Hydrocarbons Flashcards
What are the two methods by which alkanes can be produced?
Alkanes are hydrocarbons that can be produced by the addition reaction of hydrogen to an alkene or by cracking of longer alkane chains.
How can alkanes be produced from hydrogenation?
The addition reaction of alkanes with hydrogen is called hydrogenation. When hydrogen gas and an alkene are heated and passed over a finely divided Pt/Ni catalyst the addition reaction produces an alkane. The Pt/Ni catalyst is finely divided to increase its surface area and therefore increase rates of reaction.
How can alkanes be produced from cracking?
In cracking large less useful hydrocarbon molecules found in crude oil are broken down into smaller more useful molecules. The large hydrocarbon molecules are fed into a steel chamber and heated to a high temperature (500 degrees) celsius under crystals of aluminum silicate (Al2Si2O) moderately low pressure and then passed over an aluminum oxide (Al2O3) catalyst. The chamber does not contain oxygen to prevent combustion of hydrocarbon to water and carbon dioxide.
Are the reactions to produce alkanes endothermic or exothermic?
Cracking is endothermic and hydrogenation is exothermic.
What are the possible outcomes of cracking?
1.alkane + alkene + hydrogen
2. Alkane + C2H4
3. C2H4 + H2
What are the uses of cracking?
To meet supply and demand. To produce ethene which is an important compound used in the petrochemical industry.
Describe complete combustion of alkanes.
When alkanes are burnt in excess of oxygen, complete combustion will take place and carbon and hydrogen will be oxidized to carbon dioxide and water respectively.
Describe the incomplete combustion of alkanes
When alkanes are burnt only in a limited supply of oxygen, incomplete combustion will take place and not all the carbon is fully oxidized. The partially oxidized carbon will form carbon monoxide.
Describe carbon monoxide gas.
Carbon monoxide is a toxic gas as it will bind to hemoglobin in the blood which can then no longer bind to oxygen. As no oxygen can be transported around the body, The victims feel dizzy, lose consciousness and if not removed from carbon monoxide they can die. It is extremely dangerous as it is odorless and will not be noticed.
Which harmful pollution caused by burning hydrocarbons can take place in a car engine?
Incomplete combustion often takes place inside a car engine due to a limited supply of oxygen present. As well as releasing carbon monoxide road traffic also releases acidic nitrogen oxides. Cars also release unburned hydrocarbons often referred to as VOCs.
What are the effects of acidic nitrogen oxides?
Road traffic releases acidic nitrogen oxides mainly NO and NO2. These contribute to the problem of acidic rain. Acid rain can kill trees and aquatic animals in lakes. It also erodes limestone buildings and statues as well as corroding metals such as iron.
What are the effects of unburned hydrocarbons being released?
Cars also released unburned hydrocarbons often referred to as VOCs into the air. Some of these are carcinogens (can cause cancer) and can form PAN a contributor to photochemical smoke.
How can we reduce traffic emissions?
Motor vehicles can now be fitted with catalytic converters to reduce traffic emissions. This causes:
Oxidation of carbon monoxide to form carbon dioxide. Carbon dioxide is not toxic but a pollutant gas.
Reduction of nitrogen oxides to form nitrogen gas which takes place on the surface of the precious metal catalyst in catalytic converters. (2CO+2NO→2CO2 +N2).
Oxidation of unburnt hydrocarbons to form CO2 + H2O.
What type of substituition reactions do alkanes undergo?
Alkanes can undergo free radicle substituition in which hydrogen atoms get substitute by a halogen (Chlorine/Bromine). Ultraviolet light (sunlight) is needed for this reaction.
Describe the first step of alkane free radical substituiton.
Initiation step:The Cl-Cl or Br-Br bond is broken by energy from the uv light. This produces two radicles in a homolytic fission reaction.
