The process by which an electric current passes through a substance and causes a chemical change. Chemical change is a process in which a substance loses or gains electrons (oxidation or reduction). The electrolysis process is carried out in an electrolytic cell. The electrolytic cell consists of a positive electrode and a negative electrode immersed in a solution containing positive ions and negative ions respectively. Current (that is, electrons) flows into the negative electrode (cathode), and positive ions in the solution migrate to the cathode and combine with electrons to become neutral elements or molecules; Negative ions with negative charges migrate to another electrode (anode), giving electrons and becoming neutral elements or molecules.
[Edit this paragraph] Analysis of electrolysis principle
(Take cucl2 as an example)
CuCl2 is a strong electrolyte, which is soluble in water and ionized in aqueous solution to produce Cu2 ++ and Cl-.
CuCl2=Cu2++2Cl-
Before electrification, Cu2 ++ and Cl- move freely in water; After electrifying, these freely moving ions become directional motion under the action of electric field. The positively charged Cu2 ++ in the solution moves to the cathode, and the negatively charged chloride ion moves to the anode. At the cathode, copper ions gain electrons and are reduced to copper atoms to cover the cathode; At the anode, chloride ion loses electrons and is oxidized into chlorine atoms, and two two combination molecules form chlorine molecules, which are released from the anode.
Cathode: Cu2++2e-= Cu
Anode: Cl-2e-= Cl2 =
Chemical reaction equation of electrolytic CuCl2 solution: CuCl2=Cu+Cl2 (electrolysis) (5) Comprehensive analysis of electrolytic reaction of electrolyte aqueous solution.
In the electrolytic process of copper chloride, H+ and OH- in the solution were not mentioned. In fact, although H+ and OH- are rare, they do exist, but they do not participate in the electrode reaction. That is to say, in the copper chloride solution, besides Cu2+ and Cl-, there are also H+ and OH-. During electrolysis, the ions moving to the cathode are Cu2+ and H+. Because it is easier for Cu2+ to obtain electrons than H+ under such experimental conditions, Cu2+ obtains electrons at the cathode to precipitate metallic copper. The ions moving to the anode are OH- and Cl-, because under such experimental conditions, Cl- and OH- lose electrons easily, so Cl- loses electrons on the anode and generates chlorine gas.
Description:
(1) The process in which a cation gains electrons or an anion loses electrons, thus reducing the amount of charge carried by ions, is also called discharge.
(2) Electrodes made of materials with weak reducibility, such as graphite, gold and platinum, are called inert electrodes, because they do not react chemically under general electrifying conditions. Electrodes made of materials with strong reducibility such as iron, zinc, copper and silver are also called active electrodes. When they are used as anodes of electrolytic cells, they undergo oxidation reaction before other substances.
(3) Under the general electrolytic conditions, when the aqueous solution contains many cations, their discharge order on the cathode is: Ag+> Hg2+> Fe3+> Cu2+> (H+) > Fe2+> Zn2+; When the aqueous solution contains a variety of anions, the discharge sequence on its inert anode is S2-> I-> Br-> Cl-> OH _ (F-, NO3-,SO42-, etc. ).
(6) Electrolyzing the electrolyte aqueous solution with an inert electrode, and analyzing the electrolytic reaction usually includes the following steps:
① Analyze the composition of electrolyte aqueous solution, find out all ions and divide them into positive and negative groups;
(2) discharging the anion and cation respectively, and writing the electrode reaction formula on the two poles;
③ The total chemical equation or ionic equation of electrolytic reaction is composed of the reaction equations of two electrodes.
[Edit this paragraph] Purpose
Electrolysis is widely used in metallurgical industry, such as extracting metal from ore or compound (electrolytic metallurgy) or purifying metal (electrolytic purification), and precipitating metal from solution (electroplating). Electrolytic melting of sodium chloride produces metallic sodium and chlorine; Electrolysis of sodium chloride aqueous solution produces sodium hydroxide and chlorine. Electrolyzing water produces hydrogen and oxygen. Electrolyzing water is to decompose water into H2(g) and O2(g) under the action of external electric field. Electrolysis is a very powerful means to promote redox reaction, and many difficult redox reactions can be realized by electrolysis. For example, molten fluoride can be oxidized to elemental fluorine at the anode and molten lithium salt can be reduced to metallic lithium at the cathode. Electrolytic industry plays an important role in the national economy, including the smelting of many non-ferrous metals (such as sodium, potassium, magnesium, aluminum, etc.). ) and the extraction of rare metals (such as zirconium and hafnium) and metals (such as copper, zinc, lead, etc.). ), preparation of basic chemical products (such as hydrogen, oxygen, caustic soda, potassium chlorate, hydrogen peroxide, ethylene dinitrile, etc.). ), electroplating and electricity.
[Edit this paragraph] Electrolyte
Compounds that can conduct electricity in aqueous solution or molten state are called electrolytes. The premise of composite conductivity is that there are free-moving anions and cations in it.
Ionic compounds can conduct electricity in aqueous solution or molten state; * * * Valent compounds: Some can also conduct electricity in aqueous solution (such as HC, some are non-electrolyte).
Conductivity has nothing to do with solubility. Strong electrolyte generally includes: strong acid and alkali, most salts; Weak electrolytes generally include: (compounds that can only be partially ionized in water) weak acids (reversible ionization, step ionization. In addition, water is an extremely weak electrolyte.
Note: to judge whether a compound is an electrolyte, it is not necessarily an electrolyte that can conduct electricity, and it is not only based on whether it conducts electricity in aqueous solution, but also needs to further investigate its crystal structure and the properties of chemical bonds. For example, judging whether barium sulfate, calcium carbonate and iron hydroxide are electrolytes. Barium sulfate is insoluble in water (the solubility in water is 2.4× 10-4 g at 20℃), and the ion concentration in the solution is very small, so its aqueous solution is nonconductive and seems to be non-electrolyte. However, a small amount of barium sulfate dissolved in water is almost completely ionized (the ionization degree of barium sulfate saturated solution is 97.5% at 20℃). Therefore, barium sulfate is an electrolyte. Calcium carbonate and barium sulfate are also electrolytes. Structurally, other insoluble salts, as long as they are ionic compounds or highly polar valence compounds, are electrolytes even if they are insoluble.
The situation of iron hydroxide is more complicated. The chemical bond between Fe3+ and OH- has valence property, and its solubility is less than that of barium sulfate (the solubility in water at 20℃ is 9.8× 10-5 g). A small part of the part that falls into the water may form colloid, and the rest may be ionized into ions. But iron hydroxide is also an electrolyte.
To judge whether an oxide is an electrolyte, it is also necessary to analyze it in detail. Non-metallic oxides, such as SO2, SO3, P2O5, CO2, etc. , is a * * * valence compound, which is not conductive in liquid state, so it is not an electrolyte. Some oxides are not electrolytes, although they can conduct electricity in aqueous solution. Because these oxides react with water to generate new conductive substances, it is not the original oxides that conduct electricity in the solution. For example, SO2 itself cannot ionize, but it reacts with water to generate sulfurous acid, which is electrolyte. Metal oxides, such as Na2O, MgO, CaO, Al2O3, etc., are ionic compounds and can conduct electricity in the molten state, so they are electrolytes.
It can be seen that the electrolyte includes ions or strongly polar valence compounds; Non-electrolytes include weakly polar or nonpolar valence compounds. An aqueous electrolyte solution can conduct electricity because the electrolyte can be dissociated into ions. Whether a substance can ionize in water is determined by its structure. Therefore, it is the essence of the problem to distinguish electrolyte and non-electrolyte with material structure.
In addition, some conductive substances, such as copper and aluminum, are not electrolytes. Because they are not conductive compounds, but simple materials, which does not meet the definition of electrolyte.
Electrolyte refers to compounds that can conduct electricity in aqueous solution or molten state, such as acid, alkali, salt, etc. All compounds that cannot conduct electricity under the above conditions are called non-electrolytes, such as sucrose and alcohol.
Law of electrolytic products
Sixteen-character key:
Yin gains and Yang loses: during electrolysis, the cathode gains electrons and carries out reduction reaction, while the anode loses electrons and carries out oxidation reaction;
Yin is refined and Yang is coarse: in the process of copper smelting, refined copper is used as cathode, crude copper is used as anode, and finally the anode is gradually dissolved to produce anode mud;
Anion-base-cation acid: after electrolytic reaction, the oxyacid salt of inactive metal will generate acid at the anode, while the anaerobic acid salt of active metal will generate alkali at the cathode;
After the electrolytic reaction, the cathode produces solid and reducing gas, while the anode produces strong oxidizing gas.