At the beginning of the XIX century , scientistsNicholson and A. Carlyle decomposed water into oxygen and hydrogen, which caused a great surge of interest in the influence of electric current on chemical reactions. Subsequently , such a process is Faraday he called it electrolysis and characterized it as the decomposition of a compound into elements by means of an electric current. Then the concept was expanded: electrolysis is a chemical reaction occurring in solutions or melts electrolytes under the action of a direct electric current.
Faraday did a lot of work on the study of electrolysis, introduced the basic concepts to describe this phenomenon and he deduced two basic laws of electrolysis.
To understand the terms of electrochemistry, let us turn to the history of electrolysis research. The basis for the discovery of electrolysis the voltaic pole served: Alessandro Volta placed zinc and copper plates in a container with acid and connected their wire. As a result, the zinc plate began to dissolve, and bubbles stood out on the copper plate gas. Thus Volta created the first galvanic cell - a current source. The galvanic cell consisted of strips of copper, cloth with acid and strips of zinc placed on top of each other in the form of a column. If such galvanic cells are consistently mounted on this column, the voltage will increase. Thus, at the top of the pillar there was a negative charge and a positive one at the bottom of the column.
The word electrode comes from the ancient Greek hodos, which means "path", hence the electrode is the path of the electron. The prefixes cat- and an- also came from the ancient Greek words down and up, catho - down (catastrophe, catacomb), accordingly, on the Volt column, the anode is at the top - this is zinc, the cathode is at the bottom - copper. The words anion and cation are formed in a similar way. A negatively charged anion moves sideways a positive anode, and a positive cation towards a negatively charged electrode - cathode.
Faraday's laws for electrolysis
- The mass of the substance deposited on the electrode during electrolysis is directly proportional to the amount of electricity, passed through this electrode.
- For a given amount of electricity, the mass of the substance deposited on the electrode is directly proportional to equivalent to the mass of the substance.
The equivalent mass of the substance was derived earlier in the reactions of acids and bases. The mass of the substance that reacts with one mole of hydrogen is called the equivalent mass of the substance. Then, with the use of electricity, the equivalent mass was assigned the amount of substance that is released or deposited on the electrode when 1 mole of electrons passes through this substance.
Electrochemical current sources
An electrochemical current source is a device capable of obtaining electrical energy from chemical reactions. In such reactions, we will be interested in the processes of electron transfer.
Electrochemical sources are of two types: a galvanic cell and an electric battery. Electric a battery is a device capable of storing electrons in some form and subsequently giving them away.
Galvanic cell
A spontaneous redox reaction occurs in a galvanic cell. The principle of operation is based on on the interaction of two metals and/or their oxides in the electrolyte. There are three types of galvanic cells:
Primary galvanic cell
Primary galvanic cells work by converting the energy of reagents into electrical energy. Such devices work until they exhaust all the starting substances, and have a limited lifetime. The most familiar example of a primary galvanic cell in our life is an ordinary battery.
Secondary galvanic cell
A secondary galvanic cell is a device in which the initial conditions of the system are easily restored: the electrical energy applied to the device is converted into chemical and vice versa. Secondary galvanic cells are batteries, the most popular are lead and nickel-cadmium batteries.
Fuel cell
The fuel cell operates at the expense of fuel and oxygen coming from the outside. As a result of the redox reaction of fuel combustion in oxygen, an electric current is generated.
Corrosion
Corrosion is the destruction of metals due to any impact other than mechanical. In the process of corrosion, a chemical reaction of the metal with the environment occurs, which entails deterioration of the properties of the metal and at the same time the metal goes from a normal state to a bound state. The result of the reaction is, as a rule, an oxide, a salt or another compound.
Chemical corrosion
Chemical corrosion occurs as a result of direct contact of metal and an aggressive environment, as a result, a layer of a new compound is formed on the metal surface, which serves as a partition between the metal and the surrounding environment. Chemical corrosion occurs, as a rule, at high temperatures when the metal comes into contact with a heated gas.
Corrosion of iron and steel
Chemical corrosion of iron and steel in the air occurs under normal conditions (rust formation due to slow oxidation of iron with water and oxygen in the air). The formed connection does not always prevent further metal oxidation, since iron oxides are loose. Thus, steels in which iron is the main component it is necessary to be galvanized, chrome plated or coated with paint/varnish.
Electrochemical corrosion
As a result of the contact of the metal with the electrolyte, a galvanic cell is formed, in which the metal is an anode that is oxidized during the reaction. At the same time, if the metal acts as a cathode, the reaction will not occur. So, for example, in order to protect the metal from corrosion, we can pass a current through it so that when interacting with the medium, the metal will be the cathode.
