created by c. ippolito march 2007 updated march 2007 chapter 22 electrochemistry objectives:...
TRANSCRIPT
Created by C. IppolitoMarch 2007
Updated March 2007
Chapter 22 Electrochemistry
Objectives:1. describe how an electrolytic cell
works2. describe how galvanic (voltaic) cell
works3. determine net voltage from paired
standard half-cells in a galvanic cell4. predict products using standard
reduction potentials and an activity series
Created by C. IppolitoMarch 2007
Updated March 2007
Electrochemistry
• the relationship between the flow of electric current and chemical changes– ElectrolysisElectrolysis
• changes electrical energy into chemical energy– charging a car battery– electroplating a metal
• involves reactions with partial gain/loss of electrons
– Electrochemical CellElectrochemical 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. IppolitoMarch 2007
Updated March 2007
Half Reactions
• All redox reactions – reduction + oxidation2Na(s) + Cl2(g) 2NaCl(s)
– Half-reactions• oxidation (OIL)
Na0 Na+
• reduction (RIG)Cl2 2Cl-
Created by C. IppolitoMarch 2007
Updated March 2007
Half-Reaction Equations
• All redox reactions – reduction + oxidation2Na(s) + Cl2(g) 2NaCl(s)
– Half-reaction equations• represent chemical changes of redox
– oxidation reaction2Na0 2Na+ + 2e- (OIL)
– reduction reactionCl2 + 2e- 2Cl- (RIG)
Created by C. IppolitoMarch 2007
Updated March 2007
Electric Current
• flow of electric charge– Metallic ConductionMetallic Conduction
• movement of loosely held valance electrons
– Ionic ConductionIonic 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. IppolitoMarch 2007
Updated March 2007
Electrolysis• electric current causes
redox in the electrolyte in an electrolytic cell
1. source of current2. electrodes
– cathode (- terminal) – site of reduction
– anode (+ terminal) – site of oxidation
3. electrolyte– aqueous or liquid permits ions
to move between electrodes
Created by C. IppolitoMarch 2007
Updated March 2007
Electroplating
• use of electrolysis to coat a material with a layer of metal– Copper strip – anode– coin – cathode– electrolyte – copper sulfate
Created by C. IppolitoMarch 2007
Updated March 2007
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. IppolitoMarch 2007
Updated March 2007
Zinc-Copper Voltaic Cell
Created by C. IppolitoMarch 2007
Updated March 2007
Zinc-Copper Voltaic Cell
OxidationOxidation
ZnoZn2+ + 2e- ReductionReduction
Cu2+ + 2e- Cuo
External CircuitExternal Circuit
Internal CircuitInternal Circuit
Created by C. IppolitoMarch 2007
Updated March 2007
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. IppolitoMarch 2007
Updated March 2007
Dry Cells
• Voltaic cell with “paste” electrolyte
Created by C. IppolitoMarch 2007
Updated March 2007
Lead Storage Battery
Created by C. IppolitoMarch 2007
Updated March 2007
Electric Potential
• measures cell’s ability to produce current
• results from a competition for electrons– reduction potentialreduction potential – tendency of a given
half reaction to occur as reduction oxidation• reduction occurs in the cell with the greater
reduction potential
– cell potentialcell potential – difference between the reduction potentials
E E Ecell reduction oxida tion0 0 0
Created by C. IppolitoMarch 2007
Updated March 2007
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 -
EH 0
Created by C. IppolitoMarch 2007
Updated March 2007
Standard Reduction Potentials
• determined using standard hydrogen electrode and the equation for standard cell potentialE E Ecell reduction oxida tion
0 0 0
E E Ecell ZnH
0 0 02
0 7 6 0 0 0 20. .V V EZn
0 7 6 20. V EZn
Created by C. IppolitoMarch 2007
Updated March 2007
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 = -0.76VAg+ + e- Ag(s) E0 = +0.80V E0 = 0.80V-(-0.76V) =
+1.56V
E is positive reaction SPONTANEOUS
E is nega tive reaction NONSPONTANEOUScell
ce ll
0
0
E E Ecell reduction oxida tion0 0 0
Created by C. IppolitoMarch 2007
Updated March 2007
Corrosion• the deterioration and wearing away of
metals usually through “oxidation”• Prevention:
1. coat with paint to stop water and oxygen contact
2. electroplate with less reactive metal3. alloy with another metal (stainless steel –
Fe & Cr)4. protect metal by making it the “cathode”
Mg strips on ship hulls corrode instead of the hull