Transcript Utilizes relationship between chemical potential energy & electrical

Electrochemical cells: utilize relationship
between chemical potential energy &
electrical energy
Everyday Redox Reactions
•
•
•
•
battery to start car
prevent corrosion of metals
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!
all are
redox rxns
Predicting Redox Reactions
• Table J: used to predict if given redox
reaction will occur
• single metal donates electrons to ions
of metals below itself (oxidaizes)
• single non-metal takes electrons from
ion of non-metals below itself (reduces)
Predicting Redox Reactions
A + BX  B + AX
If metal A above metal B (Table J):
rxn is spontaneous
If metal A below metal B:
rxn is NOT spontaneous
X + AY  Y + AX
If non-metal X above non-metal Y (Table J):
rxn is spontaneous
If non-metal X below non-metal Y:
rxn is NOT 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
placed Cu(s) in
beakers:
one contains Zn(NO3)2(aq)
other contains AgNO3(aq)
Which beaker
contains which ions?
contains Zn ions?
A
&
contains Ag ions?
B
Overview of Electrochemistry
TWO kinds of
Electrochemical Cells
1.
galvanic or Voltaic cells:
(Regents: electrochemical cells)
– use spontaneous rxn to produce flow of
electrons (electricity)
– exothermic
2. electrolytic cells:
– 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 3 as electrochemical cells
- not exactly accurate
galvanic cells/Voltaic cells
(NYS: electrochemical cells)
• use spontaneous SR redox rxn:
produce flow of electrons
• electrons flow from oxidized substance to
reduced substance
Cell Set-up
• components arranged so e- forced to flow
through wire
• when e- travel through a wire, can make them
do work: light a bulb, ring a buzzer
• oxidation & reduction reactions must be
separated physically
Parts of a galvanic/Voltaic Cell
2 half-cells:
- one for oxidation rxn
- one for reduction rxn
each ½ cell consists of:
container with aq soln:
• +/- ions
• electrode
– surface where e- transfer takes place
– wire connects 2 electrodes
– salt bridge connects 2 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: location for oxidation half-rxn
• Reduction – Cathode
–cathode: 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 e- Flow (through wire):
Anode → Cathode
Direction of (+) Ion Flow (salt bridge):
Anode → Cathode
negative electrode (anode):
e- originate here: Zn electrode in this picture
• Zn electrode decreases in mass
• Zn+2 ions increase in concen
aq solns contain ions of same element as electrode

positive electrode (cathode):
e- attracted here: Cu electrode in this picture
• 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 Battery
Mercury Battery
Corrosion
to resist corrosion: coat one metal with stronger more durable metal
What’s wrong with this picture?