From a chemistry point of view, propane is a saturated hydrocarbon with typical alkane properties. However, in some areas of production, propane is understood as a mixture of two substances - propane and butane. Next, we will try to figure out what propane is, and why it is mixed with butane.
Molecule structure
Each propane molecule consists of three carbon atoms bonded to each other by simple single bonds, and eight hydrogen atoms. It has a molecular formula of C 3 H 8 . The – bonds in propane are nonpolar covalent, but in the – pair, carbon is a little more electronegative and slightly pulls the common electron pair onto itself, which means that the bond is covalent polar. The molecule has a zigzag structure due to the fact that carbon atoms are in a state of sp 3 hybridization. But, as a rule, they say that the molecule is linear.
The butane molecule contains four carbon atoms C 4 H 10 , and it has two isomers: n-butane (has a linear structure) and isobutane (has a branched structure). Often, they are not separated after receipt, but exist as a mixture.
Physical properties
Propane is a gas without color or any odor. It dissolves very poorly in water, but it is good in chloroform and diethyl ether. It melts at a temperature t PL = -188 ° C, and boils at t bales = -42 ° C. It becomes explosive when it reaches a concentration in air above 2%.
The physical properties of propane and butane are very close. Both butanes also have a gaseous state under normal conditions and are odorless. Practically insoluble in water, but interact well with organic solvents.
The following characteristics of these hydrocarbons are also important in industry:
- Density (ratio of weight to body volume). The density of liquid propane-butane mixtures is largely determined by the composition of hydrocarbons and temperature. With increasing temperature, volume expansion occurs, and the density of the liquid decreases. With increasing pressure, the volume of liquid propane and butane is compressed.
- Viscosity (the ability of substances in a gaseous or liquid state to resist shear forces). It is determined by the forces of adhesion of molecules in substances. The viscosity of a liquid mixture of propane and butane depends on temperature (with its increase, viscosity decreases), but the change in pressure does not significantly affect this characteristic. Gases, however, increase their viscosity with increasing temperature.
Being in nature and methods of obtaining
The main natural sources of propane are oil and gas fields. It is contained in natural gas (from 0.1 to 11.0%) and in associated petroleum gases. Quite a lot of butane is obtained in the process of oil rectification - its separation into fractions, based on the boiling points of its components. Of the chemical methods of oil refining, catalytic cracking is most important, during which a chain of high molecular alkanes breaks. In this case, propane forms about 16-20% of all gaseous products of this process:
CΗ 3 -CΗ 2 -CΗ 2 -CΗ 2 -CΗ 2 -CΗ 2 -CΗ 2 -CΗ 3 -> CΗ 3 -CΗ 2 -CΗ 3 + CΗ 2 = CΗ-CΗ 2 -CΗ 2 -CΗ 3
Large amounts of propane are formed during the hydrogenation of various types of coal and coal tar; they reach 80% of the volume of all gases formed.
The production of propane according to the Fischer-Tropsch method is also widespread, which is based on the interaction of CO and H 2 in the presence of various catalysts at elevated temperature and pressure:
nCO + (2n + 1) Η 2 -> C n Η 2n + 2 + nΗ 2 O
3CO + 7Η 2 -> C 3 Η 8 + 3Η 2 O
Industrial volumes of butane are also isolated during oil and gas processing by physical and chemical methods.
Chemical properties
The physical and chemical properties of propane and butane depend on the structural features of the molecules. Since they are saturated compounds, they are not characterized by addition reactions.
1. Substitution reactions. Under the influence of ultraviolet radiation, hydrogen is easily replaced by chlorine atoms:
CH 3 -CH 2 -CH 3 + Cl 2 -> CH 3 -CH (Cl) -CH 3 + HCl
When heated with a solution of nitric acid, the H atom is replaced by a NO 2 group:
Η 3 -Η 2 -Η 3 + ΗNO 3 -> Η 3 -Η (NO 2 ) -Η 3 + H 2
2. Cleavage reactions. When heated in the presence of nickel or palladium, two hydrogen atoms are split off to form a multiple bond in the molecule:
Η 3 -Η 2 -Η 3 -> Η 3 -Η = Η 2 + Η 2
3. Decomposition reactions. When a substance is heated to a temperature of the order of 1000 ° C, a pyrolysis process occurs, which is accompanied by the breaking of all chemical bonds in the molecule:
C 3 H 8 -> 3C + 4H 2
4. Combustion reactions. These hydrocarbons burn with a non-smoking flame with the release of a large amount of heat. What is propane is known to many housewives who use gas stoves. During the reaction, carbon dioxide and vaporous water are formed:
C 3 H 8 + 5O 2 -> 3CO 2 + 4H 2 O
Propane combustion under conditions of oxygen deficiency leads to soot and the formation of carbon monoxide molecules:
2C 3 H 8 + 7O 2 -> 6CO + 8H 2 O
C 3 H 8 + 2O 2 -> 3C + 4H 2 O
Application
Propane is actively used as fuel, since 2202 kJ / mol of heat is released during its combustion, which is a very high indicator. In the process of oxidation, many substances necessary for chemical synthesis are obtained from propane, for example, alcohols, acetone, carboxylic acids. It is necessary to obtain nitropropanes used as solvents.
As a propellant it is used in the food industry, it has the code E944. In a mixture with isobutane, it is used as a modern refrigerant that does not harm the environment.
Propane-butane mixture
It has many advantages over other fuels, including natural gas:
- high efficiency;
- easy return to a gaseous state;
- good evaporation and burning at ambient temperature.
Propane fully meets these qualities, but butanes evaporate somewhat worse when the temperature drops to -40 ° C. Additives help to fix this, the best of which is propane.
The propane-butane mixture is used for heating and cooking, in gas welding of metals and their cutting, as fuel for vehicles and for chemical synthesis.