CAcT HomePage

Electrochemistry - review and quiz

Chemical reactions involving transfer of electrons are called oxidation and reduction reactions or redox reactions. When properly set up, these reactions generate power in a Battery, or galvanic cell. A galvanic cell consists of at least two half cells, each of which consists of an electrode and a electrolyte solution.

A redox reaction can be divided into two Half Reactions, an oxidation reaction and a reduction half reaction.

Each half reaction can be set up as a Half Cell and putting two half cells together makes a simple battery. A galvanic cell can be considered as a battery, but batteries include packages of galvanic cells in series to supply higher voltages than a single galvanic cell.

Oxidation of a species (atom, ion, or molecule) is a loss of electron(s), and reduction of a species a gain of electron(s). Some conventions are used to define the Oxidation States, and oxidation of an atom causes a increase in its oxidation state. Conversely, reduction causes a decrease in its oxidation state.

Balance Redox Equations is a complicated task, but the use of half reactions gives a very good strategy for balancing redox reactions.

Energy is the driving force for chemical reactions. Energies of oxidation and reduction reactions are related to the electrochemical potentials (E), Cell EMF, of the galvanic cells. The standard reduction potentials for half cells are values that make sensible comparisons, because a set of conventions have been followed. The energy (called Gibb's free energy G) is the electric work of the reaction. Therefore,

DGo = n F DEo. The difference of Gibb's free energy (DGo) between products and reactants equals to the charge n F times the potential difference DEo. Since n electrons are transferred in the equation, n F (F is the Faraday constant of 96485 C) is the charge involved for the equation in terms of moles.

The equilibrium constant K is related to DGo,

DGo = - R T ln K. For the reaction, a A + b B = c C + d D

The Nernst equation is a natural result,

            R T    [C]c[D]d
DE = DEo - ---- ln ---------
            n F    [A]a[B]b

Notations Used

In the summary given above, we have used many energy and potential related notations. For completeness, the notations are given as follows: DG - difference in Gibb's free energy
DGo - difference in Gibb's free energy at standard condition
Eo - standard reduction cell potential
E* - standard oxidation cell potential
emf or EMF - Electromotive force
DEo - standard potential of a battery or galvanic cell
DE - Potential of a galvanic cell not necessary at standard conditions
F = 96485 C, Faraday constant
Q - charge, unit coulomb (C)
I = Q / t, current Ampere
R = 8.3145 J/mole.K, gas constant
T - temperature in K
ln(10) R T / F = 0.0592 at 298 K

Electrochemistry skills expected in Quiz

  1. Explain the meaning of terms used in electrochemistry and properly use the cell notations. For example, galvanic cells or voltaic cells are devices that use spontaneous chemical reactions to produce electric currents. Other terms are reduction potentials, oxidation potentials, oxidation state, redox reactions, cathode, anode, cell potentials, Nernst equations, etc.

    The definition for anode and cathode apply to both galvanic cells and electrolytic cells.
    Oxidation takes place on anodes, for example,

      reaction: Zn -> Zn2+ + 2 e     E* = 0.762
      notation: Zn | Zn2+            E* = - Eo
    
    Note that we have defined and used E* to represent the oxidation potential which has the same absolute value as the reduction potential Eo, but a different sign. This notation is not used in most text books.

    Reduction takes place on cathodes, for example,

      reaction: Hg22+ + 2 e -> 2 Hg   E0 = 0.796
      notation: Hg2+ | Hg
    
    The details regarding oxidation and reduction half reactions have been given on the page of Half-Cell reactions.

  2. Calculate DEo and DE for galvanic cells according to the conventions used for cell notations.

    The standard cell potential DEo is the sum of the reduction potential of the cathode and the oxidation potential of the anode,

    DEo = Eo + E*. The data for calculating DEo may be given using cell notations or in the forms of equation. If a cell notation is given, you must be able to search for proper reduction and oxidtation couples for the cell and then evaluate Eo's from a table of standard reduction potentials. Proper use of such tables is a basic skill, and some usages have been given in the modules Battery or Galvanic Cells, and Electromotive Force.,

  3. If the cells are not at standard conditions, you will be required to calculate the Gibb's free energy, DG, or the cell potential DE.

    The standard Gibb's free energy, DGo, is the negative of the maximum available electrical work. The electrical work We is the product of the charge and the potential of the cell.

    We = q E
    DG = - q E
    = - n F E
    where, q is the charge in C and E is the potential in V. Note also that q = n F, and F = 96485 C is the Faraday constant, whereas n is the number of moles of electrons in the reaction equation.

    The change of Gibbs free energy at standard condition is derived in the following way:

    DGo = - We
    = - q Eo
    = - n F Eo;
    This equation is for a galvanic cell under the standard conditions. The notations Eo and DGo are used for cells under standard conditions. When the cells are not at standard conditions, DE and DGo are used for these quantities.

    Note also that DGo is the theoretical amount of energy per reaction equation as written. The amount of energy for a given system depends on the quantities of the reactants and their conditions (concentration or pressure etc.).

  4. The Nernst equation allows us to evaluate the cell potentials when the condition of the cell is not at standard conditions. The details have been given on the page of Nernst equation.

  5. For the calculation of Gibb's energies and quantities generated in Electrolysis and electroplating, skills in solving stoichiometric problems are also required.

Confidence Building Questions

We have used mostly the Zn/Cu cell in these questions. A similar set of questions can be set up using any two couples of reduction potentials. Next page: Sulfur

Constructed by cchieh@uwaterloo.ca

©cchieh@uwaterloo.ca