Identify the industrial source of ethylene from the cracking of some of the fractions from the refining of petroleum
Several forms of cracking are possible. However, simply learning catalytic cracking is adequate. Be prepared to write at least one equation to demonstrate this.
Ethylene is certainly one of the most useful products derived from the refining of petroleum. As such, a process known as catalytic cracking is often used to break down the higher molecular weight hydrocarbons into more useful, lower molecular weight hydrocarbons such as ethylene.
In the process of cracking, special catalysts called zeolites made of inorganic compounds are used. These zeolites are porous, such that many cavities exist within the structures, thereby increasing their surface area and thus effectiveness. Zeolites are typically made of compounds of aluminium, oxygen and silicon.
Catalytic cracking requires atmospheric pressure, an absence of air, and temperatures of approximately 500◦C. Long hydrocarbon chains are repeatedly broken down into smaller chains, typically one alkane and one alkene, until the desired product such as ethylene is created.
Breaking down a hydrocarbon chain into the smaller products of decane and ethylene:
Remember- The purpose of catalytic cracking is to produce ethylene, which holds unlimited potential in the petrochemical (plastics) industry.
Identify that ethylene, because of the high reactivity of its double bond, is readily transformed into many useful products
The crux of this dotpoint is the presence of the double bond within the ethylene molecule. Make sure you understand what the terms ‘saturated’ and ‘unsaturated’ mean, and try to understand why the double bonds are so reactive, not simply that they are. An explanation of electronegativity is provided below to help with this.
A hydrocarbon can be either saturated or unsaturated. A saturated hydrocarbon contains only single bonds, and no more atoms can be added to it. In contrast, an unsaturated hydrocarbon can have double, or even triple bonds, giving the possibility of further atoms or molecules joining the existing hydrocarbon chain.
Unlike alkanes, alkenes are unsaturated, as they contain a double bond which readily allows them to undergo addition reactions. The double bond of alkenes such as ethylene is inherently more unstable than a single bond, and thus breaks relatively easily to bond with other atoms and/or molecules.
In addition, the double bond present in ethylene is also a site of high electron density. Thus elec- tronegative species such as halogens readily react with ethylene
Electronegativity is a property which describes the strength of an atom’s pull on electrons. Halo- gens are highly electronegative, and thus an area of high electron density would naturally attract a halogen.
Remember- The high electron density of the double bond present within ethylene that the molecule becomes such a useful building block in the petrochemical industry.
Identify data, plan and perform a first-hand investigation to compare the reactivities of appropriate alkenes with the corresponding alkanes in bromine water
The bromine water experiment is relatively simple, demonstrating the differences between the reactivities of alkanes and alkenes through observations. The trick is to identify which solutions decolourise the brown-coloured bromine water.
- Pour 10mL of bromine water each into two small
- Using a dropper bottle, place a few drops of hexane into one of the beakers, noting its Stir gently with a glass stirring rod.
- Repeat with a dropper bottle of
- Repeat the procedure with a variety of alkanes and With solutions that alkanes have been added to, place them near a bright window and observe the effect over time.
Alkanes do not react with bromine water, meaning that the brown solution does not decolourise. However, the presence of ultraviolet light will result in a reaction, thus decolourising will occurring. Although this reaction may be hard to see, as the reaction can be somewhat slow, it does still occur.
Below are examples of reactions between an alkane and bromine water.
Due to the reactivity of the double bonds in alkenes, alkenes will decolourise the brown-coloured bromine water.
Remember- The presence of UV light will decolourise the bromine water solution if an alkane is added. If an alkene is used, UV light is not required.
Identify that ethylene serves as a monomer from which polymers are made
Ethylene may undergo addition or substitution reactions with other monomer units to form polymers such as polyethylene, polyvinyl chloride (PVC), polystyrene, polypropylene, as well as many others. As such, ethylene is a monomer from which polymers are made.
One example is the formation of polyethylene. This is achieved through the breaking of the double bond so that each monomer unit can now attach to one another to form a chain of monomer units
– a polymer.
Remember- Due to the high reactivity of the double bonds, ethylene serves as an extremely versatile monomer in the polymerisation of products such as polyethylene, PVC, polystyrene, and many other polymers.
Identify polyethylene as an addition polymer and explain the meaning of this term
Although these dotpoints introduce polymerisation with the process of addition polymerisation, it is important to remember that other processes such as condensation polymerisation also exist. A brief explanation of both is provided below.
An addition polymer is a polymer which is formed by the joining of individual monomers without the loss of any atoms. This differs from a condensation polymer, where a small molecule is usually removed from the chain for every monomer unit present. Polyethylene is one such example of an addition polymer as the double bonds present in the monomer unit ethylene allow for the addition of many ethylene molecules to form polyethylene.
−− −> (- − -)n
This may be hard to visualise, so take full advantage of any molecular modelling kits your school may have in order to understand how the polymers are formed.
Remember- Polyethylene is an addition polymer, formed as monomer units join together. This is not to be confused with condensation polymerisation, as there is no loss of any atoms or molecules.
Outline the steps in the production of polyethylene as an example of a commercially and industrially important polymer
There are two main processes in the production of polyethylene, each producing a distinct product which reflects the process used. Learn each process well, noting that the density of each product is resultant upon the structure of the polymer determining by the process.
In addition, be prepared to relate the use of each product to their characteristics, such as low or high density. Examples of LDPE products include trays and containers, as well as flexible and pliable components. Examples of HDPE products include plastic bags and pipes as well as plastic bottles.
Low-density polyethylene (LDPE)
For the production of LDPE, pressures of approximately 2000 atmospheres and temperatures of 300◦C are used along with an organic peroxide called an ‘initiator’. This process, known as the older gas phase as well as free radical polymerisation relies upon the initiator to open the double bonds in the monomer units, which then combine. This process results in alkyl groups periodically branching out where hydrogen atoms are usually present, producing a polymer that is low in density.
High-density polyethylene (HDPE)
The Ziegler-Natta process is used for the production of HDPE, where pressures about 20atm and temperatures of approximately 60◦C are used along with a catalyst usually made from titanium chlo- rides and trialkyl aluminium compounds. This allows for surface polymerisation to occur, producing closely-packed polytethylene molecules without the presence of alkyl branches. As such, HDPE is considerably denser than LDPE.
Analyse information from secondary sources such as computer simulations, molecular model kits or multimedia resources to model the polymerisation process
This dotpoint is useful for students unable to see the reactions taking place simply through the chemical equations. More importantly, it is useful to note the benefits and limitations of models, as this question does appear from time to time in various papers. For this reason, a list of the benefits and limitations of modelling is provided below.
- Provides a physical representation of the type and quantity of atoms involved in a molecule
- Demonstrates the difference between the various bonds in a molecule
- Provides a simple representation to aid understanding
- Relative sizes of, and distances between the atoms are unrealistic
- The dynamic nature of various molecules and their bonds is not shown
- Oversimplifies the model
Remember- On top of noting these benefits and limitations, also refer to dotpoint 1.1.5 on page 4 for further information regarding the addition and condensation polymerisation processes.
Identify the following as commercially significant monomers: vinyl chloride and styrene- by both their systematic and common names
Describe the uses of the polymers made from the above monomers in terms of their propertiesThe polymerisation of the monomers vinyl chloride and styrene yield polyvinylchloride and polystyrene respectively. Each polymer is of great commercial significance due to the properties they display.
Remember- Vinyl Chloride is also known by its systematic name of chloroethene and polyvinyl chloride is referred to as polychoroethene. Styrene can be known by both phenylethylene and ethenylbenzene. By the same token, polystyrene may be referred to as polyphenylethene as well as polyethnylbenzene.
Always relate the uses of the polymer with its properties. State one property, such as the water resistant nature of PVC, and then relate this to its use as raincoats and shower curtains. Simply listing properties followed by a list of users will not get you the best possible marks
Polyvinylchloride is both water and flame resistant, as well as relatively durable as it does not readily react with many chemicals. It is also rigid, strong, and does not conduct heat or electricity. As a result of these properties, PVC is commonly used for insulation and drain pipes, as well as raincoats and shower curtains. This range of PVC products, from thin films to rigid items, demonstrates the versatility of PVC.
Polystyrene is an effective heat, cold, and electrical insulator. When these characteristics are taken into consideration along with the ability for gas to be blown into the polymer (Producing a rigid and low-density material), uses such as insulators and packaging are self-evident. Polystyrene is also not chemically reactive, allowing for safe use in plates and foam cups. Because polystyrene has few crystals, it can also be made transparent, thus enabling uses such as CD cases and various containers.