Download Utilizes relationship between chemical potential energy & electrical

Electrochemical cells: utilize relationship
between chemical potential energy &
electrical energy
Redox Reactions
•
•
•
•
battery to start car
prevent corrosion
cleaning with bleach (oxidizing agent)
Na, Al, Cl prepared or purified by redox
reactions
• breathing
• O2  H2O and CO2
Redox Reactions
• synthesis rxns
• decomposition rxns
• SR rxns
• DR rxns are NOT redox rxns!
redox rxns
Predicting Redox Reactions
• Table J: used to predict if given redox
reaction will occur
• metals donate electrons to ion of metals
below itself
• non-metal takes electrons from ion of
non-metal below itself
Predicting SR Redox Reactions
Element + Compound 
New Element + New Compound
If element above swapable ion, reaction is
spontaneous
If element below swapable ion, reaction is
NOT spontaneous
Predicting Redox Reactions
A + BX  B + AX
If metal A above metal B (Table J):
reaction is spontaneous
X + AY  Y + AX
If non-metal X above non-metal Y (Table J):
reaction is spontaneous
Spontaneous or not?
• Li + AlCl3  Yes
• Cs + CuCl2  Yes
• I2 + NaCl  No
• Cl2 + KBr  Yes
• Fe + CaBr2  No
• Mg + Sr(NO3)2  No
• F2 + MgCl2  Yes
Started with:
• Zn(NO3)2 & Cu
• AgNO3 & Cu
Which beaker
had Zn ions &
which had Ag
ions?
A
Overview of Electrochemistry
TWO kinds of cells:
1.galvanic or Voltaic (NYS – electrochemical)
– use spontaneous rxn to produce flow of
electrons (electricity)
– Exothermic
2.electrolytic
– use flow of electrons (electricity) to force
non-spontaneous rxn to occur
– endothermic
Vocabulary
• galvanic cell
• Voltaic cell
• electrochemical cell
Board of Regents considers
all be the same type of cell
Electrochemical Cells
• use spontaneous SR redox rxn:
– produces flow of electrons
• electrons flow from oxidized substance to
reduced substance
• names: galvanic cells, Voltaic cells, or
electrochemical cells (NYS)
Electrochemical Cells
• Redox rxn arranged so electrons forced to flow
through wire
• when electrons travel through a wire, can make
them do work - light a bulb, ring a buzzer
• oxidation & reduction reactions must be
separated physically
Half-Cell
• place where each half-reaction takes place
– ½ cells:
• 2 needed for complete redox rxn
• connected by wire so electrons flow from 1st to 2nd cell
• connected by salt bridge to maintain electrical neutrality
Schematic of Galvanic/Voltaic Cell
Parts of a Galvanic/Voltaic Cell
• 2 half-cells:
– one for oxidation rxn
– one for reduction rxn
– each consists of:
• container with aqueous solution &
• electrode
– surface where electron transfer takes place
– wire connects electrodes
– salt bridge connects solutions
How much work can you get out of
this reaction?
• can measure voltage by allowing
electrons to travel through voltmeter
• galvanic cell is a battery
– not easy battery to transport or use in real-life
applications
Electrode: surfaces at which oxidation or
reduction half-reaction occur
anode: oxidation
surface decreases in mass
cathode: reduction
surface increases in mass
An Ox ate a Red Cat
• Anode – Oxidation
–anode is location for oxidation half-rxn
• Reduction – Cathode
–cathode is location for reduction half-rxn
Anode / Cathode
• how know which electrode is which?
• Table J: use to predict which
electrode is anode and which
electrode is cathode
Anode
• Anode = Oxidation = Electron Donor
–anode composed of metal higher on
Table J
Cathode
• Cathode = Reduction = Electron Acceptor
– cathode composed of metal lower on
Table J
Zn above Cu: Zn is anode; Cu is cathode
cathode
Direction of Electron Flow
(through wire):
Anode → Cathode
Direction of Positive Ion Flow
(salt bridge):
Anode → Cathode
Negative electrode (anode):
where electrons originate: here it’s Zn electrode
•Zn electrode decreases in mass
•Zn+2 ions increase in concen
aq solns contain ions of same element as electrode
Positive electrode (cathode):
where electrons attracted: here it’s Cu electrode
•Cu electrode increases in mass
•Cu+2 ions decrease in concen
aq solns contain ions of same element as electrode
Salt Bridge
• allows migration of ions between half-cells
– necessary to maintain electrical neutrality
• reaction can not proceed without salt bridge
Half-Reactions
ox:
Zn  Zn+2 + 2e-
red: _________________________
Cu+2 + 2e-  Cu
Zn + Cu+2  Zn+2 + Cu
Which electrode is dissolving? Zn
Which electrode is gaining mass? Cu
Which species is increasing its concen? Zn+2
Which species is getting more dilute? Cu+2
When the reaction reaches
equilibrium
• voltage is 0!
–electrons no longer flow
Construct Galvanic Cell with Al & Pb
• Use Table J to identify anode & cathode
• Draw Cell:
– put in electrodes & solutions
• Label:
– anode, cathode, positive electrode, negative
electrode, direction of electron flow in wire, direction
of positive ion flow in salt bridge
[remember:
negative electrode: where electrons originate
positive electrode: attracts electrons]
Electron flow 
wire
Al:
anode
(-)
Positive ion flow 
Salt bridge
Pb:
cathode

Al+3 & NO3-1
Pb+2 & NO3-1
Oxidation:
Al  Al+3 + 3e-
Reduction:
Pb+2 + 2e-  Pb
Overall Rxn
+3 + 3e-)
(Al

Al
2
+ 3 (Pb+2 + 2e-  Pb)
_____________________________
2Al + 3Pb+2 + 6e- 2Al+3 + 3Pb + 6e2Al + 3Pb+2  2Al+3 + 3Pb
Application: Batteries
Dry Cell
Mercury battery
Corrosion
Corrosion Prevention
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