 C):

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**Calculating an Equilibrium Constant from a Cell Potential**

Zn + Cu2+ Zn2+ + Cu E0 = V

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**Both sides have the same components but at different concentrations.**

??? Concentration Cell Both sides have the same components but at different concentrations. Step 1: Determine which side undergoes oxidation, and which side undergoes reduction.

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**Both sides have the same components but at different concentrations.**

??? Concentration Cell Both sides have the same components but at different concentrations. Anode Cathode The 1.0 M Zn2+ must decrease in concentration, and the 0.10 M Zn2+ must increase in concentration Zn2+ (1.0M) + 2e- Zn (reduction) Zn Zn2+ (0.10M) + 2e- (oxidation) Zn2+ (1.0M) Zn2+ (0.10M)

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**Both sides have the same components but at different concentrations.**

Concentration Cell ??? Concentration Cell Both sides have the same components but at different concentrations. Anode Cathode Step 2: Calculate cell potential using the Nernst Equation (assuming 25 C). Zn2+ (1.0M) Zn2+ (0.10M)

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Nernst Calculations Zn2+ (1.0M) Zn2+ (0.10M)

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**Electrolytic Processes**

Electrolytic processes are NOT spontaneous. They have: A negative cell potential, (-E0) A positive free energy change, (+G)

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**Electrolysis of Water In acidic solution Anode rxn: Cathode rxn:**

-1.23 V Cathode rxn: -0.83 V -2.06 V

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**Electroplating of Silver**

Anode reaction: Ag Ag+ + e- Cathode reaction: Ag+ + e- Ag Electroplating requirements: 1. Solution of the plating metal 2. Anode made of the plating metal 3. Cathode with the object to be plated 4. Source of current

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**Solving an Electroplating Problem**

Q: How many seconds will it take to plate out 5.0 grams of silver from a solution of AgNO3 using a 20.0 Ampere current? Ag+ + e- Ag 5.0 g 1 mol Ag 1 mol e- C 1 s 1 mol e- 20.0 C g 1 mol Ag = 2.2 x 102 s

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