The basic concept of electrochemistry - electrode potential!

First, the formation of electric double layer

The metal lattice consists of neatly arranged metal cations and electrons flowing between them. When a metal is immersed in a human electrolyte solution, metal ions on the metal surface will be hydrated due to the action of polar water molecules. If the hydration energy generated during hydration is sufficient to overcome the gravitational force between the metal ions and the electrons in the metal lattice, some of the ions will break away from the metal character and enter the liquid layer in contact with the metal surface to form hydration ions. The electrons that remain electrically neutral with these ions remain on the metal, which is the oxidation reaction. As the electrons accumulate on the metal surface, a reverse reaction of the above reaction occurs, that is, the metal ions return to the metal surface to coincide with the electrons, which is the reduction reaction. When the oxidation reaction and the reduction reaction are equal, that is, equilibrium is reached, there is a certain amount of electron excess on the surface of the metal, which is closely arranged on the surface of the metal, and an equal amount of excess hydrated metal ions are arranged in the liquid layer near the surface of the electrode. This type of charge layer having the same size and opposite charge sign at the interface between the electrode and the solution is called an electric double layer. Many negatively charged metals such as iron, zinc, cadmium, etc., form this type of double layer electricity in an aqueous solution containing the metal salt.

If the hydration energy of the metal ions is insufficient to overcome the attraction between the metal ions and the electrons in the metal lattice. Then the metal ions in the solution may be attracted to the electrons on the electrode and enter the crystal lattice, so that the surface is positively charged, and the liquid layer near the surface of the electrode has an excess of negative ions, which is negatively charged, thus forming a double with the above The electric layer is charged with the opposite polarity of the double layer. For example, copper is contained

In the solution of the copper salt, silver forms a double layer of this type in a solution containing a silver salt.

A schematic diagram of two types of electric double layers, as shown in FIG. 1 to 2-7.

Second, the reversible electrode and the irreversible electrode

It is known from the generation of the electric double layer that in the electrolyte solution, the oxidation reaction and the reduction reaction are simultaneously performed on any of the electrodes. Under equilibrium conditions (ie, no current is passed through the electrode or the current passed through is infinite), if the oxidation reaction and the reduction reaction are reversible, the electrode is a reversible electrode. For example, pure zinc is placed in a zinc sulfate solution, and when the oxidation reaction and the reduction reaction rate are phase, the following reaction equation is satisfied:

That is to say, when the oxidation reaction and the reduction reaction rate are equal, the substance and the charge are exchanged in the opposite direction and at the same speed at the interface, that is, the substance exchange and the charge exchange are both reversible, so the oxidation reaction and the reduction reaction are reversible. This electrode is a reversible electrode.

An electrode, a hydrogen electrode, a calomel electrode, or the like, in which a metal is placed in a solution containing the metal salt, is a reversible electrode.

Electrodes that do not meet the above conditions for material exchange and charge exchange are reversible. It is called an irreversible electrode. For example, pure zinc is placed in dilute hydrochloric acid with zinc dissolution at the beginning: zn→zn2++2e. However, there is no zinc ion at the beginning of the solution, only H+ ions. Therefore, the reverse reaction at this time is the reduction of H+ ions: H+ + e→H. However, as the zinc dissolves, the zinc ions in the solution increase, and the zinc ion reduction reaction starts: zn2++2e→zn, but the reduction rate is lower than the oxidation rate of zinc. At the same time, the reduced hydrogen atoms also begin to reoxidize to hydrogen ions: H → H + + e. However, its oxidation rate is also lower than the reduction rate of hydrogen ions. Thus, there are two pairs of reactions on the electrode (note: there is only one pair of reactions on the reversible electrode), and the dissolution and deposition rates of zinc are not equal, the oxidation of hydrogen and

The rate of reduction is also not equal, that is, the exchange of matter is irreversible and therefore an irreversible electrode.

The metal is placed in an acid, alkali or salt solution, and the reversible electrode can be formed only under the condition that the reversible electrode is satisfied; otherwise, an irreversible electrode is generally formed.

Third, the electrode potential

Due to the presence of the electric double layer, a potential difference is generated between the metal and the solution interface, and this potential difference is called the electrode potential.

However, the electrode potential values ​​we often use are not the absolute values ​​of such potential differences, because such absolute values ​​are not yet sufficiently reliable to test. However, the value and the value can be accurately determined by comparison. The electrode potential value we often use is actually the exact relative potential value.

Therefore, the electrode potential can be defined as follows: an electrode and a standard hydrogen electrode constitute a special primary battery, wherein the standard hydrogen electrode is defined as a negative electrode, and the measured electromotive force of the primary battery is referred to as the electrode potential of the electrode, or It is the hydrogen standard electrode potential and is represented by the symbol Φ.

The electromotive force of the primary battery mentioned above refers to the potential difference between the two electrodes when the primary battery is lifted (when no current flows): E=Φ+-Φ-

Where: E is the electromotive force (V) of the primary battery, Φ+ is the electrode current (V) of the positive electrode, and Φ- is the electrode potential (V) of the negative electrode.

Since the electrode potential of the standard hydrogen electrode is artificially set to zero, the electromotive force of the actual side is equal to the electrode potential of the electrode.

Fourth, balanced potential and non-equilibrium potential

When no current passes, the electrode potential of the reversible electrode is called the equilibrium electrode potential, which is called the equilibrium potential, also called the reversible potential. The equilibrium potential is related to the temperature and the effective concentration (activity) of the substance in the solution. The value can be determined not only by experiment, but also by the Nernst equation:

Where: Φ is the equilibrium potential, Φ. For the standard electrode potential, R is the gas constant, equal to 8.313J / ° C; T is the absolute temperature; Z is the number of electrons participating in the electrode reaction, F is the Faraday constant, a oxidation state is the average activity of the oxidation state, a reduction state It is the average activity of the reduced substance.

For pure metals, a reduction state = 1 so the Nernst equation can be simplified to;

Where: a is the average activity of the metal ions.

The electrode potential of an irreversible electrode in the absence of current is called an unbalanced potential, also called an irreversible potential.

The non-equilibrium potential generally changes as the electrode process progresses. If a completely stable value is reached, the non-equilibrium potential is called the stable bit, otherwise it is called the unsteady potential. The non-equilibrium potential cannot be calculated using the Nernst equation and can only be determined experimentally.

The non-equilibrium potential of certain metals in the graded sodium solution is listed in Tables 1 through 2-7.

5. Standard electrode potential and electrochemical sequence

The activity (or gas fraction) of the reactants and products in the electrode reaction is equal to the equilibrium potential at 1 and is called the standard electrode potential. It can be seen from the Nernst equation that if both the a oxidation state and the a reduction state are equal to 1, then Φ is flat = Φ. Therefore, Φ. It is the standard electrode potential.

The standard electrode potentials of the various electrode reactions are arranged in the order in which their algebraic values ​​are increased. It is called the electrochemical sequence.

The metals in the table, which are located in hydrogen, are often referred to as negatively charged metals, and their standard electrode potentials are negative: metals below hydrogen are called positively charged metals, and their standard electrode potentials are positive. The ability to oxidize or reduce the metal can be judged from the electrochemical sequence. For electroplating , the more positive the metal, such as gold, silver, copper, etc., the easier it is to reductively precipitate on the cathode, and the more negative the potential, such as aluminum, magnesium, titanium, etc., is not easily plated.