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Chapter 22 Electrochemistry. Objectives: describe how an electrolytic cell works describe how galvanic (voltaic) cell works determine net voltage from paired standard half-cells in a galvanic cell predict products using standard reduction potentials and an activity series. Electrochemistry.

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chapter 22 electrochemistry

Chapter 22 Electrochemistry

Objectives:

describe how an electrolytic cell works

describe how galvanic (voltaic) cell works

determine net voltage from paired standard half-cells in a galvanic cell

predict products using standard reduction potentials and an activity series

Created by C. Ippolito

March 2007

electrochemistry
Electrochemistry
  • the relationship between the flow of electric current and chemical changes
    • Electrolysis
      • changes electrical energy into chemical energy
        • charging a car battery
        • electroplating a metal
      • involves reactions with partial gain/loss of electrons
    • Electrochemical Cell
      • changes chemical energy into electrical energy
        • dry cells running iPod, cell phone etc
      • involves reactions with complete gain/loss of electrons

Created by C. Ippolito

March 2007

half reactions
Half Reactions
  • All redox reactions – reduction + oxidation

2Na(s) + Cl2(g)  2NaCl(s)

    • Half-reactions
      • oxidation (OIL)

Na0 Na+

      • reduction (RIG)

Cl2  2Cl-

Created by C. Ippolito

March 2007

half reaction equations
Half-Reaction Equations
  • All redox reactions – reduction + oxidation

2Na(s) + Cl2(g)  2NaCl(s)

    • Half-reaction equations
      • represent chemical changes of redox
        • oxidation reaction

2Na0 2Na+ + 2e- (OIL)

        • reduction reaction

Cl2 + 2e- 2Cl- (RIG)

Created by C. Ippolito

March 2007

electric current
Electric Current
  • flow of electric charge
    • Metallic Conduction
      • movement of loosely held valance electrons
    • Ionic Conduction
      • movement of positive and negative ions along a path
  • Direct Current
    • electrons flow in only one direction
    • from negative terminal to positive terminal

Created by C. Ippolito

March 2007

electrolysis
Electrolysis
  • electric current causes redox in the electrolyte in an electrolytic cell
    • source of current
    • electrodes
      • cathode (- terminal) – site of reduction
      • anode (+ terminal) – site of oxidation
    • electrolyte
      • aqueous or liquid permits ions to move between electrodes

Created by C. Ippolito

March 2007

electroplating
Electroplating
  • use of electrolysis to coat a material with a layer of metal
    • Copper strip – anode
    • coin – cathode
    • electrolyte – copper sulfate

Created by C. Ippolito

March 2007

electrochemical cell
Electrochemical Cell
  • Galvanic (voltaic) Cells
    • electric current from spontaneous redox rxns
    • chemical energy  electrical energy
  • Battery
    • multiple voltaic cells act as a unit
  • Electromotive Force (emf)
    • voltage between the electrodes
      • affected by:
        • temperature
        • metals used
        • electrolyte concentration

Created by C. Ippolito

March 2007

zinc copper voltaic cell
Zinc-Copper Voltaic Cell

Created by C. Ippolito

March 2007

zinc copper voltaic cell1
Zinc-Copper Voltaic Cell

External Circuit

Oxidation

ZnoZn2+ + 2e-

Reduction

Cu2+ + 2e- Cuo

Internal Circuit

Created by C. Ippolito

March 2007

zinc copper voltaic cell2
Zinc-Copper Voltaic Cell
  • Zinc-Copper Voltaic Cell
    • Zn(s)|ZnSO4(aq)||CuSO4(aq)|Cu(s)
    • oxidized half cell is always written first

Created by C. Ippolito

March 2007

dry cells
Dry Cells
  • Voltaic cell with “paste” electrolyte

Created by C. Ippolito

March 2007

lead storage battery
Lead Storage Battery

Created by C. Ippolito

March 2007

electric potential
Electric Potential
  • measures cell’s ability to produce current
  • results from a competition for electrons
    • reduction potential – tendency of a given half reaction to occur as reduction oxidation
      • reduction occurs in the cell with the greater reduction potential
    • cell potential – difference between the reduction potentials

Created by C. Ippolito

March 2007

standard cell potential
Standard Cell Potential
  • measured when
    • ion concentrations = 1M
    • 25oC and 1 atmosphere (101 kPa)
  • Standard hydrogen electrode used with others to determine reduction potentials
    • assigned reduction potential 0.00 V -

Created by C. Ippolito

March 2007

standard reduction potentials
Standard Reduction Potentials
  • determined using standard hydrogen electrode and the equation for standard cell potential

Created by C. Ippolito

March 2007

calculating standard cell potentials
Calculating Standard Cell Potentials
  • Use Table of Reduction Potentials to predict the half-cells of reduction and oxidation.
  • Given reaction:

Zn(s) + 2Ag+(aq)  Zn2+ + 2Ag(s)

  • Write half-reactions and look up E0

Zn(s)  Zn2+(aq) + 2e- E0 = -0.76V

Ag+ + e- Ag(s) E0 = +0.80V

E0 = 0.80V-(-0.76V) = +1.56V

Created by C. Ippolito

March 2007

corrosion
Corrosion
  • the deterioration and wearing away of metals usually through “oxidation”
  • Prevention:
    • coat with paint to stop water and oxygen contact
    • electroplate with less reactive metal
    • alloy with another metal (stainless steel – Fe & Cr)
    • protect metal by making it the “cathode”

Mg strips on ship hulls corrode instead of the hull

Created by C. Ippolito

March 2007