The electrode potential is the potential difference between the electrode plate and the solution in the electrode.In order to obtain the electrode potential values of various electrodes, the electrode potential of a certain electrode is usually used as the standard to form a battery with other electrodes to be measured. The relative electrode potential of various electrodes is determined by measuring the electromotive force of the batteryEValue.In 1953, the International Union of Pure and Applied Chemistry (IUPAC) suggested that the standard hydrogen electrode should be used as the standard electrode, and the electrode potential of the standard hydrogen electrode should be set to zero artificially.
German chemistNernst ProposedElectric double layer theory(Electrical double layers theory) Explain the cause of electrode potential.When the metal is put into the solution, on the one hand, the metal ions in thermal motion in the metal crystal leave the metal surface and enter the solution under the action of polar water molecules.The more active the metal property is, the greater this trend will be;On the other hand, metal ions in the solution are attracted by the electrons on the metal surface and deposited on the metal surface. The greater the concentration of metal ions in the solution, the greater the trend.After reaching equilibrium in a solution of a certain concentration, a double electric layer with opposite charges is formed on the interface of metal and solution. Although the thickness of the double electric layer is very small (about 10-8Centimeter order), but it creates a potential difference between the metal and the solution.Usually people call the potential difference between the double electric layers between metal and salt solution the electrode potential of metal, and describe the relative strength of the electrode's ability to gain and lose electrons.The electrode potential is represented by the symbol E (Mn+/M) Is expressed in V.If the electrode potential of zinc is equal to E (Zn2+/Zn) indicates that the electrode potential of copper is expressed as E (Cu2+/Cu).
The size of electrode potential mainly depends on the nature of the electrode, and is affected by temperature, medium, ion concentration and other factors.
Standard status
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Electrode symbol: Pt | Htwo(100kPa)|H+(1mol/L)
Electrode reaction:
The symbol "⊖" in the upper right corner represents the standard state.
The standard state requires that the electrode is under the standard pressure (100kPa or 1bar), and the solid or liquid substances constituting the electrode are pure substances;The partial pressure of gaseous substances is 100kPa;The concentration of relevant ions (including the medium participating in the reaction) constituting the electric pair is 1mol/L (the strict concept is activity).Generally, the measured temperature is 298K.
useStandard hydrogen electrodeAnd the electrode to be tested to form a battery in a standard state, measure the electromotive force value of the battery, and passDC voltmeterDetermine the positive and negative poles of the battery, that is, according to Epool=E(+)-E(-)Calculate theStandard electrode potentialThe relative value of.
For example, at 298K, use a potentiometer to measure the electromotive force (Epool）0.76V, Zn/Zn calculated according to the above formula2+The standard electrode of the electric pair is -0.76V.Cu can be measured by the same method2+/The electrode potential of Cu electric pair is+0.34V.
E of electrode⊖A positive value indicates that the oxidizing substance constituting the electrode has a greater tendency to obtain electrons than H in the standard hydrogen electrode+, such as Cu in copper electrode;If the value of the electrode is negative, the tendency of the oxidizing substance constituting the electrode to obtain electrons is less than that of H in the standard hydrogen electrode+, such as Zn in zinc electrode.
In practical application, some electrodes with relatively stable potential, such asSaturated calomel electrodeandSilver silver chloride electrodeAsReference electrodeForm a battery with other electrodes to be tested, and calculate the potential of other electrodes.The electrode potential of saturated calomel electrode is 0.24V.The electrode potential of silver silver chloride electrode is 0.22V.
The standard electrode potential meter of electrode reaction is obtained by arranging the standard electrode potential values of different redox electric pairs in order from small to large.Its characteristics include:
(1) Generally, the reduction potential of electrode reaction is used, and the electrode reaction of each electrode is written in the form of reduction reaction, namely: oxidation type+ne-=Restore the prototype.
(2) The standard electrode potential is the equilibrium potential, and each electric pair E⊖The positive and negative sign of the value does not change with the direction of electrode reaction.
（3）E⊖The value can be used to judge the relative strength of the oxidation capacity of the oxidizing substances and the reduction capacity of the reducing substances in the electric pair under the standard state, which is independent of the amount of the substances participating in the electrode reaction.For example:
（4）
The value is only suitable for the electrode reaction in the water solution in the standard state.For non-aqueous, high-temperature, solid phase reactions, there are certain limitations.For non-standard reactionsNernst equationConversion.
influence factor
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The factors affecting the electrode potential are ion concentration, acidity and alkalinity of solution, precipitant and complexing agent, and the factors judging are Nernst equation.
Nernst equation: the standard electrode potential is measured under the standard state.If the conditions change, the electrode potential of the electric pair will also change.The size of the electrode potential depends on the nature of the electrode first, which is reflected by the standard electrode potential.Secondly, the change of ion concentration (or partial pressure of gas) and temperature in solution will cause the change of electrode potential.The quantitative relationship between them can be expressed by Nernst equation.
Related applications
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Judge the relative strength of oxidant and reducing agent
The relative strength of oxidant and reducing agent in standard state can be directly compared
The size of the value.
The electrode with a lower value (e.g. Li: -3.03V) is more likely to lose electrons in its reducing substance, which is a stronger reducing agent. The corresponding oxidizing substance is more difficult to obtain electrons, which is a weaker oxidant.E⊖The higher the value of the electrode, the easier it is for the oxidizing substance to get electrons, which is a stronger oxidant, while the corresponding reducing substance is more difficult to lose electrons, which is a weaker reductant.
Judge the direction of redox reaction
(1) According to
Value to judge the direction of redox reaction under standard conditions.
Under normal conditions, the redox reaction always proceeds from stronger oxidants and reducers to weaker oxidants and reducers.From the value of electrode potential, the reaction can only be carried out when the potential of oxidant electric pair is greater than that of reductant electric pair.The reaction is carried out in the direction of "oxidation type with high potential and reduction type with low potential".When judging whether the redox reaction can be carried out spontaneously, it usually refers to the positive reaction.
(2) According to the battery electromotive force
Pool value to judge the direction of redox reaction.
In principle, any redox reaction can be designed as a primary cell.The direction of redox reaction can be judged by the electromotive force of the primary cell.The primary cell consists of redox reaction. Under the standard state, if the standard electromotive force of the cell is>0, the cell reaction can occur spontaneously;If the standard electromotive force of the battery is<0, the battery reaction cannot be carried out spontaneously.In the non-standard state, the electromotive force in this state is used to judge.
From the relationship between the electromotive force of the primary cell and the electrode potential, only when the electromotive force of the primary cell is>0, the redox reaction can spontaneously proceed to the positive reaction direction.That is to say, the electrode potential of the electric pair where the oxidant is located must be greater than the electrode potential of the electric pair where the reductant is located to meet theE>0.
In terms of thermodynamics, the electromotive force of the battery is the driving force for the battery reaction.When the electromotive force of the battery formed by redox reaction
When it is greater than zero, the redox reaction can be carried out spontaneously.Therefore, the battery electromotive force is also a criterion to judge whether the redox reaction can be carried out.
The free energy of the system is reduced when the battery generates electric energy through redox reaction.Under constant temperature and pressure, the reduction value of free energy (- △G）Equal to the maximum useful electric work that the battery can make（Welectric）：
-△G=Welectric=QE=nFEpool
That is:
In the standard state, the above formula can be written as:
When
When the value is positive,
Is negative, and the redox reaction is positive and spontaneous under the standard state;When
When the value is negative,
It is a positive value. Under the standard state, the reaction is positive and non spontaneous, and the reverse reaction is spontaneous.E or
The larger the positive value, the greater the tendency of the redox reaction to proceed spontaneously.E pool or
The greater the negative value, the greater the tendency of spontaneous reverse reaction.
Judge the limit of reaction
The degree of completion of a chemical reaction can be quantitatively judged from the size of the equilibrium constant of the reaction.Therefore, the standard equilibrium constantKφAnd thermodynamicsGibbs free energyConnect.
△Gφ=-2.303RTlgKφ
△Gφ=-nFEφ
benFEφ=2.303RTlgKφ
Standard equilibrium constantKφAnd standard electromotive forceEφThe relationship between them is lgKφ=
RIs the gas constant,TIs the absolute temperature,nIs the number of electron transfer in the redox reaction equation,FIs Faraday constant.
The equation shows that at a certain temperature, the equilibrium constant of the redox reaction is related to the electromotive force of the standard battery, and is independent of the concentration of reactants.EφThe larger the equilibrium constant, the more complete the reaction.Therefore, you can useEφValue to estimate the extent of the reaction.Generally speaking,EφThe equilibrium constants of redox reactions ≥ 0.2~0.4V are greater than 10six, indicating that the reaction has been carried out completely.KφThe value can indicate the degree of reaction, but cannot determine the reaction rate.
Potential diagram
Most non-metallic elements and transition elements can have several oxidation values, and there are corresponding standard electrode potentials between the oxidation values.The various oxidation values can be arranged in the order of high to low (or low to high). The two oxidation values are connected by a straight line and the standard electrode potential value of the corresponding electrode reaction is marked on the straight line. Such a graph represents the relationship between the various oxidation values of an elementelement potential diagram , which is also called Ratmutu because it was discovered by Ratmutu.According to the different pH values of solutions, they can be divided into two categories: (A means acidic solution) means pH=0;(B indicates alkaline solution) indicates pH=14 of the solution.When writing the potential diagram of an element, you can list all the oxidation values or some of them as needed.
At the far right end of the element potential diagram is a reduced substance, such as Cl-The leftmost end is oxidizing substances, such as ClO-。The substance in the middle is oxidized relative to the substance at the right end and reduced relative to the substance at the left end, for example, Cl is relative to Cl-It is oxidation type, relative to ClO-It is a prototype.
Judge whether the disproportionation reaction can be carried out:
Disproportionation reactionIt refers to self oxidation and reduction reaction:Redox reactionIn, oxidation and reduction occur on elements with the same oxidation value within the same molecule, that is, the atoms (or ions) of the element are oxidized and reduced at the same time.
Two electric pairs composed of three substances with different oxidation values of an element are arranged from left to right according to their oxidation values.
