خواص فیزیکی و شیمیایی هیدروژن
First, the physical properties of hydrogen
The usual elemental form of hydrogen is hydrogen, which is a colorless, odorless and odorless diatomic gas molecule. The density of hydrogen is very small, and it is the smallest gas in nature, much smaller than that of air. Under standard conditions (temperature 0℃, pressure 101.325 kPa), the mass of 1 liter of hydrogen is 0.089 g. Compared with the same volume of air, the mass of hydrogen is about 1/14 of that of air. Taking advantage of this property, people used to use hydrogen balloons as a means of transportation. Because the density of hydrogen is too low, the hydrogen on the earth gradually rises in the atmosphere and finally evaporates into the universe.
Hydrogen is a very difficult gas to liquefy. At 101.325 kPa, hydrogen can become colorless liquid at -252.8℃. Liquid hydrogen has superconducting properties. At -259.2℃, liquid hydrogen can change into snowflake-like solid hydrogen.
The solubility of hydrogen in general liquid is relatively small. Under a certain temperature and pressure, the highest amount of gas dissolved in a certain amount of solvent is called gas solubility. The solubility of gas is not only related to the nature of gas and solvent, but also to temperature and pressure. Generally, its solubility decreases with the increase of temperature. Because the volume of gas changes greatly when it dissolves, its solubility increases significantly with the increase of pressure. Solubility is usually expressed by the maximum volume dissolved in one volume of solvent at a certain temperature. For example, if 1.82 ml of hydrogen can be dissolved in 100 ml of water at 20℃ under 1 atmosphere (pure hydrogen environment), it means 1.82%. According to the molar concentration, the concentration of pure hydrogen dissolved in water at 1 atmosphere at 20℃ is 0. 92 mM. It is suggested that, unlike many gases, the solubility of hydrogen may increase with the increase of temperature.
As for the solubility of gas in liquid, in 1803, British chemist Henry summed up an empirical law based on the study of gas dilute solution, which is called Henry's law. According to Henry's law, at a certain temperature and pressure, the solubility of a gas in a liquid is directly proportional to the equilibrium pressure of the gas. That is to say, the solubility of gas in liquid increases proportionally with the increase of the partial pressure of the gas. The solubility of 100% hydrogen in liquid under the same conditions is 50 times that of 2% hydrogen. The solubility of hydrogen in water (0.017%) is slightly larger than that of nitrogen (0.013%). The solubility of hydrogen in fat (0.036%) is smaller than that of nitrogen (0.067%), which is about half of that of nitrogen. At 25℃, the solubility of hydrogen in ethanol is 0.089%, which is four times that of water. Although the solubility of hydrogen in water and fat is very small, it is very large in metals such as nickel, palladium and molybdenum. One volume of palladium can dissolve several hundred volumes of hydrogen.
Due to the low molecular weight of hydrogen, the permeability of hydrogen is very strong. At normal temperature, it can penetrate rubber and latex tube, but at high temperature, it can penetrate palladium, nickel, steel and other metal films. A filled hydrogen balloon often spends one night, and the next day it can't fly because of its air leakage. This is because hydrogen can drill through tiny holes in rubber that people can't see. Moreover, under high temperature and pressure, hydrogen can even pass through thick steel plates. Moreover, hydrogen has strong permeability, so when steel is exposed to hydrogen at a certain temperature and pressure, atomic hydrogen permeating into the lattice of steel causes embrittlement in slow deformation. This property of hydrogen brings great difficulties to the storage and transportation of hydrogen.
Hydrogen has high specific heat and good thermal conductivity. The thermal conductivity of hydrogen is 7 times greater than that of air. Under the same pressure, the specific heat of hydrogen is 13.6 times that of nitrogen and 2.72 times that of helium. Therefore, compared with other gases, hydrogen has stronger heat absorption and thermal conductivity.
The diffusion rate of hydrogen is fast. According to the law of gas diffusion, the diffusion rate of gas in liquid is inversely proportional to the square root of the molecular weight of the gas. In liquid or human tissues, the diffusion rate of hydrogen is 3.74 times that of nitrogen and 1.41 times that of helium (0.138).
Hydrogen transmits sound quickly. Under standard conditions, the sound transmission speed of air is 331 m/s, that of helium is 972 m/s, and that of hydrogen is 1286m/s. Therefore, if people breathe hydrogen, their voice will change obviously, and divers can also change their voice when they breathe hydrogen and oxygen.
Second, the chemical properties of hydrogen
The chemical property of hydrogen is stable at room temperature, and the stable chemical property is mainly determined by the strong covalent bond between the two hydrogen atoms that make up hydrogen.
Hydrogen is flammable. Under the conditions of ignition or heating, hydrogen can easily react with various substances. When pure hydrogen is ignited, it can burn quietly, emitting light blue flame, releasing heat and generating water. If a dry and cold beaker is covered on the flame, water droplets can be seen on the wall of the beaker. The concentration range at which hydrogen burns is 4-74%. If it is lower or higher than this concentration, it will not burn or explode even under high pressure. In oxygen environment, the combustion concentration of hydrogen ranges from 4% to 94%. When the oxygen concentration is lower than 4%, even under very high pressure conditions, the mixture of hydrogen and oxygen will not burn. People can use this characteristic of hydrogen for diving, and they can also use these characteristics of hydrogen to design equipment for safely breathing hydrogen.
Hydrogen has reducibility. The chemical property of hydrogen is active, and it reacts with oxygen to produce water, which is prone to combustion and explosion. Combustibility is also the embodiment of the reducibility of hydrogen, which is determined by the nature of hydrogen reducing oxygen. Hydrogen can react not only with oxygen, but also with oxygen in some compounds. For example, by passing hydrogen through hot copper oxide, red metallic copper can be obtained, while water is generated. In this reaction, hydrogen takes the oxygen from copper oxide and generates water; Copper oxide loses oxygen and is reduced to red copper, which proves that hydrogen has reducibility and is a good reducing agent. Hydrogen can also reduce other metal oxides, such as tungsten trioxide, ferroferric oxide, lead oxide and zinc oxide.
Although hydrogen has reducibility, it does not mean that hydrogen has the same properties in solution or organism. For example, the combustion of hydrogen and oxygen requires the concentration of hydrogen to be above 4% and the ignition point to be 400℃. In human environment, even in pure hydrogen environment, the dissolved concentration of hydrogen is only 1.8%. However, since the temperature of the body is only 37℃, this condition is far from the condition where hydrogen and oxygen react, so hydrogen and oxygen cannot react in human body. This is the important reason why people have used hydrogen as physiological inert gas for a long time.
Hydrogen is not only reductive, but also oxidizing. Hydrogen is a diatomic molecule covalently formed by hydrogen atoms, and each hydrogen atom can obtain one electron to form negative hydrogen ions. This situation can be seen in the reaction with strongly reducing metals, and its effect is similar to chlorine. In this kind of reaction, hydrogen is an oxidant, which can oxidize metals into metal ions. Strictly speaking, the product of the reaction between hydrogen and metal is hydride, which is characterized by strong reducibility and is very easy to react with water to release a large amount of hydrogen.