Inorganic and Applied Chemistry

Download free books at BookBooN.com

Inorganic and Applied Chemistry

Example 6- M:

Electrolysis of water

We have seen earlier how hydrogen and oxygen may be combined and spontaneously form water

molecules and that this reaction produces energy which may be used in fuel cells. The opposite process

where hydrogen and oxygen are formed from water molecules is not spontaneous but requires an

electrolytic process. The following half cell reaction takes place at the anode:

2 H 2 O(l)  O 2 (g) + 4 H+(aq) + 4 e-

with corresponding half cell potential of^0 = 1.23 volt while that following half cell reaction takes place at

the cathode:

4 H 2 O(l) + 4 e-  2 H 2 (g) + 4 OH-(aq)

having a half cell potential of^0 = -0.83 volt. Hereby the total overall cell reaction becomes:

2 H 2 O(l)  2 H 2 (g) + O 2 (g)

and the total cell potential thereby^0 = -2.06 volt. These potential calculations requires of cause that there

is a concentration of H+ ions of 1 M in the half cell with the anode while there is a concentration of OH-

ions of 1 M in the half cell with the cathode. This is naturally not the case in pure water with pH = 7.0

which is why we expect a potential for the overall process of -1.23 volt.

6.8 Summing up on chapter 6

We have seen in this chapter of important parts of the electrochemistry. We started by looking at the basic

principles of oxidation- and reduction reactions and introduced the levels of oxidation and the rules for

giving levels of oxidation. This was used to balance redox-reactions.

Further we looked at galvanic cells where it was possible to extract electrical energy from chemical reactions.

We looked into cell potentials and standard reduction potentials which are both central and necessary for the

electrochemical calculations. We also looked at concentration dependence of cell potentials and introduced

the Nernst-equation stating the combination of the reaction fraction and cell potentials. The use of the Nernst

equation was presented through examples where er also saw how the equation may be used to determine

equilibrium constants.

In order to highlight the application of the chemistry in practice we went through three types of batteries

being the lead battery, the dry cell battery and the fuel cell. We further looked at corrosion and saw through

examples on how steel may be protected from corrosion in terms of electrochemical treatment with a

scarifying other metal. Lastly we looked at electrolysis of metal ion solutions and the electrochemical

fractioning of water molecules.

Electrochemistry