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C-Chain Making & Breaking Study Booklet

Name :………………………………………………… Class:……………

Repeat Unit

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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 CnH2n. 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

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“Chain Making and Breaking” Glossary

Isomer 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

Alkane A hydrocarbon which is saturated and has the general formula

CnH2n+2. Alkanes make good fuels.

Alkene A hydrocarbon which is unsaturated. There is one double bond

between two carbon atoms. Alkenes are more reactive and,

therefore, more useful than alkanes.

Homologous

Series

A family of compounds with a similar general formula (e.g. CnH2n+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 CH2 unit

Saturated Contains only C-C single bonds

Unsaturated Contains one or more C=C double bonds

Covalent Bond 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.

Cracking The process used to break up large alkane molecules into smaller

more useful molecules. A high temperature and a catalyst

(aluminium oxide) is required:

Molecular

formula

Shows the total number of atoms present in a molecule of the

compound, e.g. C3H8

Displayed

Formula

Shows all the bonds and atoms present,

e.g.

Feedstock Starting material for making all sorts of useful chemicals.

Addition

Reaction

The joining of two molecules, one of which has a C=C double bond,

to form a single product molecule

Polymer A substance made of long molecular chains, built up from many

small molecules

Monomer The small molecule used to build a polymer molecule

Polymerisation The chemical reaction that takes place when many monomers link

together to form a long chain molecule

Addition

Polymer

A polymer made from monomers containing C=C double bonds.

There is just one product – the polymer

Condensation

Polymer

A polymer formed from a reaction that leaves behind a small

molecule, often water – so there are two products

Biodegradable Can be broken down by micro-organisms

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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.

How many isomers of pentane, C5H12 can you make with molymods? Draw the displayed formula of

each isomer below:

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:

pentane methylbutane

1. Give the molecular formula of this alkane molecule ……..

2. What’s this alkane called? ……………..

Heat,

catalyst

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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.

Thermal cracking plant in China:

decane (large alkane molecule) ethene (an alkene) octane (component of petrol)

Q. What type of hydrocarbon is octane? ……………………….

crack

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 600oC) using a catalyst made from alumina or silica. This is called catalytic cracking or “cat cracking”

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)

Fractional distillation of crude oil

+

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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

C2H4

2

4

Propene

Butene

Pentene

Hexene

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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.

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Where do alkenes come from?

Cracking

Alkane molecules can be cracked using just heat (thermal cracking) or heat and a catalyst (cat-

cracking) 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?

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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: C10H22 ............... + C2H4 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:

C14H30 C4H8 + C2H4 + ..........

7. Write the molecular formula and name the hydrocarbon which was cracked in the below reaction.

.............. C3H6 + C2H4 + C7H16

8. In the following cracking reactions, (a) underline the alkane molecules and (b) circle the alkene molecules

C12H26 → 2C5H10 + C2H6

C12H26 → C2H4 + 2C3H6 + C4H10

C12H26 → 3C2H4 + 2C3H6 + H2

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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 1cm3 of oil.

Think time 1. Why are the first bubbles of gas not collected?

....................................................................................................................................................

2. Why is the porcelain heated strongly before the oil is warmed?

....................................................................................................................................................

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?

Bromine water is a

good test for a C=C

double bond. Simply

add and watch it

decolourise instantly

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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

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Blank Page for Making Notes

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Addition Reactions

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

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.

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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 Br2 is no different to Br—Br in the previous question

3. + Br2

4. + Cl2

5. + H—Br

7. + H2

Write the product as a molecular formula

8. C3H6 + HCl

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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”

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.

Ethene is the monomer

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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

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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:

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Case Study: Nylon

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.

The Nylon Rope Trick

Nylon was the first commercially successful synthetic polymer. It was first used commercially in a nylon-bristled 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.

An amide group

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:

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.

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)

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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 (NH2) 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

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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 (-NH2) group in an adjacent molecule. Let’s make our molecule longer by reacting another monomer:

+ H2O 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 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?

Q1. Why can we not call this an addition reaction?

Nylon is a polyamide

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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)

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

Method

Two test tubes contain 3cm3 of hexane

One of the test tubes is completely wrapped in foil

1cm3 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

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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 Br2) 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. CH4 + Cl2 → CH3Cl + …….

Q8. CH4 + ….. → CH3Br + HBr

Q9. ……… + Cl2 → C2H5Cl + HCl

Q10. ……… + ….. → C6H13Br + ……. 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

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Practice Questions

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ANSWERS to Practice Question

8A = 2

8B = 4

8C = 3

8D = 3

9A = 3

10(i) (di)amine

(ii)

Allow: