Carbon dioxide, a carbon oxide compound with a chemical formula of CO2 and a Molar mass: 44.01 g/mol. It is a colorless and odorless (no smell) at normal temperature and pressure.
Carbon dioxide with a slight acidity is also a common greenhouse gas and one of the components of air (0.03%-0.04% of the total volume of the atmosphere).
About Carbon Dioxide
In terms of physical properties, carbon dioxide has a melting point of -78.5 ° C, a boiling point of -56.6°C, a density greater than air density (under standard conditions), and is soluble in water.
In terms of chemical properties, It has high thermal stability (only 1.8% decomposition at 2000°C), and cannot be burned, usually it does not support combustion, is an acidic oxide, has the permeability of acidic oxides, Since carbonic acid is formed by reaction with water, it is an acid anhydride of carbonic acid.
It can generally be obtained by calcining limestone at high temperature or by reacting limestone with dilute hydrochloric acid, mainly for refrigerating perishable foods (solid), as a refrigerant (liquid), for making carbonated soft drinks (gaseous) and as a solvent for homogeneous reaction ( Supercritical state) and so on.
The shape of the CO2 molecule is linear and its structure was once considered to be: O = C = O.
However, the carbon-oxygen bond in the CO2 molecule is 116 pm, which is between the carbon-oxygen double bond (the bond length is 124 pm) and the carbon-oxygen triple bond (the bond length is 113 pm), so the carbon-oxygen bond in the CO2 has a certain degree.
Modern scientists generally believe that the central atomic carbon atoms of CO2 molecules are sp hybridized, and the two sp hybridization orbitals overlap with the 2p orbitals (containing one electron) of two oxygen atoms to form two σ bonds, which are perpendicular to each other. The p orbitals respectively form two large π bonds with p orbitals parallel to the two oxygen atoms.
Physical and chemical properties
Carbon dioxide is a colorless and odorless gas at normal temperature and pressure and is soluble in most organic solvents such as water and hydrocarbons. The relevant physical constants are as follows:
|nature||Condition or symbol||unit||data|
|Melting point||Celsius (°C)||-78.5|
|Boiling point||527kPa||Celsius (°C)||-56.6|
|Relative density||-79 ° C, water = 1||1.56|
|Relative vapor density||Air=1||1.53|
|Saturated vapor pressure||-39 ° C||Kilowatt (kPa)||1013.25|
|Critical temperature||Celsius (°C)||31.3|
|Critical pressure||Megapascal (MPa)||7.39|
|Octanol/water partition coefficient||0.83|
|Molar refractive index||6.98|
|Viscosity||21 ° C, 5.92 MPa||Millipascal seconds (mPa·s)||0.0697|
|Evaporation heat||sublimation||Kilojoules per mole (kJ/mol)||25.25|
|Heat of fusion||Kilojoules per mole (kJ/mol)||8.33|
|Generating heat||Kilojoules per mole (kJ/mol)||394.40|
|Specific heat capacity||20 ° C, constant pressure||KJ per kilogram Kelvin [kJ/(kg·K)]||2.8448|
|Vapor Pressure||5.9 to 14.9 ° C||Megapascal (MPa)||4.05~5.07|
|Thermal conductivity||12 to 30 ° C||Watts per meter Kelvin [W/(m·K)]||0.10048～83.74×10 -7|
|Body expansion coefficient||-50～0°C||Every Kelvin (K -1 )||0.00495|
|0～20oC||Every Kelvin (K -1 )||0.00991|
|Molar volume||Milliliter per mole (mL/mol)||44.7|
|Surface Tension||Dyne per centimeter (dyne/cm)||3.4|
|Polarizability||10 -24 cm 3||2.76|
Carbon dioxide is one of the carbon oxides. It is an inorganic substance that is non-flammable and usually does not support combustion. It is non-toxic at low concentrations.
It is also carbonic anhydrides, are an acidic oxide, the acidic oxide having continuity, wherein the carbon valence of +4 valence, the valence of carbon at the highest, so the reduction of carbon dioxide has no oxidation, but oxidation Not strong.
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The generality of acidic oxides
React with water
Carbon dioxide can be dissolved in water and react with water to form carbonic acid, while unstable carbonic acid is easily decomposed into water and carbon dioxide. The corresponding chemical reaction equation is:
Reaction with basic oxides
Under certain conditions, carbon dioxide can react with basic oxides to form corresponding salts, such as:
Reacting with a base
1 reacts with calcium hydroxide
The addition of carbon dioxide to the clarified lime water will clarify the clarified lime water and produce a calcium carbonate precipitate (this reaction is often used to test carbon dioxide). The corresponding chemical reaction equation is:
When carbon dioxide is excessive, calcium bicarbonate is formed.
The second step:
The general formula:
Since the calcium bicarbonate has a large solubility, it is found that the precipitate gradually disappears when carbon dioxide is introduced into the turbid lime water for a long time.
2 react with sodium hydroxide
Carbon dioxide will degrade caustic soda, and the corresponding chemical reaction equation is:
When carbon dioxide is excessive, sodium bicarbonate is formed.
The second step:
The general formula:
- Weak oxidizing
- Carbon single substance reduction
Under high-temperature conditions, it can react with carbon to form carbon monoxide. The corresponding chemical reaction equation is:
- Magnesium reduction
Under ignited conditions, the magnesium strip can be burned in carbon dioxide, and the corresponding chemical reaction equation is:
- Hydrogenation reduction
Carbon dioxide and hydrogen react under the action of a catalyst to form a series of reactions such as methanol, carbon monoxide, and methane. The chemical reaction equations of several reactions are:
- Electrochemical reduction
Electrochemical reduction of carbon dioxide is a process in which is reduced in the anode of an electrolytic cell by electric energy to oxidize hydroxide ions in the electrolytic cell to oxygen.
Since the activation energy required for reducing carbon dioxide is high, this process requires a certain high voltage.
It can be realized, and the degree of hydrogen evolution reaction occurring at the cathode increases with the increase of voltage, which inhibits the reduction of carbon dioxide.
Therefore, the efficient reduction of carbon dioxide requires a suitable catalyst, so that the electrochemical reduction of carbon dioxide is often an electrocatalytic reduction. Process. The simple mechanism of this process is:
(1) Electrolytic cell cathode: In the initial stage, carbon dioxide is adsorbed on the surface of the cathode catalyst to form an intermediate product (Reaction formula 1); then electrons are transferred under the action of the potential difference between the two electrodes, and the number of transitions may be It is 2, 4, 6, 8, and 12. The reduction product may be carbon monoxide, formate, formic acid, etc. depending on the number of electron transfer (reaction formula 2-4).
(2) Electrolytic cell anode: Hydrogen evolution reaction occurs in the aqueous solution to generate hydrogen (Reaction formulas 5 and 6).
- React with peroxide
And carbon dioxide to be sodium peroxide (of Na₂O₂) reaction of sodium carbonate (of Na₂CO.’S.3) and oxygen (O₂), the respective chemical equation is:
- React with format reagents
Under acidic conditions, carbon dioxide can be reacted with a format reagent in anhydrous diethyl ether to form a carboxylic acid. The corresponding chemical reaction equation is:
Description: In the formula, R is an aliphatic hydrocarbon group or an aromatic hydrocarbon group, X is a halogen, and Etheranhydrous means anhydrous diethyl ether.
- Insertion reaction with epoxy compounds
Carbon dioxide can react with epoxy compounds under electrocatalysis to form cyclic carbonates. The corresponding chemical reaction equation is:
- Preparation of diamond (displacement reaction)
At 440 °C (713.15 K) and 800 atmospheres (about 808 MPa), carbon dioxide can react with sodium metal to form a diamond. The corresponding chemical reaction equation is:
- Photosynthesis dark reaction
Carbon dioxide is involved in the dark reaction of photosynthesis. It is an indispensable raw material for photosynthesis of green plants. The reaction process involved is called “fixation of carbon dioxide”. The corresponding chemical reaction equation is:
Description: In the formula, C5 is ribulose 1,5-diphosphate, and 2C3 is 2 molecules of 3-phosphoglycerate.
Carbon Dioxide Cycle
Carbon dioxide gas is a part of the atmospheric composition (0.03%-0.04% of the total volume of the atmosphere). It is abundant in nature and has the following main routes:
1 organic matter (including animals and plants) is decomposed, fermented, rotted, and deteriorated. Carbon dioxide is released during the process.
2 In the process of burning petroleum, paraffin, coal and natural gas, carbon dioxide should also be released.
3 Oil and coal will also release carbon dioxide in the process of producing chemical products.
4 All feces and humic acid can release carbon dioxide during fermentation and ripening.
5 All animals should breathe oxygen to exhale carbon dioxide during the breathing process.
- Calcination method
The carbon dioxide gas generated during high-temperature calcination of limestone (or dolomite) is washed, decontaminated, and compressed to produce gaseous carbon dioxide:
- Fermentation gas recovery
The carbon dioxide gas produced during the ethanol fermentation process is washed, decontaminated, and compressed to obtain carbon dioxide gas.
- By-product gas recovery method
During the production process of ammonia, hydrogen and synthetic ammonia, there is often a process of decarburization (that is, removal of carbon dioxide from the gas mixture), so that the carbon dioxide in the mixed gas is subjected to pressure absorption and desorption under reduced pressure to obtain high-purity carbon dioxide gas.
- Adsorption expansion method
Generally, by-product carbon dioxide is used as raw material gas, high-purity carbon dioxide is extracted from the adsorption phase by adsorption expansion method, and the product is collected by a cryogenic pump, it can also be obtained by adsorption rectification method, and the adsorption distillation method uses silica gel, 3A molecular sieve and activated carbon as adsorbent. remove some impurities, after rectification can produce high purity carbon dioxide products.
- Charcoal kiln method
Carbon dioxide is obtained by purifying the gas obtained by carbon kiln gas and methanol cracking.
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- Preparation of marble and dilute hydrochloric acid
Laboratory production of carbon dioxide, marble and dilute hydrochloric acid. Both types of soda are not used, and the speed is too fast to control.
Without sulphuric acid, magnesium salts are not as good as calcium salts. Nitric acid is easy to decompose when it is light, and it is not flammable when the bottle is full.
Marble or limestone (main component CaC03₃) and dilute hydrochloric acid. (Laboratory carbon dioxide, marble and dilute hydrochloric acid)
Preparation apparatus set
Solid-liquid is not heated (as shown).
Since the carbon dioxide density is larger than that of air, it is soluble in water and can react with water, so the upward air method is adopted.
The generated gas was passed to clarified lime water, and the lime water became cloudy, which proved that the gas was carbon dioxide.
Use a burning strip of wood to be in the mouth of the gas cylinder (not into the bottle). If the flame is extinguished, the proof is full.
1. Hydrogen chloride (HCl) gas which may be volatilized during the reaction and the hydrogen chloride gas in the generated gas can be removed by a saturated sodium hydrogen carbonate (NaHCO3) solution.
2. If necessary, the water vapor in the generated gas may be removed by using a gas scrubbing bottle containing concentrated sulfuric acid.
3. Cannot be reacted with calcium carbonate and concentrated hydrochloric acid, the reason: concentrated hydrochloric acid is easy to volatilize a large amount of hydrogen chloride gas, so that sodium bicarbonate cannot be completely removed, the purity of carbon dioxide produced will decrease.
4. Is marble (CaCO3 in the laboratory reaction) dilute hydrochloric acid and carbon dioxide from the system.
5. Not use of Na ₂ CO.’S₃ (soda) and NaHC03 instead of CaC03 (baking soda) with acid to the reaction system to take carbon dioxide, reasons: of Na₂ CO.S₃ and NaHC03 speed too fast with the hydrochloric acid reaction, carbon dioxide is generated quickly Escape, difficult to control, and not easy to operate. (Do not use both sodas, it is difficult to control too fast)
6. Cannot replace with dilute sulfuric acid, hydrochloric acid, reasons: marble with dilute sulfuric acid (CaC03) reactions produce the calcium sulfate dissolved in water micro (CaS04) precipitates covering the surface of the marble, hindered the progress of the reaction, The reaction is very slow. (without sulphuric acid hydrochloric acid)
7. Not use of MgCO₃ (magnesium) on behalf of CaC03 (calcium salt), the reason: although of MgCO₃ with hydrochloric acid and CaC03 with a hydrochloric acid reaction similar, but because of MgCO₃ fewer sources, as CaC03 readily available and inexpensive. (Magnesium salt is not as good as calcium salt)
8. Cannot replace hydrochloric acid with nitric acid, the reason: nitric acid is easy to decompose
If in place of hydrochloric acid and nitric acid, the resulting CO. S₂ in will be a small amount of NO₂ and O. In addition, the price of nitric acid is more expensive than hydrochloric acid, so it is usually not necessary to replace hydrochloric acid with nitric acid. (Nitric acid is easy to decompose)
9. Because carbon dioxide can extinguish the fire, the burning match can be placed in the gas cylinder mouth. If the flame is extinguished, it proves that carbon dioxide is full of gas cylinders. (Identify matches can’t burn)
- Heating to decompose sodium bicarbonate
The sodium hydrogen carbonate was sufficiently dried, placed in a hard glass tube, filled with glass wool at the nozzle, sealed, and evacuated with an air pump. Then, heating is carried out to decompose sodium hydrogen carbonate.
The initial carbon dioxide can be released. The gas generated by the decomposition is introduced into an ice-cooled conduit to condense the water vapor in the gas, and the gas is successively introduced into a U-shaped tube each containing calcium chloride and phosphorus pentoxide to be dried. At 100 °C, the decomposition pressure of sodium hydrogen carbonate was 97.458 kPa, and at 120 ° C was 166.652 kPa.
When baking soda (mainly sodium bicarbonate) and white vinegar are mixed together, a metathesis reaction occurs, and carbon dioxide gas is evolved. The corresponding chemical reaction equation is:
Human body health
Studies have shown that when the concentration of it in the air is less than 2%, there is no obvious harm to humans. Exceeding this concentration can cause damage to human respiratory organs.
Under normal circumstances, This gas is not a toxic substance, but when the concentration of carbon dioxide in the air exceeds a certain limit. At the time, the body will be poisoned, and high concentrations of carbon dioxide will cause suffocation.
Animal experiments have shown that in the air with normal oxygen (20%), the higher the concentration of carbon dioxide, the higher the mortality rate of animals. At the same time, pure carbon dioxide causes animal death to be more rapid due to lower oxygen.
In addition, some people think that in the case of low oxygen, 8% to 10% of carbon dioxide can cause the death of humans and animals in a short time.
Principle of poisoning
High concentrations of carbon dioxide itself have stimulating and anesthetic effects and can cause hypoxia and suffocation in the body.
Mild: generally feel dizziness, headache, muscle weakness, body weakness, and another discomfort.
Moderate: Dizziness will have a tendency to fall to the ground, chest tightness, unbearable pain in the nose and throat, tight breathing, chest pressure, and suffocation, severe headache, tinnitus, muscle weakness, redness of the skin, elevated blood pressure, fast pulse And strong.
Severe: Sudden dizziness cannot support and fall to the ground, suffocation, difficulty breathing, palpitations, unconsciousness, coma, skin lips and nails bruising, blood pressure drop, weak pulse to reach, pupil dilated.
The light reflection disappears, the whole body is soft, the glottis expands, and the heartbeat stops until the death. Some symptoms such as lethargy and memory loss can be left after the acute phase.
1. Quickly remove the poisoned person from the high concentration of it to the fresh air, loosen the collar of the poisoned person, artificially assist the breathing to inhale oxygen as soon as possible, if necessary, treat with hyperbaric oxygen, rescue personnel should wear effective breathing Guard.
2. Injection of respiratory stimulants, secondary infection with antibiotics, the binding should be intravenous infusion of sodium bicarbonate or sodium lactate, limbs can try to take a larger dose of sedative, long-term high fever and convulsions can be used for sedative drugs, other such as lung Treatment of edema, cerebral edema, etc.
Before entering a working area containing a higher concentration of it, check whether the concentration of carbon dioxide in the air exceeds 2%.
If it exceeds, it is necessary to take effective safety measures, such as 1 for ventilation and detoxification, replacing workplace air, and making carbon dioxide concentration in the air.
Not more than 2%, 2 wearing a gas mask, self-priming catheter gas mask, oxygen breathing apparatus, and other common gas masks.
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Carbon dioxide is suitable for storage in a cool, ventilated, non-combustible warehouse, suitable for shipment in liquid or solid form.
When storing and transporting, please pay attention to the following points:
1 Keep away from fire and heat source, the storage temperature should not exceed 30 °C.
2 separate storage with easy (can) fuel, avoid mixing.
3 storage area should be equipped with leakage emergency treatment equipment.