electrochemistry part ii: the galvanic cell
DESCRIPTION
Electrochemistry Part II: The Galvanic Cell. Jespersen Chap. 20 Sec 1. Dr. C. Yau Spring 2013. 1. 1. What is a Galvanic Cell?. A galvanic cell is a spontaneous electrochemical cell in which electricity is produced by a spontaneous redox reaction. - PowerPoint PPT PresentationTRANSCRIPT
11
ElectrochemistryPart II: The Galvanic Cell
Dr. C. YauSpring 2013
Jespersen Chap. 20 Sec 1
1
2
What is a Galvanic Cell?• A galvanic cell is a spontaneous
electrochemical cell in which electricity is produced by a spontaneous redox reaction.
• The resulting electron transfer is forced to take place through a wire.
• It is also known as a voltaic cell.• Do not confuse it with the electrolytic cell
(discussed later) where an electrochemical reaction is forced to take place by passing electricity through the cell.
3
Comparison of Types of Cell
Galvanic or Voltaic Cell:Spontaneous electrochemical rxnElectricity produced
Electrolytic Cell:Non-spontaneous electrochemical rxnElectric current is passed thru a wire to force the reaction to take place.
44
Anatomy Of A Galvanic Cell
• Half-cells (compartments containing reactants for each half- reaction)
• Electrodes to conduct current through the solution.• Salt bridge to allow ion movement to keep solns neutral.• Supporting electrolyte (spectator ions: NO3
-)• Connecting external circuit (wire and voltmeter)
Zn Zn2+ + 2 e− Cu2+ (aq) + 2e− Cu (s)
Note: Textbk is inconsistent with which is on the left (cathode or anode).There is no convention which is on the left side in a sketch of this sort.
e-e-
anodecathode
55
Electrochemical Cells• In all cells, electrons transfer between the
cathode (the reduction half-cell) and the anode (the oxidation half-cell)
REMEMBER! "Red-Cat and An-Ox”
Reduction at the Cathode& Oxidation at the Anode
66
A closer look at the electrodes:
Zn2+
Zn2+
Zn2+
Zn2+
Zn2+
Zn anode
Cu2+
Cu2+
Cu2+
Cu2+
Cu2+
Cu2+
Cu2+
Cu cathode
Oxidation of Zn to Zn2+
Leaves e- behind on the electrode (soln becomesmore positive)
Reduction of Cu2+ to Cue- extracted from electrode (soln becomesmore negative)
Zn Cu
77
Electrochemical Cells• Electrical current is conducted via the
movement of electrons and ions.• To prevent charge buildup, a salt bridge
allows ions to move between the cells.REMEMBER! • Electrons flow from anode to cathode
through the wire. a to c• Cations move towards the cathode.• Anions move towards the anode.• Red-Cat and An-Ox.
88
There is a buildup of what charges at each cell?
The salt bridge often made of KCl or KNO3
(unreactive ions – spectator ions)
What ions in the salt bridge move to which cell?
Zn2+
Zn Cu
KCl or KNO3
99
Towards which compartment will electrons flow in an electrochemical cell?
A. Toward the cathode B. Toward the anode C. It depends on the reaction
Through which components of the cell will ions not flow?
A. The electrodes B. The solution C. The salt bridgehttp://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/galvan5.swf
anode to cathode
1010
Standard Cell Notation (Line Cell Notation)
• Cell reactions separated by || that represents the salt bridge with ANODE on left, CATHODE on right.
• Electrodes appear at the outsides• Reaction electrolytes in inner section• Phases (phys. States) separated with |• Species in the same state separated with ;• Concentrations shown in ( )
Zn (s) | Zn2+ (aq) || Cu2+ (aq) | Cu (s)
salt bridgeanode cathode
anode electrode
cathode electrode
anodeelectrolyte
cathodeelectrolyte
1111
Standard Cell Notation (Line Cell Notation)
Write the half reactions for the galvanic cell shown above.
Cu (s) Cu2+ (aq) + 2e-
Ag+ (aq) + e- Ag (s)Make a sketch of the galvanic cell and label
it fully.
1212
Now, consider the reaction of Al3+
(aq) + Zn (s) Al(s) + Zn2+(aq)
Write the half-reactions.Balance the electrons and write the balanced
net ionic equation.Sketch the galvanic cell (electrochemical
cell). Label it fully.Write the standard cell notation.
Do Prac Exer 1 & 2 on p. 924
13
Given: Mg(s) | Mg2+(aq) || Sn2+(aq) | Sn(s)
Sketch the galvanic cell corresponding to this standard cell notation. Label it fully.
Practice with p. 969 #20.50, 20.5113
1414
Now, consider the reaction of Fe3+ + Zn Fe2+ + Zn2+
Write the half-reactions.Balance the electrons and write the
balanced net ionic equation.Write the standard cell notation.Sketch the galvanic cell (electrochemical
cell). Label it fully.How can you have an electrode that is an
ion (such as Fe3+)?
15
Where there are no conductive metals involved in a process, an inert electrode is used. C(gr) and Pt are often used.
2Fe3+ + Zn 2Fe2+ + Zn2+
Zn (s) Zn2+(aq) Fe3+
(aq) Fe2+(aq)
Zn(s) |Zn2+(aq) || Fe3+
(aq); Fe2+(aq)|Pt(s)
Zn inert anode cathode (where Fe3+ reduces to Fe2+ at the surface of the Pt electrode)
1616
Balance and identify the cathode and anodeH2O2(aq) + CO2(g) → H2C2O4(aq) + O2(g) (acidic)
H2O2(aq) + 2CO2(g) → H2C2O4(aq) + O2(g) (acidic)
H2O2(aq) → O2(g) +2H+ + 2e-
2H+ + 2e- + 2CO2(g) → H2C2O4(aq) + (acidic)oxid
reduc
Which is at the cathode? At the anode?
1717
Write Line Notation for the cell:H2O2(aq) + CO2(g) → H2C2O4(aq) + O2(g) (acidic)
Standard Cell notation for the reaction:
C(gr)| H2O2(aq) ;H+|O2(g)||CO2(g)|H2C2O4(aq); H+|C(gr)
oxidationreduction
1818
Balance and identify the cathode and anodeCrO3(s) + MnO2(s)→MnO4
-(aq) + Cr3+
(aq) (basic)
CrO3(s) + MnO2(s) + H2O(l) →MnO4-(aq) + Cr3+
(aq) + 2OH-(aq)
CrO3(s) + 3H2O(l) + 3e- → Cr3+(aq) +6OH-(aq)
MnO2(s) + 4OH- → MnO4-(aq) + 2H2O + 3e-
Balancing redox equations by half-reaction method is given in Sec 6.2 (p. 222)
19
Galvanic Cells without Metal Electrodes
Equation from previous slide:CrO3(s)+ MnO2(s) + H2O(l) →MnO4
-(aq)+Cr3+
(aq) + 2OH-(aq)
Write the Standard Cell Notation:
C(gr);MnO2(s)|MnO4-(aq)||CrO3(s)|Cr3+
(aq);OH-|C(gr)
p. 970 #20.52