CAcT HomePage

Electrolysis

Skills to develop

Electrolysis

Recently, some laptop computers come with rechargeable lithium ion batteries, (LIBs). Electrolysis played an important part in the research and development of these batteries. How good are they? Are they worth the money? How do they work? What are the technology behind them? What is electrolysis? What do I need to know in order to understand the jargon and technology on the website about electrochemistry of rechargable LIBs )?

This picture is from Asahi's website. It shows the Asahi Chemical's ion exchange process for salt electrolysis.

The following discussion provides an introductory guide regarding electrolysis, which is also used to produce and refine metals for industries, making jewelries, protecting metal from corrosions and more. We deal with the fundamental science of electrolysis here.

Electrolysis

Chemical reactions in batteries or galvanic cells provide the driving force for electrons to struggle through loads. This is how chemical energy is transformed into electric energy.

Electrolysis can be carried out in solutions or molten salts (liquid). Because the atoms and ions have to move physically, the medium has to be a fluid. The products, like the reactants in a galvanic cell, can be in a solid, liquid, or gas state.

Electrolysis of Molten Salts

Electrolysis is a process by which electrons are forced through a chemical cell, thus causing a chemical reaction. The positive charge usually attracts electrons, and the electrode providing electrons is called cathode, because reduction takes place on it.

Reduction always takes place at the cathode, by definition. In the electrolysis of molten salt, NaCl, the cathode and anode reactions are:

Cathode (reduction): Na+ + e- = Na
  Anode (oxidation): 2 Cl- = Cl2 + 2 e-

or
Anode oxidation DE Cathod reduction
2 Cl- = Cl2 + 2 e- e- pump 2 Na+ + 2 e- = 2 Na
2 Cl- + 2 Na+ = Cl2 + 2 Na
If one mole of electrons (96485 C or 1 Faraday) passes from the anode to the cathode, one mole of Na (23 g) will be deposited, and half a mole of chlorine gas Cl2 (or one mole of Cl atoms) will be collected from the reaction at the anode.

In the above setup, if the current is 1.0 A, the rate at which Na deposits at the cathode will be 1.0E-5 (= 1/96485) mol (or 0.24 mg) per second. Of course, it takes 96485 seconds (or 26.8 hrs) to deposit one mole (23 g) of sodium metal.

Electrolysis of Water

Pure water does not conduct electricity, because the numbers of H+ and OH- ions are small (10-7 mol/L each). In the presence of an acid, water can be decomposed.

A potential of -2.06 V is the standard cell potential for,

Pt | H2O, [H+] = 1 M | O2 || H2O [OH-] = 1 M | H2 | Pt And when a potential greater than 2.06 V is applied such that the forward cell has a positive potential, the following reactions take place.
Anode oxidation DE Cathod reduction
H2O = 4 H+ + 4 e + O2 >2.06 V 4 H2O + 4 e = 2 H2 + 4 OH-
2 H2O = 2 H2 + O2
Note that the acid and base must be separated by a salt bridge to prevent the neutralization reaction. This cell potential is different from the cell potential for the reverse reaction in pure water, for which [H+] = [OH-] = 10-7, and E° is 1.23 V.

The Hall process

Aluminum (Al) is the third the most abundant elements on Earth crust, in the form of bauxite or alumina Al2O3. Because it is very reactive, this metal remained unknown to mankind until 1827. By then, Wohler obtained some Al metal by reducing Al2O3 with potassium vapore.

In 1886, two young men working in two continent apart electrolyzed molten cryolite Na3AlF6 (melting point 1000° C). Alminium was not produced when pure cryolite was used. Electrolysis is successful only if the ions move to the electrodes, and the reactions take place. That was not the case for molten cryolite.

However, both Hall and Heroult tried to mix about 5% alumina in their molten cryolite, and their discovery is now known as the Hall-Heroult process, which is a commercial process. In a modern process, the reactions are:

AlF63- + 3 e = Al + 6 F- . . . Cathode
2 Al2OF62- + C(s) + 12 F- = + 4 AlF63- + CO2 + 4 e . . . Anode

2 Al2O3 + 3 C = 4 Al + 3 CO2 . . . Overall cell reaction
The overall reaction is simple despite the complicated mechanism in the electrolysis. In the example 1 below, we use simple formulation for the cathode and anode reaction to illustrate the shoichiometry of electrolysis.

Electrochemical Stoichiometry

Electrolysis causes chemical reactions. Amounts of reactants, products, energy, and charge are inter-related. The following examples illustrate the stoichiometry of electrolysis.

  1. The Hall process can be oversimplified by these reactions, Al3+ + 3 e = Al . . . Cathode
    C(s) + 2 O2- = CO2 + 4 e . . . Anode
    How many Faradays and how many coulombs must be passed through a molten mixture of Al2O3 and Na3AlF6 to produce 1 Kg of Al metal?
    The reactions are unrealistic because the ions containing Al are not bare Al3+ ions. However, we use the simplified reaction for stoichiometry relationships only.

    Solution

    Study the following conversion method to get from 1 Kg of Al to number of Faradays and coulombs. Note that values in the numerators are equivalent to those in the denominators in the factors.

              1000 g   1 mol   3 F(araday)
     1 kg Al -------  -------  --------
              1 Kg    26.98 g  1 mol Al
    
                  96485 C
       =  111 M  --------
                   1 M
    
       =  1.1e7 C.
    
    Producing Al is an expensive process.

  2. Thirty minutes (30 m) of electrolysis of a solution of CuSO4 produced 3.175 g Cu at the cathode. How many Faradays and how many Coulombs passed through the cell? What is the current?

    Solution

    Using the same method as indicated above, you have

                 1 mol    2 F
     3.175 g Cu ------- --------   (At.wt. Cu = 63.5)
                63.5 g  1 mol Cu
    
                   96485 C
       =  0.100 M ---------
                     1 M
    
       =  9650 C
    
    To calculate the current, you divide the charge (C) by the time period (sec).
     I = 9650 C/(30*60 sec.)
       = 5.36 A.
    
  3. An electrolysis cell with Fe(NO3)3 solution is operated for 2.0 hrs at a constant current of 0.10 A, how much Fe metal is plated out if the efficiency is 90%? (At.wt. Fe=55.8)

    Solution

    The charge passed the cell is

     0.10 (C/sec)*2*3600 sec = 720 C.
    
              1 M    1 mol Fe  55.8 g Fe
     720 C  -------  --------  ---------  0.90
            96485 C   3 M      1 mol Fe
    
      =  0.12 g Fe.
    
    Note that the last factor corresponds to 90% efficiency.

  4. An electrolysis cell contains MSO4 solution is operated for 1.0 hr at constant current of 0.200 A. If the current efficiency is 95%, and 0.399 g of M plates out, what is the atomic weight of the element M?

    Solution

    Taking the current efficiency of 95% into consideration, the effective charge passed through the cell is

     0.200 (C/sec) * 3600 sec * 0.95 = 684 C
    

    1 M 1 mol M 684 C ------- -------- 96485 C 2 M = 3.54E-3 mol M

    The atomic weight of M is thus,
     0.399 g
     -------- = 112.7 g/mol
     3.54E-3
    
    Checking the results against a table indicates that the element is cadmium Cd.

Electroplating

Electroplating different from electrolysis in that the metal deposited from electrolysis plates out on the surface of another metal. The electrolyte contains the plating metal in the form of dissolved ions and the annode usually is made of the plating metal. The object to be plated is the cathode.

An Industrial Chemistry on electroplating considers a more complete syllabus.

As I search for recent information on electroplating, I came accross a websit for Jobs - Plating Chemist / Technician.

Electroplating technology is frequently use in metal finishing, metallic coating and finishing, and salt water pool chlorination.

If you are interested in electroplating companies, here are some websites:

The Samson Technology Corporation sells portable electroplating system. The following paragraph is from Samson Samson's portable electroplating systems are built for continuous heavy duty use, light weight and easy to operate. Auto emblems can be plated without removing them from the car. Faucets can be plated on the sink. 24K gold, sterling silver, nickel and copper can be brush plated with ease using Samson's electroplating systems. Samson offers a complete line of chemicals and supplies for electroplating, buffing and polishing.

Confidence Building Questions

©cchieh@uwaterloo.ca