1. Prepared by Odyssa Natividad RM. Molo
Electrochemistry
Prepared by
Odyssa Natividad RM. Molo

2. Electrochemistry
Branch of chemistry that deals with electricity and its relation to chemical reaction
Electrochemical processes
Redox rxns in which the energy released by a spontaneous rxn is converted to electricity or in which electricity is used to drive a nonspontaneous chemical rxn

3. Redox Reaction Oxidation-reduction reaction
Chemical reactions in which the oxidation state of one or more substances changes
Oxidation state/number
a (+) or (-) whole # assigned to an element in a molecule or ion on the basis of a set of formal rules; to some degree it reflects the (+) or (-) character of that atom

4. A Spontaneous Redox Rxn

5. Redox reaction Oxidation Reduction Oxidizing agent/oxidant
Involves loss of electron
Reduction
Involves gain of electrons
Oxidizing agent/oxidant
Substance responsible for another substance to be oxidized; one that undergoes reduction
Reducing agent/reductant
Gives up electrons/undergoes oxidation; caused another substance to be reduced

6. How to determine if redox or not?
Keep tract of the oxidation number of all the elements involved in the reaction.
Activity: Rules on assigning oxidation #s (p44 LM)
Ex:
Zn(s) + 2 H+(aq)  Zn2+(aq) + H2(g)
In any redox rxn, both oxidation & reduction must occur. If one substance is oxidized, another must be reduced.

7. Balancing Redox Reaction
The law of conservation of mass must be observed.
+ the gains & losses of electrons must be balanced( Electrons are neither created nor destroyed in any chemical rxn)
Two methods:
Oxidation number method (LM p 44)
Ion-electron method (LM p48)

8. Electrochemical Cell

9. Electrochemical cell
Apparatus which converts chemical energy from spontaneous reaction to produce electricity
Also called voltaic cell or galvanic cell
Transfer of electron takes place through an external pathway rather than directly between reactants
Based on the principle formulated by Alessandro Volta (Italian physicist) & Luigi Galvani (Italian physiologist)

10. Voltaic cell diagram

11. Components of Electrochemical Cell
Electrodes
simply where redox occurs (anode: oxidation; cathode: reduction)
Types: inert, membrane & metallic
Charge carriers
Metal wire (usually wire with alligator clips)
Where electrons pass through as current that be measured by a voltmeter
Salt bridge
Where electrons travel through its ion to transfer from the anode to cathode
Usually made up of U-tube with an electrolyte soln that will not react with other specie in the cell
Maintains the electrical neutrality of the cell by transferring its cations or anions

12. Types of Electrode Inert electrode Membrane electrode
Usually made of C(graphite) or Pt (inert & nonreactive to components that undergo redox)
Usually used when gases or liquids are formed as by-products of redox since the charge carrier cannot be connected to these
Membrane electrode
Specialized; also called ion-selective electrode
Detects electron transfer from or to a specific species
Metallic electrode
Composed of metal strip & its metallic ion soln
Not only act as sites of rxn but also participate in redox rxn
Also called active electrodes

13. Cell emf
Q: Why does electrons flow spontaneously in the way that they do?
A: There is a “driving force” that pushes the electrons through an external circuit in a voltaic cell
Comparison:
Electron Flow to Flow of Water in Waterfall

14. Electron Flow to Flow of Water in Waterfall
Water flows over the waterfalls because of its PE is lower at the bottom of the falls that at the top. Likewise, if there is an electrical connection between the anode & cathode of a voltaic cell, electrons flow from anode to cathode to lower their PE

15. Electromotive force, emf
Electromotive = “causing electron motion”
Potential difference between two electrodes of a voltaic cell which provides the driving force that pushes electron through the external circuit

16. Cell emf/cell potential
Standard electrochemical cell potential; aka cell voltage
Measured in volts
E°cell = E°red(cathode) - E°red(anode)
Standard condition: (with note sign)
1M conc for R & P in soln & 1 atm pressure for those that are gases at 25°C

17. Cell emf cont… Factors that cell emf depends on:
Specific rxns that occur at the cathode & anode
Conc of reactants & products
Temp (assumed to 25°C unless noted)
Standard Reduction Potential (SRP), E°red
Standard electrode potential for reduction half rxn
Standard Hydrogen Electrode, SHE
Reference half-rxn
2 H+(aq, 1M) + 2e-  H2(g, 1atm) E°red = 0V

18. SHE

19. SRP

20. Points to remember on SRP
SRPs are intensive properties or has intrinsic properties. Thus changing the stoichiometric coefficient in a half-rxn does not affect the value of SRP
Ex: Zn2+(aq) + 2e-  Zn(s) E°red = -0.76
2 Zn2+(aq) + 4e-  2 Zn(s) E°red = -0.76V
The more positive the value of E°red, the greater the tendency for the reactant to be reduced, undergo reduction, & oxidize another specie

21. Practice Exercise Exercise 18.3 page 479 (refer to SRP page 478)
Compute corresponding E°cell
Indicate which rxn occurs in the anode & cathode
Which electrode is consumed?
Which is the OA? RA?
Is the rxn spontaneous or not?
Write the cell diagram

22. Spontaneity of Redox Rxn
Determining std change in Gibb’s free energy of the rxn (G) makes it possible to determine the spontaneity of a rxn using this eqn:
G = -nFEcell
where n = # of e- transferred during rxn
F = Faraday’s constant, C/mol
G is (-) spontaneous; G is (+) nonspontaneous

23. Spontaneity cont…
Voltaic cells use redox rxns that proceed spontaneously. Any rxn that can occur in a voltaic cell to produce a (+) emf must be spontaneous.
General statement:
“A (+) value of E° indicates a spontaneous process & a (-) value indicates a nonspontaneous one”
E°= emf under std condition
E= emf under nonstandard condition

24. Cell Diagram
Conventional notation that shows the components of an electrochemical cell
Conventions: (Std rule: ABC)
Anode = left side
Cathode = right side
Boundary
between diff phase (ex electrode & soln) = single vertical line (│)
between half-cell compartment (salt bridge) = double vertical line (││)
between diff species within the same soln = comma (,)
Species in aqueous soln are placed on either side of the double vertical line

25. Practice Exercise
Example 18.1 page 482
Exercise 18.4 page 483

26. Effect of Concentration on Cell emf
As a voltaic cell is discharged, the Reactant/s of the rxn are consumed & the Product/s are generated so the conc of these substances change. The emf progressively drops until E=0, at which point, we say the cell is “dead”. At that point the conc of the R & P cease to change; they are at eqlbm.
The emf generated under nonstandard conditions can be calculated using an eqn first derived by Walther Nernst, a German chemist who established many of the theoretical foundations of electrochemistry.

27. Nernst Equation

28. Practice Exercise
Calculate the emf at 298K generated by the cell below:
Zn(s) + Cu2+(aq)  Zn2+(aq) + Cu(s) where [Cu2+] = 5.0M & [Zn2+] = 0.050M
2 Al(s) + 3 I2(s)  2Al3+(aq) + 6I-(aq) where [Al3+] = 4.0 x 10-3M & [I-]=0.010M
Zn(s) + Cd2+(aq)  Zn2+(aq) + Cd(s) where [Cd2+] = M & [Zn2+] = 0.950M

29. Concentration Cells
Cell based solely on the emf generated because of a difference in a concentration.
more dilute soln = anode; more conc = cathode.
Applications: pH meter; regulation of heartbeat in mammals.

30. Cell emf & Chemical Equilibrium
From Nernst eqn: As R are converted to P, the value of Q inc, so the value of E dec. The cell emf eventually reaches E = 0, where G = 0; system is at eqlbm; Q = K.
Rearranging Nernst eqn T = 298K):