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The source material is dissolved in an appropriate solvent, or melted, so that constituent ions are available in the solution. An electrical potential is applied across a pair of conductors immersed in the liquid. The negatively charged conductor is called the cathode, and the positively charged conductor is called the anode. Each conductor attracts the ions of the opposite charge. Therefore, positively charged ions (cations) move towards the cathode while negatively charged ions (anions) move to the anode. The energy required to separate the ions, and increase their concentration at the electrodes, is provided by an electrical power supply that maintains the potential difference across the electrodes. At the electrodes, electrons are absorbed or released by the ions, forming concentrations of the desired element or compound. For example, when water is electrolyzed, hydrogen gas (H2) will form at the cathode, and oxygen gas (O2) at the anode. This was first discovered by William Nicholson, an English chemist, in 1800.
The amount of electric energy that must be added equals the change in Gibbs free energy of the reaction plus the losses in the system. The losses can (theoretically) be arbitrarily close to zero, so the maximum thermodynamic efficiency equals the enthalpy change divided by the free energy change of the reaction. In most cases the electric input is larger than the enthalpy change of the reaction, so some energy is released in the form of heat. In some cases, for instance in the electrolysis of steam into hydrogen and oxygen at high temperature, the opposite is true. Heat is absorbed from the surroundings, and the heating value of the produced hydrogen is higher than the electric input. In this case the efficiency can be said to be greater than 100%. (It is worth noting that the maximum theoretic efficiency of a fuel cell is the inverse of that of electrolysis. It is thus impossible to create a perpetual motion machine by combining the two processes.)
The following technologies are related to electrolysis:
- Electrochemical cells, including the hydrogen fuel cell, use the reverse of this process.
- Gel electrophoresis is an electrolysis where the solvent is a gel: it is used to separate substances, such as DNA strands, based on their electrical charge.
Scientific pioneers of electrolysis included:
First law of electrolysis
In 1832, Michael Faraday reported that the quantity of elements separated by passing an electrical current through a molten or dissolved salt was proportional to the quantity of current passed through the circuit. This became the basis of the first law of electrolysis.
Second law of electrolysis
Faraday also discovered that the mass of the resulting separated elements was directly proportional to the atomic masses of the elements when an appropriate integral divisor was applied. This provided strong evidence that discrete particles of electricity existed as parts of the atoms of elements.
- Manufacture of aluminium, lithium, aspirin.
- Manufacture of hydrogen for hydrogen cars and fuel cells.
- High-temperature electrolysis is also being used for this.
- Coulometric techniques can be used to determine the amount of matter transformed during electrolysis by measuring the amount of electricity required to perform the electrolysis.
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