Electrode potential electrode potential
0 The potential difference generated when the electrode comes into contact with the solution. Parameters that measure the trend of electrodes in response to . Also known as electrode potential, it is referred to as electrode potential for short. Because the electrochemical reactions on the electrode are all redox reactions , the electrode potential is also called redox potential.
When a metal electrode is immersed in a salt solution of the metal, the atoms in the metal tend to leave the lattice and convert into metal positive ions to enter the solution. The metal ions in the solution also tend to be reduced to atoms on the surface of the metal electrode and deposited. In this way, the electrode surface is negatively charged due to the accumulation of electrons, and the solution close to the electrode surface is positively charged due to the accumulation of metal ions; vice versa. Therefore, there is a certain potential difference between the electrode and the solution .
The potential difference of different metals in their different concentrations of salt solutions is also different. There is no possibility and no need to measure the absolute value of the electrode potential of various substances. Instead, the potential difference between the platinum sheet saturated by 1 atmospheric pressure (Pa) and the hydrogen ion solution with a concentration of 1 mole/liter is specified to be 0 volts (this electrode is called a standard hydrogen electrode), that is, it is stipulated that the equilibrium potential of the following reduction reaction is equal to 0 volts:
(1)
Then connect other metals directly or indirectly to the standard hydrogen electrode formed by its salt solution to form a chemical primary battery, and measure the electromotive force of the battery, that is, the relative value between the metal electrode potential and the hydrogen electrode potential is obtained. For example, the electrode composed of M metal and ionic solution:
(2)
is connected to a standard hydrogen electrode to form a battery for measurement. The electromotive force of the battery measured by the experiment is determined as the electric potential of the electrode. When the ion concentration in the solution is 1 mole/liter at 1 atmospheric pressure, that is, the potential value of the battery in a standard state is the standard potential of the electrode. Generally, the electrode reaction is in the form of reduction reaction in formula (2), and the corresponding electrode potential is called the standard reduction potential. The reduction potential of those substances that are easier to reduce has a positive value, while those that are more difficult to reduce have a negative value.
Except for metals, the reduction potential of other substances is also obtained by referring to the standard hydrogen electrode potential of 0 volts. Their standard reduction potential refers to the equilibrium potential when the concentration of the oxidation state and the reduced substance participating in the reaction are in the standard state. For the reduction potential in the non-standard state, it can be expressed by the formula of Nest : in the formula of
, it is the standard reduction potential, R is the gas constant, T is the absolute temperature, n is the number of electron transfers in the reduction reaction, F is the Faraday constant, and the activity of the oxidation state substance and the reduced substance respectively. When the concentration of the substance in the reaction is equal to 1 mole/liter, that is, the equilibrium potential is the standard reduction potential.
Standard reduction potential of various substances in acidic solutions is shown in the table. If the semi-reaction in the table is combined into the form of oxidation reaction , the potential becomes the standard oxidation potential, and its value is the same as the original reduction potential, but the symbol is opposite.
The greater the reduction potential of a substance, the easier it is to reduce, that is, the easier it is to obtain electrons from other substances, thereby oxidizing other substances. In some semi-reactions listed at the bottom of the table, the substances on the left are strong oxidants, such as halogen, chlorate , permanganate , hydrogen peroxide , etc.; while the substances on the right of the semi-reactions listed at the top are some strong reducing agents, such as alkali metal , alkaline earth metal , aluminum, etc.
Excerpted from: "China Encyclopedia (2nd Edition)" Volume 5, China Encyclopedia Press , 2009
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