Docstoc

Electrochemistry

Document Sample
Electrochemistry Powered By Docstoc
					    Electrochemistry
  Chapter 18-Masterton

The study of the interchange of
chemical and electrical energy.
Review of Terms-REDOX

oxidation-reduction (redox) reaction:
involves a transfer of electrons from the
reducing agent to the oxidizing agent.
Redox Reaction-video
How “beaters” rust-video
Review of Terms-OXIDATION

LEO: oxidation: loss of electrons

  - OXIDATION occurs at the
    ANODE of electrochemical cell
  -Anions/negative ion move TO the anode
 Review of Terms-REDUCTION

GER: reduction: gain of electrons

  - REDUCTION occurs at the
    CATHODE of electrochemical cell.
  -Cations/positive ions move TO the
    Cathode
ANODE and CATHODE-video
              Half-Reactions
The overall reaction is split into two half-reactions,
one involving oxidation and one reduction.

 8H+ + MnO4 + 5Fe2+  Mn2+ + 5Fe3+ + 4H2O

Reduction: 8H+ + MnO4 + 5e  Mn2+ + 4H2O

Oxidation: 5Fe2+  5Fe3+ + 5e
Half-Reactions-video
           Electrolysis

. . . forcing a current through a cell to
      produce a chemical change for which
      the cell potential is negative.

… use chemical energy to make non-
   spontaneous reaction take place
Electrolysis-video
         Electrolysis of Water
Overall Reaction:
    2H2O (l)            2H2 (g) + O2 (g)

Oxidation/Anode half-reaction:
    2H2O (l)          O2 (g) + 4H+ + 4e -
Reduction/Cathode half-reaction:
    2H2O (l) + 4e -       2H2 (g) + 4OH -
     Galvanic/Voltaic Cell

-A device in which chemical energy is
changed to electrical energy.

-A spontaneous chemical reaction
occurs to generate electrical energy.
Voltaic cell-Zn/Zn2+||Cu2+/Cu
             video
      Cell Potential

Cell Potential or Electromotive
Force (emf): The “pull” or driving
force on the electrons.
   Standard Reduction Potentials-
       Table 18.1 Page 481

The E values corresponding to reduction
half-reactions with all solutes at 1M and all
gases at 1 atm.

Cu2+ + 2e  Cu           E = 0.34 V vs. SHE

SO42 + 4H+ + 2e  H2SO3 + H2O
                      E = 0.20 V vs. SHE
    Voltaic cell-Zn/Zn2+||Cu2+/Cu
                         (anode1st||cathode 2nd)
             Calculation of E
ETotal = Ered + Eox
Reduction: Cu2+ + 2e  Cu        E red = +0.339 V
Oxidation: Zn  Zn2+ + 2e       E ox = +0.762 V


ETotal= +0.339 V + +0.762 V = +1.101 volts
      Strength of Oxidizing and
          Reducing Agents
The more positive Ered , the stronger the
   oxidizing agent.



The more positive Eox , the stronger the
   reducing agent.
            SPONTANEITY
          of Redox Reactions
If ETotal= +, then the reaction is spontaneous.

ETotal= -,then the reaction is non-spontaneous.

Therefore, cell-Zn/Zn2+||Cu2+/Cu, with

   ETotal= +1.101 volts is spontaneous.
          Batteries

A battery is a galvanic cell or,
more commonly, a group of
galvanic cells connected in series.
Alkaline Battery-video
Car Battery-video
                Fuel Cells
. . . galvanic cells for which the reactants are
      continuously supplied.

          2H2(g) + O2(g)  2H2O(l)

anode: 2H2 + 4OH  4H2O + 4e

cathode: 4e + O2 + 2H2O  4OH
               Corrosion
Some metals, such as
copper, gold, silver
and platinum, are
relatively difficult to
oxidize. These are
often called noble
metals.
    Stoichiometry of Electrolysis
-    How much chemical change occurs with
     the flow of a given current for a specified
     time?

     current and time  quantity of charge 
     moles of electrons  moles of analyte 
     grams of analyte
            emf and Work


                                  work ( J )
emf  potential difference (V) 
                                 ch arg e (C)
      Free Energy and Cell
            Potential

             G = nFE
n = number of moles of electrons
F = Faraday = 96,485 coulombs per mole of
   electrons
  Calculation of Equilibrium
Constants for Redox Reactions

At equilibrium, Ecell = 0 and Q = K.

                     nE
         log( K )         at 25 C
                    0.0592
     Concentration Cell

. . . a cell in which both compartments
      have the same components but at
      different concentrations.
     The Nernst Equation

We can calculate the potential of a cell in
which some or all of the components are not
in their standard states.

                 0.0592
        E  E         logQ
                    n

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:26
posted:1/17/2012
language:English
pages:34