Electrochemistry
A comparison of the E°s would lead us to predict that the reduction (ii) would be favored over that of (i). This is certainly the case from a purely energetic standpoint, but as was mentioned in the section on fuel cells, electrode reactions involving O2 are notoriously slow (that is, they are kinetically hindered), so the anodic process here is under kinetic rather than thermodynamic control. The reduction of water (iv) is energetically favored over that of Na+ (iii), so the net result of the electrolysis of brine is the production of Cl2 and NaOH ("caustic"), both of which are of immense industrial importance:
Since chlorine reacts with both OH– and H2, it is necessary to physically separate the anode and cathode compartments. In modern plants this is accomplished by means of an ion-selective polymer membrane, but prior to 1970 a more complicated cell was used that employed a pool of mercury as the cathode. A small amount of this mercury would normally find its way into the plant's waste stream, and this has resulted in serious pollution of many major river systems and estuaries and devastation of their fisheries.
- Electrolytic Refining Of Aluminum
Aluminum is present in most rocks and is the most abundant metallic element in the earth's crust (eight percent by weight.) However, its isolation is very difficult and expensive to accomplish...
- Reference Electrodes
In most electrochemical experiments our interest is concentrated on only one of the electrode reactions. Since all measurements must be on a complete cell involving two electrode systems, it is common practice to employ a reference...
- Electrochemistry
Learning Objectives(a) describe and explain redox processes in terms of electron transfer and/or of changes in oxidation number (oxidation state) (b) define the terms: (i) standard electrode (redox) potential (ii)...
- How To Balance Ionic Equations In Alkaline Medium?
(I) Standard Electrode Potential What information can we obtain from SEP? Sign – oxidation or reduction favoured Magnitude – the extent the reaction is favoured How to measure SEP? - connecting the redox couple (half cell) to SHE. - learn to draw...
- Electrochemistry : 10.1 Galvanic Cell (continued)
SPONTANEOUS REACTION occurs as the result of different ability of metal to give up their electron to flow through the circuit. CELL POTENTIAL (ECell) different in electrical potential of electrodes also called voltage or electromotive...
Electrochemistry
Industrial electrolytic processes.
The chloralkali industry
The electrolysis of brine is carried out on a huge scale for the industrial production of chlorine and caustic soda (sodium hydroxide). Because the reduction potential of Na+ is much higher than that of water, the latter substance undergoes decomposition at the cathode, yielding hydrogen gas and OH–.| anode reactions | 2 Cl– → Cl2(g) + 2 e– | -1.36 v | i |
| 4 OH– → O2(g) + 2 H2O + 4 e– | -0.40 v | ii | |
| cathode reactions | Na+ + e– → Na(s) | -2.7 v | iii |
| H2O + 2 e– → H2(g) + 2 OH– | +.41 v | iv |
A comparison of the E°s would lead us to predict that the reduction (ii) would be favored over that of (i). This is certainly the case from a purely energetic standpoint, but as was mentioned in the section on fuel cells, electrode reactions involving O2 are notoriously slow (that is, they are kinetically hindered), so the anodic process here is under kinetic rather than thermodynamic control. The reduction of water (iv) is energetically favored over that of Na+ (iii), so the net result of the electrolysis of brine is the production of Cl2 and NaOH ("caustic"), both of which are of immense industrial importance:
2 NaCl + 2 H2O → 2 NaOH + Cl2(g) + H2(g)
Since chlorine reacts with both OH– and H2, it is necessary to physically separate the anode and cathode compartments. In modern plants this is accomplished by means of an ion-selective polymer membrane, but prior to 1970 a more complicated cell was used that employed a pool of mercury as the cathode. A small amount of this mercury would normally find its way into the plant's waste stream, and this has resulted in serious pollution of many major river systems and estuaries and devastation of their fisheries.
- Electrolytic Refining Of Aluminum
Aluminum is present in most rocks and is the most abundant metallic element in the earth's crust (eight percent by weight.) However, its isolation is very difficult and expensive to accomplish...
- Reference Electrodes
In most electrochemical experiments our interest is concentrated on only one of the electrode reactions. Since all measurements must be on a complete cell involving two electrode systems, it is common practice to employ a reference...
- Electrochemistry
Learning Objectives(a) describe and explain redox processes in terms of electron transfer and/or of changes in oxidation number (oxidation state) (b) define the terms: (i) standard electrode (redox) potential (ii)...
- How To Balance Ionic Equations In Alkaline Medium?
(I) Standard Electrode Potential What information can we obtain from SEP? Sign – oxidation or reduction favoured Magnitude – the extent the reaction is favoured How to measure SEP? - connecting the redox couple (half cell) to SHE. - learn to draw...
- Electrochemistry : 10.1 Galvanic Cell (continued)
SPONTANEOUS REACTION occurs as the result of different ability of metal to give up their electron to flow through the circuit. CELL POTENTIAL (ECell) different in electrical potential of electrodes also called voltage or electromotive...