Chapter18

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Transcript Chapter18

Chapter 18
REDOX AND ELECTROCHEMISTRY
SUROVIEC
SPRING 2015
I. Electron Transfer Reaction
 Study of interconversion
of electrical and chemical
energy
 2 types of cells
I. Electron transfer reactions
Cu°(s) + 2Ag+(aq)  Cu2+(aq) + 2Ag°(s)
A. Balancing in Acidic solution
 Balance the following in acid:
Al(s) + Cu2+(aq)  Al3+(aq) + Cu(s)
B. Balancing in Base
 Balance the following in base
I-(aq) + MnO4-(aq)  I2(aq) + MnO2(s)
II. Voltaic Cells
Lets take: Cu°(s) + 2Ag+(aq)  Cu2+(aq) + 2Ag°(s)
and make a voltaic cell around it
B. Other Salt Bridge Cells
III. Standard Voltages
 The driving force behind a spontaneous reaction in a
voltaic cell is measure by cell voltage.
A. E°RED + E°ox
 Any redox reaction can be split into 2 ½ reactions
each with its own E° and therefore its own
A. E°cathode and E°anode
 Notice on the table only E°RED is listed
B. Strength of oxidizing and reducing agents
 Oxidizing agents =
species that can electrons
 Reducing agents =
species that supply
electrons
C. Calculations of E° and spontaneity
 To determine if a reaction is spontaneous look at the
sign of E°
IV. E° , DG° and Keq
A. E° , DG°
It can be shown that DG° = -nFE°
B. E° and Keq
 Redox equations also have a state of equilibrium
which means that
DG° = -RT lnKeq
Example
 For this reaction, determine DG° and Keq at 25°C
3Fe2+(aq) + NO3-(aq) +4H+(aq)  NO(g) + 2H2O(l) + 3Fe3+(aq)
V. Concentration and Voltage
 What if we are not at standard state?
 When the concentrations change the voltage must
also change
Example
 Given this reaction where all the concentrations are
2.5x10-2 M, calculate Q, n, E0 and E
Zno(s) + 2H2O(l) + 2OH-(aq)  [Zn(OH)4]2-(aq) + H2(g)
VI. Electrolytic Cells
 Non-spontaneous reactions made to occur by
pushing electrical energy through the system
Example
 Chromium can be electroplated onto other surfaces:
Cr2O72- (aq) + 14H+ (aq) + 12e-2Cr (s) + 7H2O (l)
Using a current of 8.00A and 3.5V, how many grams of
chromium can be plated is the experiment is run for a
hour?
Example
 A baby spoon with an area of 6.25cm2 is plated with
silver from AgCl (s) using 2.00A for 2 and ¼ hours.
If the current efficiency is 82% how many grams of
Ag is plated?
AgCl (s) + e-  Ag (s) + Cl- (aq)