What is the concept of chemistry, disproportionation and passivation?

In the reaction, if oxidation and reduction occur on an element that is in the same oxidation state inside the same molecule, one part of the atoms (or ions) of the superoxide dismutase (SOD) element is oxidized and the other part is reduced. This own redox reaction is called disproportionation. This reaction is also known as the Cannizzaro reaction.

For example, Cl2+H2O=HCl+HCl

In this reaction, Cl2 is originally 0 valent.

After the reaction, one rises to +1 and the other decreases to -1.

Disproportionation is a type of chemical reaction in which the valency of an element rises and falls. As opposed to the centering reaction.

Iron and aluminum can be quickly dissolved in dilute Hno3 or dilute H2SO4, but the dissolution phenomenon in concentrated Hno3 or concentrated H2SO4 is almost completely stopped, carbon steel is usually easy to rust, if an appropriate amount of Ni and Cr is added to the steel, it will become stainless steel. The phenomenon that the chemical stability of a metal or alloy is significantly enhanced due to some factors is called passivation. The phenomenon of metal passivation caused by certain passivators (chemicals) is called chemical passivation.

Oxidants such as concentrated HNO3, concentrated H2SO4, HCO3, K2Cr2O7, KMno4 and other oxidants can passivate metals.

Disproportionation is the reaction in which the valency of the same element is originally the same, and the valency of the same element increases or decreases after the reaction, such as the decomposition of hydrogen peroxide.

Passivation refers to the formation of a dense oxide film on the surface of the reactant during the reaction, or the fact that it is covered with solids, which prevents or slows down the progress of the reaction.

For example, iron is passivated in cold, concentrated sulphuric acid, preventing further reactions.

1. This is the last process step in chemical cleaning, which is a key step, and its purpose is to prevent corrosion of materials.

2. Passivation refers to the process of oxidizing the metal by strong oxidant or electrochemical method to make the surface become inactive, that is, the blunt state, which is a method to transform the metal surface into a state that is not easy to be oxidized and delay the corrosion rate of the metal. In addition, an active metal or alloy, in which the chemical activity is greatly reduced, and becomes a state, is also called passivation.

Categories: Education Science >> Admission >> College Entrance Examination Problem Description:

Why do some metals with moderate activity "passivate" with concentrated Hno3 at room temperature?

Analysis: For example, metals such as iron and aluminum can be passivated in concentrated nitric acid at room temperature. There are usually two theories to explain this problem:

One is adsorption theory. It is believed that the passivation of metals is due to the formation of an oxygen adsorption layer composed of oxygen molecules and OH- and O2- ions on the middle surface of nitric acid, and there are four possibilities for the passivation of metals caused by the oxygen adsorption layer: (1) the oxygen adsorption layer increases the superelectric potential of metal oxidation, which increases the difficulty of metal oxidation; (2) It is believed that the unsaturated bonds of atoms on the metal surface reach saturation after adsorption of oxygen, so the metal surface loses its activity; (3) It is believed that the oxygen adsorption layer crowds out other chemical media that the metal surface is in contact with; (4) It is believed that the oxygen atoms adsorbed on the metal surface are induced into an oxygen dipole by the electrons on the metal surface, with the positive end on the metal surface and the negative end towards the solution, thus forming an electric double layer that retards the oxidation of the metal.

The other is the thin film theory. It is believed that when the concentration of metal ions exceeds the solubility product of metal hydroxide in concentrated HNO3, a film of hydroxide is formed on the surface of the metal, which aborts the further reaction between the metal and concentrated nitric acid. The results of polarization analysis showed that the reason for the passivation of iron in concentrated nitric acid was due to the formation of a nano -Fe2O3 film on the surface of iron, which aborted the further reaction between iron and concentrated Hno3.

Aluminum is also passivated in concentrated HNO3, and the passivation film of aluminum is analyzed to be a layer of non-porous -Al2O3, which is also covered with a layer of porous Al2O3·3H2O, so the stability of aluminum in concentrated Hno3 is better than that of iron (in the storage and transportation of concentrated Hno3, people often use aluminum containers instead of iron containers).

It is worth mentioning that the passivated iron, aluminum and other metals have high stability, for example, after the iron is passivated in concentrated nitric acid, it will no longer be oxidized by nitric acid when it is put into dilute HNO3; Putting passivated iron into the CuSO4 solution does not replace copper.

Passivation phenomenon. Some of the more reactive metals in the motorized process become inert in some specific environmental media. For example, iron corrodes rapidly in dilute nitric acid, and its corrosion rate increases rapidly with the increase of nitric acid concentration, and when the nitric acid concentration increases to 30 40, the dissolution rate reaches a maximum.

If the nitric acid concentration continues to increase (>40), the dissolution rate of iron decreases suddenly and dramatically until the reaction is close to stopping (Fig. 5-1). At this time, the metal becomes very stable and can remain stable for a period of time even if it is placed in dilute nitric acid. Iron loses its original chemical activity in concentrated nitric acid or after treatment with concentrated nitric acid, an anomaly called passivation.

The passivation of iron in concentrated nitric acid was discovered as early as the thirties of the 19th century.

The concept of passivation:

Passivation refers to the formation of a dense oxide protective film on the surface of metals such as iron and aluminum under the action of concentrated oxidizing hidden acids (sulfuric acid, nitric acid) to prevent them from being further oxidized or other chemical reactions occurring on the bench, which is called passivation.

Passivation is a method of reducing the corrosion rate of a metal surface to a state that is not easily oxidized.

To put it simply, when some metals encounter oxidation-stealing substances, oxide films formed by oxides are produced on the surface.

The image says little by little, the metal wears a layer of clothing, shielding it from possible corrosion.

The more common substances that are passivated are Al and Fe.

When Fe encounters cold concentrated sulfuric acid, the reaction proceeds very slowly, and the reason for this is the formation of an oxide film on the surface.

When AL is exposed to air, an AL2O3 oxide film is formed on the surface, which delays the process of oxidation.

It is a phenomenon in which a dense oxide film is formed on the surface of the metal after being put into concentrated acid, preventing the acid from continuing to react with the metal.

Purification is the removal of rust, and passivation is the production of dense rust.