construct word and balanced formulae equations of chemical reactions as they are encountered
There are three important steps involved:
1. Show all reactants and all products in the word equation. 2. Write the correct formula for each reactant and each product. 3. Balance the formula equation by placing coefficients (numbers) in front of formulas so that you have the same total number of each kind of atom on both the reactant side and the product side. Remember that in chemical reactions atoms are just rearranged, not created or destroyed.
identify the industrial source of ethylene from the cracking of some of the fractions from the refining of petroleum
Ethylene is manufactured from various fractions produced by the fractional distillation of crude oil. The fractions used are usually naptha and LPG (liquefied petroleum gas). The naptha fraction is the remainder of the petrol fraction not used for petrol. It consists of a complex mixture of C5 to C10 straight and branched chain molecules of alkenes, cycloalkanes and aromatic hydrocarbons (benzenes). The process to produce ethylene and other small chain hydrocarbons is called cracking.
Two main processes – steam (thermal) cracking and catalytic cracking. Steam cracking involves heating hydrocarbon feedstock to about 750 – 9000C in long coils inside a furnace. 3 main steps: INITIATION (formation of free radicals) - hydrocarbon chain is split at high temperatures into fragments. These fragments (radicals) are reactive as they have unpaired electrons. PROPAGATION (decomposition of free radicals) - free radicals decompose to produce smaller free radicals and release alkenes such as ethene and propene. TERMINATION - free radicals react with other free radicals to form hydrocarbon molecules.
e.g. Decane -( 2 pentyl radicals
Pentyl radical ( propyl radical + ethene
Propyl radical + propyl radical ( hexane
Catalytic cracking is used to crack large hydrocarbon molecules into smaller ones. Heavy crude oil is heated in the presence of a catalyst composed of a zeolite (aluminium silicate). The use of a catalyst allows a lower temperature (about 5000C). Cracking is also important for producing alkenes from alkanes.
e.g. decane ( ethylene +2-methylheptane
identify that ethylene, because of the high reactivity of its double bond, is readily transformed into many useful products
Alkenes are much more reactive than alkanes because of the presence of the double bond. The double bond is a site of high electron density and molecules that have electron-attracting centres readily combine with alkenes at this point. Thus highly electronegative atoms react readily with alkenes.
Alkenes often react with other molecules via an addition reaction. Common addition reactions involving ethylene include hydrogenation, halogenation and hydrohalogenation.
Hydrogenation of ethylene is the addition of hydrogen molecules across the double bond. This produces ethane.
Halogenation of ethylene is the addition of halogen molecules across the double bond. Addition of chlorine to ethylene produced ethylene dichloride or 1,2 –dichloroethane – a petrol additive. Addition of bromine forms 1,2 –dibromoethane – again a petrol additive.
Hydrohalogenation of ethylene involves the addition of a HX molecule where X is a halogen. Addition of HCl to ethylene produces chloroethane. Addition of HBr to ethylene produces bromoethane. These are both used as solvents and refrigerants.
Some of the useful products made from ethylene are:
|Product |Formula |Use | |polyethylene |(CH2)n |plastic | |ethylene oxide |(CH2)2O |steriliser | |ethanol |C2H5OH |solvent | |ethanoic acid...