C-Chain Making & Breaking Study Booklet Repeat Unit Name : Class: 1
date By the end of this topic this is what I should know. Alkane molecules can be straight or branched. Molecules that have the same molecular formulae but different structures are called isomers During the process of cracking, large less useful molecules get broken down into smaller more useful molecules. Catalytic cracking uses silica (silicon oxide) or alumina (aluminium oxide) as the catalyst and requires a temperature in the range of 600 700 C Cracking can be classified as a thermal decomposition reaction, since heat (and a catalyst) is required to break down molecules. Cracking of hydrocarbons always produces alkene molecules (as well as smaller alkane molecules) Alkenes are hydrocarbons with the general formula C n H 2n. They are unsaturated and have at least one double bond between two carbon atoms. They form a homologous series called the alkenes (see glossary) Alkenes are more useful than alkanes because they re more reactive. It s that double bond that makes them more reactive. Recall a test for unsaturation. E.g. add bromine water and it changes from orange to colourless. Understand how and why alkenes undergo addition reactions Recognise another example of an addition reaction - addition polymerisation. In this reaction many monomer molecules join together to make a long molecule called a polymer. Recognise the monomer needed to make a polymer, when given the displayed formula of the polymer. understand that some polymers, such as nylon, form by a different process called condensation polymerisation understand that condensation polymerisation produces a small molecule, such as water, as well as the polymer Many polymers are not biodegradable, so they are not broken down by microorganisms and this can lead to problems with waste disposal. Although bromine water is an excellent test for alkenes, alkanes also react with bromine - more slowly and only in the presence of UV light. A substitution reaction takes place 2
Chain Making and Breaking Glossary Isomer Alkane Alkene Homologous Series Saturated Unsaturated Covalent Bond Cracking Molecular formula Displayed Formula Isomers are compounds with the same molecular formulae but are structurally different in some way the atoms are arranged differently. They, therefore, have a different displayed formula A hydrocarbon which is saturated and has the general formula C n H 2n+2. Alkanes make good fuels. A hydrocarbon which is unsaturated. There is one double bond between two carbon atoms. Alkenes are more reactive and, therefore, more useful than alkanes. A family of compounds with a similar general formula (e.g. C n H 2n+2 ), possessing similar chemical properties due to the presence of the same functional group (e.g. they might all have a C=C double bond). Each member differs from the next by a CH 2 unit Contains only C-C single bonds Contains one or more C=C double bonds A chemical link between two atoms in which electrons are shared between them. The atoms in alkanes and alkenes are held together by this type of bond. You can have single, double and even triple covalent bonds. The process used to break up large alkane molecules into smaller more useful molecules. A high temperature and a catalyst (aluminium oxide) is required: Shows the total number of atoms present in a molecule of the compound, e.g. C 3 H 8 Shows all the bonds and atoms present, Feedstock Addition Reaction Polymer Monomer Polymerisation Addition Polymer Condensation Polymer Biodegradable e.g. Starting material for making all sorts of useful chemicals. The joining of two molecules, one of which has a C=C double bond, to form a single product molecule A substance made of long molecular chains, built up from many small molecules The small molecule used to build a polymer molecule The chemical reaction that takes place when many monomers link together to form a long chain molecule A polymer made from monomers containing C=C double bonds. There is just one product the polymer A polymer formed from a reaction that leaves behind a small molecule, often water so there are two products Can be broken down by micro-organisms 3
Straight Not Good Enough! Many of the molecules in the gasoline fraction of crude oil have straight chains. Straight chain molecules have a large surface area and burn rapidly in the air at high temperatures. 1. Give the molecular formula of this alkane molecule.. 2. What s this alkane called?.. If petrol contained only straight chain hydrocarbons, it would burn too quickly for the engine to work efficiently. How can we slow down the burn? One way is isomerisation. Isomers are compounds with the same molecular formulae but the atoms are arranged differently As soon as crude oil has been through fractional distillation, the gasoline fraction is piped to another part of the refinery where some of the straight chain molecules are converted to branched chain molecules. The straight chain molecules are isomerised. This requires high temperatures and a catalyst. Hydrocarbons molecules that are branched have less surface area so are less likely to collide with oxygen molecules in a given time. Therefore, a fuel made from branched hydrocarbons will burn more slowly than a fuel made from unbranched hydrocarbons. Example of isomerisation: when pentane is converted to methylbutane: Heat, catalyst pentane methylbutane How many isomers of pentane, C 5 H 12 can you make with molymods? Draw the displayed formula of each isomer below: 4
Problems of meeting demand Cracking is the name given to the breaking up of large hydrocarbon molecules into smaller and more useful bits. This is achieved by using high pressures and temperatures without a catalyst, or lower temperatures and pressures in the presence of a catalyst. Fractional distillation of crude oil Fractional distillation does not provide enough gasoline to meet world demand for automobile fuels. The solution is to take those fractions where this is less demand and split them into more useful smaller molecules. When alkanes are heated to high temperatures in the absence of air, they crack or split into smaller molecules. This is called thermal cracking. Alkanes can also be cracked at lower temperatures (about 600 o C) using a catalyst made from alumina or silica. This is called catalytic cracking or cat cracking Thermal cracking plant in China: When a large molecule is cracked, an alkane and an alkene is produced. Alkenes are hydrocarbons which contain a C=C double bond. Alkenes are more useful than alkanes because they are more reactive you can do more useful chemistry with them. It seems then that cracking not only produces smaller alkane molecules (which are needed for petrol), they also produce alkene molecules which are very useful chemical feedstocks. (A chemical feedstock is a useful starting material for manufacturing other chemicals) decane (large alkane molecule) ethene (an alkene) octane (component of petrol) crack + Q. What type of hydrocarbon is octane?. 5
Hydrocarbons The Alkene Family Name of alkene Molecular formula Number of Carbon atoms Number of hydrogen atoms Full Displayed Formula (Remember, alkenes have just one double bond per molecule) Ethene C 2 H 4 2 4 Propene Butene Pentene Hexene 6 GENERAL FORMULA OF AN ALKENE = Alkenes are another homologous series of hydrocarbons. You should now be familiar with two homologous series the alkanes and the alkenes. 6
Where do alkenes come from? Cracking Alkane molecules can be cracked using just heat (thermal cracking) or heat and a catalyst (catcracking) to produce alkenes. Alkenes are more useful than alkanes because they are more reactive. Being more reactive allows you to do more chemistry with them. This means a lot of useful substances can be made from alkenes such as polymers. Notice also, that a smaller alkane molecule is produced during cracking. It s just the right size for making gasoline (petrol). In fact nearly all the hydrocarbons in gasoline come from cracking larger hydrocarbon molecules. This is because fractional distillation alone cannot meet the world s demand for petrol (gasoline). The larger (less useful) molecules in crude oil are usually cracked into smaller more useful hydrocarbon molecules. Exercise: Working in a group and using molymods make a pentane molecule. Now break (crack) the molecule into two molecules make sure all the carbon atoms always form 4 bonds you ll need two floppy bonds after you ve cracked it Q. How many ways are there of cracking pentane into 2 molecules? 7
Cracking of Hydrocarbons 1. Cracking could also be classified as... CHOOSE FROM: A. thermal decomposition B. combustion C. oxidation D. hydrogenation 2. Why do petroleum companies carry out this process? 3. The word equation for the cracking of decane is: Decane Octane + Ethene Complete the balanced equation for this reaction below: C 10 H 22... + C 2 H 4 4a. Write a word equation for the cracking of decane but this time make something different 4b. Re-write your word equation as a balanced symbol equation 5. Give two conditions needed for this reaction to take place properly. 6. Complete the equation below: C 14 H 30 C 4 H 8 + C 2 H 4 +... 7. Write the molecular formula and name the hydrocarbon which was cracked in the below reaction.... C 3 H 6 + C 2 H 4 + C 7 H 16 8. In the following cracking reactions, (a) underline the alkane molecules and (b) circle the alkene molecules C 12 H 26 2C 5 H 10 + C 2 H 6 C 12 H 26 C 2 H 4 + 2C 3 H 6 + C 4 H 10 C 12 H 26 3C 2 H 4 + 2C 3 H 6 + H 2 8
Cracking Paraffin Oil bromine water C Using a wooden splint, try to light the gas (in a tube) and the oil (on a watch glass). Do they burn? D Add a few drops of bromine water to a tube of the gas. Stopper and shake. Repeat the test on 1cm 3 of oil. Think time 1. Why are the first bubbles of gas not collected?... Bromine water is a good test for a C=C 2. Why is the porcelain heated strongly before the oil is warmed? double bond. Simply add and watch it... decolourise instantly 3. What are your conclusions from having carried out a bromine water test?... 4. What evidence is there that the molecules in the product are smaller than those in the starting material? 9
Addition Reactions Addition is the reverse of cracking. Molecules add together to make one larger product molecule Make a model of ethene and bromine using the molecular model kits. Use two flexible bonds to make the C=C double bond Using the model try and simulate the reaction above and to try and decide which products are possible Repeat the simulation and keep a tally of the number of bonds that are broken in forming each product Deciding on the product 1. How many bonds had to be broken in order to transform your reactants into each product molecule? 2. What is required to break a chemical bond? 3. Suggest which product is the most likely to form. Explain your answer 4. Explain how modelling was helpful in helping you decide the likely product when ethene reacts with bromine 10
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Addition Reactions The bromine water test takes advantage of the fact that alkenes undergo addition reactions: bromine water is added to the sample. If there is a reaction (an addition reaction), the orange colour of the bromine water immediately disappears. Alkenes decolourise bromine water immediately. Alkanes do not As the name suggests, an addition reaction is when two molecules add together to make one product molecule. Alkenes can undergo addition because they are unsaturated - it s possible for more atoms to add to the molecule - across the C=C double bond Alkenes undergo addition reactions because they have a double bond which can open up and allow two extra atoms to bond. When no more atoms can add to a molecule it is said to be saturated 12
Addition Reactions Chemistry is all about looking for patterns. Study the equation opposite and make a note of how the bromine molecule adds to the ethene molecule. Then complete the equations below: 1. + Cl Cl 2. + Br Br Don t be alarmed! In chemistry we sometimes write the displayed formula or we sometimes write the molecular formula. In this question Br 2 is no different to Br Br in the previous question 3. + Br 2 4. + Cl 2 5. + H Br 7. + H 2 8. C 3 H 6 + HCl Write the product as a molecular formula 13
Addition Polymerisation Joining many small molecules called monomers into one long molecule of many units called a polymer Alkenes can add to themselves. Addition polymers are formed by many monomers (any molecule with a C=C double bond) adding together to form one long-chain molecule called a polymer e.g. ethene poly(ethene) or polythene Ethene is the monomer This is now one polymer molecule n means many the repeat unit goes in brackets heat, catalyst & pressure i.e. Many ethene molecules one polymer molecule with many repeat units STARTER If necessary come back to this question after you have completed the exercise opposite CHALLENGE: If necessary come back to this question after you have completed the exercise opposite (c) write a chemical equation, similar to the one near the top of this page which represents the polymerisation of vinyl alcohol. 14
Do you know your monomers from your polymers? Name of monomer Displayed formula of monomer Name of polymer Displayed formula of a polymer section (3 monomers combined) Formula of polymer Ethene Polyethene Vinyl chloride (chloroethene) Styrene Propene Tetrafluoroethene 15
The down side of Polymers Polymers are really useful, but they do have their downside. Place the following disadvantages beside the appropriate picture. Polymers are non-biodegradable; polymers can release toxic fumes when burned; polymers are bulky and will take up a lot of space in landfill sites Outline three ways of reducing the problems caused by waste polymers: 16
Case Study: Nylon Nylon was the first commercially successful synthetic polymer. It was first used commercially in a nylonbristled toothbrush (1938), followed more famously by women's stockings (1940). Nylon was intended to be a synthetic alternative for silk and was its replacement in many different products after silk became scarce during World War II. It replaced silk in military applications such as parachutes and flak vests, and was used in many types of vehicle tyres. Today, nylon fibres are used in many applications, including fabrics, bridal veils, carpets, musical strings and rope. Solid nylon is used for mechanical parts such as machine screws, gears and other low- to medium-stress components previously cast in metal. A nylon molecule is made of repeating units linked by amide bonds and is frequently referred to as polyamide. An amide group The Nylon Rope Trick Nylon is not an addition polymer like polythene. Nylon is made from two monomers which join together alternately: If A represents monomer A and B represent monomer B, then the formula of nylon can be written as: -A-B-A-B-A-B-A-B-A-B-A-B-A-B-A-B-A-B-A-B-A-B-A-B-A-B-A-B- Shorthand: Making nylon also produces other by-products such as water or HCl. This is why it cannot be called an addition polymer (see p.18) In the following demonstration, the two monomers are dissolved in two different solvents. As the two solvents are immiscible (don t mix), one monomer solution floats on top of the other, and where they meet (the interface) is where polymerisation takes place. The nylon polymer can be scooped out and wound around a glass stirring rod until one or other of the monomer solutions has run out. 17
Nylon an example of a Condensation Polymer Alkenes aren t the only molecules that can polymerise. Nature found a way long before we were on the scene. Examples of naturally occurring polymers are silk, wool, DNA and proteins. However, these polymers are known as condensation polymers. Condensation Polymers During condensation polymerisation, a polymer and a small molecule is produced often water, hence the name condensation. The monomers that are involved in condensation polymerisation are not the same as those in addition polymerisation. The monomers for condensation polymerisation have three main characteristics: Instead of a C=C double bond, these monomers have functional groups (a reactive group of atoms) A condensation polymer is usually made from two different monomers Each monomer has at least two functional groups. Chemical technology has tried to mimic nature. The nearest thing the chemical industry has got to making silk is nylon an example of a polyamide. How do we explain the reaction that makes nylon? We try and explain chemical reactions by modelling them. We draw molecules which represent the particles and try and mimic the reaction on the molecular scale. Consider the following two molecules. The rectangles simply represent the rest of the molecule most likely a hydrocarbon chain. Notice how each molecule has an identical functional group at each end. Monomer with two carboxylic acid groups (COOH) at each end Monomer with two amine (NH 2 ) groups at each end When the two monomers collide, the carboxylic acid group reacts with amine group. Where the functional groups interact, we have drawn the full displayed formula so you can clearly see which bonds break and which new bond is about to form The two molecules then join together. But it is not an addition reaction. Addition reactions only make one product. In this reaction we know that water is also produced. So when we join these two 18
molecules together, we have to eliminate a small molecule of water otherwise there will be just too many bonds around the carbon atom and nitrogen atom. This is why we call the reaction condensation. So far we have managed to join just two molecules together. However, a polymer is a very long molecule with many repeating units. We still need to join many more molecules together. We can do this provided a carboxylic acid (-COOH) group in one molecule is always able to line up with an amine (-NH 2 ) group in an adjacent molecule. Let s make our molecule longer by reacting another monomer: + Q1. Why can we not call this an addition reaction? Q2. Which monomer will be the next molecule to react and add on to the molecule above? Is it: (a) a dicarboxylic acid or (b) a diamine + H 2 O If you start off with n molecules of each monomer (where n is a large integer), the chemical equation will look like this: Q3. At the start of this reaction how many monomer molecules are there altogether? Q4. How many polyamide molecules are made in this reaction? Nylon is a polyamide 19
Can Bromine React with Saturated Hydrocarbons? Quick Recap: 1. What colour is bromine water? 2. What happens to the colour of bromine water if it is added to an alkene? 3. Are alkenes saturated or unsaturated? 4. Give the special feature in an alkene molecule which allows bromine to combine with it 5. Therefore, explain whether we would expect bromine water to react an alkane TEACHER DEMO AIM To investigate the effect of light on the reaction between bromine and an alkane Results {summarise your observations) Method Two test tubes contain 3cm 3 of hexane One of the test tubes is completely wrapped in foil 1cm 3 of bromine water is added to each test tube. The bromine water does not mix with hexane so two layers form with the denser aqueous layer at the bottom A rubber bung is placed on each test tube and the mixtures shaken so bromine dissolves from the water and into the hexane we want to see if bromine reacts with hexane Both test tubes are placed in sunlight or beside a very bright lamp for five minutes. The rubber bungs are loosened slightly in case of the build up of gas pressure After 5 minutes, the bungs are removed and a bottle of concentrated ammonia is brought close to the mouth of each test tube (concentrated ammonia solution is a good test for hydrogen halides such as hydrogen chloride and hydrogen bromide. If white smoke is seen then this is confirmation of the presence of a hydrogen halide, e.g. HCl or HBr) the foil is removed and the two test tubes compared Conclusions Give two pieces of evidence that a reaction took place in the test tube exposed to light Name the gas produced in the test tube that was exposed to light 20
Molecular Interpretation Alkanes do react with bromine but compared with alkenes, the reaction is much slower and requires the presence of ultra violet radiation to initiate the reaction. Q1. All reactions have to be initiated in one form or another. Use your knowledge of chemical energetics to suggest why The previous experiment should have shown you that hydrogen bromide gas is produced whenever bromine is added to an alkane in the presence of UV light. Let s attempt to write an equation for this kind of reaction. We ll begin with the simplest alkane, methane Q2. What is the molecular formula of methane? Q3. Write the first half of the chemical equation showing methane reacting with bromine (bromine s molecular formula is Br 2 ) Q4. Now we already know that one of the products was hydrogen bromide, HBr. One other product is produced. Now try and write the full equation. You should be able to guess the other product by inspecting the atoms that are still left over : Q5. Why can we not call this an addition reaction? The reaction is in fact a substitution reaction Q6. Which atoms substitute when bromine reacts with an alkane? Alkanes don t just react with bromine; they react with any element in Group VII. Complete the following substitution reactions. Assume all these reactions are exposed to UV light: Q7. CH 4 + Cl 2 CH 3 Cl +. Q8. CH 4 +.. CH 3 Br + HBr Q9. + Cl 2 C 2 H 5 Cl + HCl Q10. +.. C 6 H 13 Br +. This was the reaction carried out opposite Compare the reaction of bromine with alkanes and with alkenes. Think of some similarities and differences and complete the table: Reaction of bromine with an alkane Reaction of bromine with an alkene 21
Practice Questions 22
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10 ANSWERS to Practice Question 8A = 2 8B = 4 8C = 3 8D = 3 9A = 3 10(i) (di)amine (ii) Allow: 24