Organic Chemistry Workbook GCSE
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4.7.1 Carbon compounds as fuels and feedstock 4.7.1.1 Crude oil, hydrocarbons and alkanes Content
Key opportunities for skills development
Crude oil is a finite resource found in rocks. Crude oil is the remains WS 1.2 of an ancient biomass consisting mainly of plankton that was buried Make models of alkane in mud. molecules using the Crude oil is a mixture of a very large number of compounds. Most of molecular modelling kits. the compounds in crude oil are hydrocarbons, which are molecules made up of hydrogen and carbon atoms only. Most of the hydrocarbons in crude oil are hydrocarbons called alkanes. The general formula for the homologous series of alkanes is CnH2n+2 The first four members of the alkanes are methane, ethane, propane and butane. Alkane molecules can be represented in the following forms: C2H6 or
Students should be able to recognise substances as alkanes given their formulae in these forms. Students do not need to know the names of specific alkanes other than methane, ethane, propane and butane.
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GCSE Chemistry 8462. GCSE exams June 2018 onwards. Version 1.0 21 April 2016
4.7.1.2 Fractional distillation and petrochemicals Content
Key opportunities for skills development
The many hydrocarbons in crude oil may be separated into WS 1.2 fractions, each of which contains molecules with a similar number of carbon atoms, by fractional distillation. The fractions can be processed to produce fuels and feedstock for the petrochemical industry. Many of the fuels on which we depend for our modern lifestyle, such as petrol, diesel oil, kerosene, heavy fuel oil and liquefied petroleum gases, are produced from crude oil. Many useful materials on which modern life depends are produced by the petrochemical industry, such as solvents, lubricants, polymers, detergents. The vast array of natural and synthetic carbon compounds occur due to the ability of carbon atoms to form families of similar compounds. Students should be able to explain how fractional distillation works in terms of evaporation and condensation. Knowledge of the names of other specific fractions or fuels is not required.
4.7.1.3 Properties of hydrocarbons Key opportunities for skills development
Content
WS 1.2, 4.1 Some properties of hydrocarbons depend on the size of their molecules, including boiling point, viscosity and flammability. These Investigate the properties of properties influence how hydrocarbons are used as fuels. different hydrocarbons. Students should be able to recall how boiling point, viscosity and flammability change with increasing molecular size. The combustion of hydrocarbon fuels releases energy. During combustion, the carbon and hydrogen in the fuels are oxidised. The complete combustion of a hydrocarbon produces carbon dioxide and water. Students should be able to write balanced equations for the complete combustion of hydrocarbons with a given formula. Knowledge of trends in properties of hydrocarbons is limited to: • boiling points • viscosity • flammability.
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4.7.1.4 Cracking and alkenes Content
Key opportunities for skills development
Hydrocarbons can be broken down (cracked) to produce smaller, more useful molecules.
WS 1.2
Cracking can be done by various methods including catalytic cracking and steam cracking. Students should be able to describe in general terms the conditions used for catalytic cracking and steam cracking. The products of cracking include alkanes and another type of hydrocarbon called alkenes. Alkenes are more reactive than alkanes and react with bromine water, which is used as a test for alkenes. Students should be able to recall the colour change when bromine water reacts with an alkene. There is a high demand for fuels with small molecules and so some of the products of cracking are useful as fuels. Alkenes are used to produce polymers and as starting materials for the production of many other chemicals. Students should be able to balance chemical equations as examples of cracking given the formulae of the reactants and products. Students should be able to give examples to illustrate the usefulness of cracking. They should also be able to explain how modern life depends on the uses of hydrocarbons. (For Combined Science: Trilogy and Synergy students do not need to know the formulae or names of individual alkenes.)
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GCSE Chemistry 8462. GCSE exams June 2018 onwards. Version 1.0 21 April 2016
4.7.2 Reactions of alkenes and alcohols (chemistry only) 4.7.2.1 Structure and formulae of alkenes Content
Key opportunities for skills development
Alkenes are hydrocarbons with a double carbon-carbon bond. The general formula for the homologous series of alkenes is CnH2n
WS 1.2
Alkene molecules are unsaturated because they contain two fewer hydrogen atoms than the alkane with the same number of carbon atoms. The first four members of the homologous series of alkenes are ethene, propene, butene and pentene. Alkene molecules can be represented in the following forms: C3H6
Recognise substances that are alkenes from their names or from given formulae in these forms. MS 5b Visualise and represent 2D and 3D forms including twodimensional representations of 3D objects.
or
Students do not need to know the names of individual alkenes other than ethene, propene, butene and pentene.
4.7.2.2 Reactions of alkenes Key opportunities for skills development
Content
Alkenes are hydrocarbons with the functional group C=C. It is the generality of reactions of functional groups that determine the reactions of organic compounds. Alkenes react with oxygen in combustion reactions in the same way as other hydrocarbons, but they tend to burn in air with smoky flames because of incomplete combustion. Alkenes react with hydrogen, water and the halogens, by the addition of atoms across the carbon-carbon double bond so that the double bond becomes a single carbon-carbon bond. WS 1.2
Students should be able to: • describe the reactions and conditions for the addition of hydrogen, water and halogens to alkenes • draw fully displayed structural formulae of the first four members of the alkenes and the products of their addition reactions with hydrogen, water, chlorine, bromine and iodine.
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Helping yourself Read the section on Atomic structure and the Periodic table in your textbook Read the relevant sections of these websites bbc.co.uk/schools/gcsebitesize/chemistry gcsescience.com docbrown.info revisioncentre.co.uk youtube for video tutorials other teachers!
Spend time every week making revision notes. The biggest help to doing well in your GCSEs is to do well in your end of topic tests!
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Organic Chemistry There are about nine million different organic compounds and they all contain ………………………………. and almost all contain ……………………………………..as well.
Organic compounds containing just hydrogen and carbon are called ……………………………..
Most Hydrocarbons come from ……………………………… Crude oil is the product of decomposing organism like plants and microbes from many millions of years ago. It is found in large reserves underground. It is dug out the ground all over the world and is used for ………………………….
Carbon’s nature Carbon has the ability to bond together with other carbon atoms to form long chains of carbon atoms. This is called ………………………………...
Bonding Carbon always forms ………………………. ………………………….bonds as they need four more electrons to complete their outer shell. We represent this using a dotcross diagram. This is just a simplistic 2-dimensional representation of the molecules. A better 3dimensional representation is shown above.
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A covalent bond is a …………………………….. of electrons and since electrons are all negatively charged they all repel each other pushing the bonds as far from each other as possible. This produces a carbon atom where all four bonds are at 109.5° to each other and they form a shape called a ……………………………. This is the four-sided shape above. Covalent bonds are also able to ……………………… along their axis and so long chain carbon atoms can appear to twist and turn into lots of different shapes without us breaking any bonds.
Families of Organic Compounds The nine million organic compounds in the world are grouped into families or ‘…………………………….. A homologous series is a series of organic compounds having the same functional group but with each successive member differing by CH2. Every member of a homologous series has a very special group of atoms which do the bulk of all the reacting. These are called the ‘………………………………….. A functional group is a group of atoms responsible for the characteristic reactions of a compound. Common functional groups include an unsaturated double bond -C=C- for the alkenes, a- OH group for alcohols and a –COOH for all organic acids. All Homologous Series: a) Follow a general formula A general formula is the simplest algebraic formula of a member of a homologous series e.g. !! !!!!! for all alkanes !! !!! for all alkenes !! !!!!! !" for alcohols Where n represents any number
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b) Differ by !!! down the series !! !! ethane −!!! methane = !!! Or, !! !! propane −!! !! ethane = !!!
c) Have the same functional group so have similar chemical properties. !!! !" methanol !! !! !" ethanol !! !! !" propanol
d) The physical properties trend down the series e.g
Melting and boiling points increase Viscosity increases Colour darkens
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4th Form
Representing Molecules Empirical formula
Molecular formula
A molecular formula simply counts the number of each sort of atom present in the molecule. It tells us nothing about how they are all put together. These are usually only used in equations when the structure of a molecule doesn’t matter. e.g.
propane is !! !! butane is !! !!" !! !! ! + 5!! ! → 3!!! ! + 4!! !(!)
Structural formula
A structural formula shows how atoms are joined together. e.g. propane !!! !!! !!! 4 carbon alkane: !!! !!! !!! !!! or !!! !" !!! !!!
Display formula
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4th Form
A display formula shows all the bonds in the molecule as individual lines. Each line represents a ……………………………. electrons.
ethanol Display
Ball and Stick
Wedge and Dash Space filled
butane Notice that the display formula of butane doesn’t show the shape of the molecule. Display formulae always show the bonds flattened out. What this means is that the display formula will often be twisted at angles but still be the same molecule. Be careful. Display formulae are only used for small molecules because it is very time consuming.
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Naming organic compounds Naming organic compounds is like putting together a code. There is a clear method to it.
1. The first half of the name (the stem) tells you how many carbon atoms are in the ………………………………………... 2. The second part of the name (suffix) tells you what …………………………………… its got. 3. If there are ……………………… or a second functional group a prefix will be used to describe them.
1. Longest Carbon Chain length Count the number of carbon atoms in the longest chain, find the correct stem and write it down. Each carbon now has a number. 1
2
3
4
5
6
CH3CH2 CH2 CH2 CH2 CH3 Stem MethEthPropBut PentHexSeptOctNonDec-
Number of carbon atoms 1 2 3 4 5 6 7 8 9 10
2. Functional Groups Identify the functional group, find the correct suffix and write it after the prefix to form the full name. Suffix -ane
Homologous Series Alkane
Functional group Saturated
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-ene -ol -oic acid
Alkene Alcohol Carboxylic Acid
-C=C-OH -COOH
Sometimes the number the functional group starts on must be given also. 5
4
3
2
1
CH3CH2CH=CHCH3 Worked examples a) Name the compound drawn below from the display formula. 1. There are two carbon atoms in the molecule. The stem will be eth2. The homologous series/functional group is alkanes, so suffix is –ane 3. Put them together you get eth-ane. ETHANE.
b) Name the compound CH2=CHCH3 from its structural formula. 1. There are three carbon atoms in the molecule. stem: Prop2. There is a double carbon-carbon bond present. Homologous series is Alkene. Suffix: -ene 3. Put together to get full name. Propene
c) Name the compound CH3CH=CHCH3 from its structural formula
3.
1.
There are four carbon atoms in the chain. Stem: But-
2.
There is a double bond in the molecule. Suffix: -ene
3.
Double bond is on carbon number 2. Therefore suffix becomes -2-ene
4.
Put it all together: But-2-ene.
Branch Chains
Hydrocarbons can have branch chains on them.
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Side chain !!! − !!! !!! − !!! !!! !!! − !!! !!! !!! !!! −
Coded Methyl Ethyl Propyl Butyl
The name of molecule is always based in the longest carbon chain. The chain positions are numbered starting from the end which will produce the smaller numbers in the name.
Methyl -
1
1
2
3
Where there are more than one side chains, you describe the position of each of them. The number is followed by a hyphen followed by the side chain name. e.g. 2-methyl… Methyl 1
2
3
Methyl
If there is more than one of the same side chains we add the latin prefix before the chain name to tell us how many there are e.g. diethyl… We then put the carbon number they are attached to infront of this. There should be a number for each side chain with a comma between them e.g. 2,3-diethyl… prefix
Number of side chains
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4th Form
Alkanes Hydrocarbons contain just two elements, carbon and hydrogen. Hydrocarbons are organic compounds which means that they are derived from living or plant matter which has broken down over a very long period of time and under large pressures. Organic compounds always contain carbon and contain covalent bonding.
Alkanes are a series of hydrocarbons or Homologous Series. General Formula is !! !!"!! Where n represents a number. The alkanes are described as Saturated. This means that they contain only single bonds between carbon atoms or they contain the maximum number of hydrogen atoms for every carbon atom.
Alkane Methane Ethane Propane
Structural Formula
Display Formula
!!! !!! !!! !!! !!! !!!
Butane
!!! !!! !!! !!!
Pentane
!!! !!! !!! !!! !!!
Hexane
!!! !!! !!! !!! !!! !!!
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4th Form Heptane Octane
!!! !!! !!! !!! !!! !!! !!! !!! !!! !!! !!! !!! !!! !!! !!!
Physical Properties 1. Lightest alkanes are gases 2. Heavier alkanes are liquids and increasingly viscose 3. Colour gets increasingly dark down the series This is because the strength of the intermolecular forces increase between the molecules as the molecules get larger. The strength of IMF is directly related to a) the number of electrons in the molecule and,
b) the size of the surface area over which two molecules can interact.
Chemical Properties Alkanes are unreactive towards acids, bases metals and other chemicals. A. Combustion
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4th Form Their most important reaction is combustion. All alkanes burn in air or oxygen. Complete combustion gives carbon dioxide and water. e.g. !!! ! + 2!! ! → !!! ! + 2!! !(!) If there isn’t enough oxygen, there is incomplete combustion of the hydrocarbon which gives poisonous carbon monoxide instead. Carbon Monoxide binds with haemoglobin in the blood in preference to oxygen starving the body of oxygen. Alkanes are chiefly used as fuels around the world. B. Halogination Alkanes react with bromine only in the presence of Ultra-Violet light. Under its influence a hydrogen is replaced by a bromine atom in what is called a substitution reaction. This is also called a photochemical reaction because it only happens in sunlight. This causes the brown bromine water to decolourise.
The product is a bromoalkane. An alkane which has a halogen substituted onto it is called a halogenoalkane. The process of adding halogens to an alkane is called Halogenation. The addition of chlorine is called Chlorination, the addition of bromine bromination and the addition of iodine iodination. !!! ! + !!! ! → !!! !" + !"# Methane + Bromine -> bromomethane + hydrogen bromide The reaction can continue till all the hydrogens have been substituted by a bromine atom !!! !" + !!! → !!! !!! + !"# Bromomethane + Bromine -> Dibromomethane + Hydrogen bromide These halogenoalkanes have many specialised uses as anaesthetics non-stick coatings, herbicides and pesticides.
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3rd Form notes
Fractional distillation of crude oil Crude oil is a complex mixture of hydrocarbon with very similar chemical properties. The one things that differentiates the components are their ………………………… points. Fractional Distillation is a process by which a mixture such as crude oil can be separated into …………………………… containing compounds with very similar boiling points. Industrially crude oil is separated in a column shown in the schematic diagram below.
Process 1. The crude oil is …………………………. The hydrocarbons in the oil are now gaseous as they have boiling points below this temperature. There are some solids dissolved in the oil that do not evaporate as their boiling points are above 400oC so they just fall to bottom of column as the residue fraction
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3rd Form notes 2.
gaseous oil rises up the ……………………………..
The column gets cooler as you go up the column, there is a temperature gradient. Each tray level is at a different height and therefore temperature range up the column. The levels gets cooler further up. 3. When a gas reaches a level in the column that is the same as its boiling point, it ………………………………… into a liquid. 4. The …………………..and is collected in the trays and run off. Eventually most of the hydrocarbon gases condense at some point apart except some which are called the petroleum gases. They are tapped off at the top. Each fraction collected is still a mixture of hydrocarbons with small differences in boiling points but it is less complex than the original oil. Fractional distillation is a process of continuous condensation and evaporation of the hydrocarbons. Below is a table of the boiling point ranges of the fractions as well as their uses: names of fractions
C atoms in the molecule
Boiling range in oC
USES of the fraction - mainly depends on its physical properties
Fuel Gas, LPG, Refinery Gas
1 to 4
-160 to 20oC
methane gas fuel, C3-4 easily liquefied, portable energy source bottled gas for cooking (butane), higher pressure cylinders (propane)
Gasoline, Petrol
5 to 11
20 to 60oC
easily vaporised, highly flammable, easily ignited, car fuel
Naphtha
7 to 13
60 to 180oC
Paraffin, Kerosene Diesel oil, Gas oil Fuel and lubricating oils and Waxes
Bitumen
10 to 16 15 to 25
120 to 240oC 220 to 250oC
no good as a fuel, but valuable source of organic molecules to make other things, cracked to make more petrol and alkenes less flammable than petrol, domestic heater fuel, jet fuel car and larger vehicle fuel
20 to 70
250 to 350oC
not so easily evaporated, not as flammable, safe to store for central heating oil, quite viscous (sticky) and can also be used for lubricating oils, clear waxes and polishes
over 70
over 350oC
forms a thick, black, tough and resistant adhesive on cooling, used as waterproofing material and to sticks rock chips on roofs or road surfaces
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4th Form Organic
Catalytic Cracking of Alkanes This is vital industrial reaction for turning larger less useful alkanes into smaller more useful saturated alkanes used for fuels and unsaturated alkenes used for plastics and organic synthesis. Cracking involves heating the larger molecules up to a high temperature whereupon they decompose into smaller ones when in the presence of a catalyst.
The temperature can be brought down by using a catalyst, usually pumice stone, aluminium oxide or Zeolite.
C10H22
→
C8H18 +
C2H 4
With a catalyst, the temperature needed is 500oC. This is called CATALYTIC CRACKING. The alkenes formed, CnH2n, are used to make polymers and alcohols. The alkanes, CnH 2n+2 , are used for making petrol. Cracking allows alkanes molecules of high demand and lower supply to be made from larger alkanes, which are of lower demand and higher supply.
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4th Form Organic
Questions Q.1 Why is the pumice stone heated?[1]
Q.2 What is the function of the pumice stone? [2]
Q.3 Fully label the diagram on the previous page.
[3]
Q.4 Why are the few first bubbles of gas not collected?
Q.5 How many tubes of gas did you collect?
[1]
[1]
Q.6 What does the bromine water test tell you about the molecules in the gas products? [2]
Q.7 If liquid paraffin contains for example C18H38 molecules, write an equation for the cracking of this molecule. [2]
Q.8 What is cracking used for?
[4]
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The Alkenes
Double Bond
These are a homologous series containing the ________________________ double bond functional group. General Formula !! !!!
Name
Molecule Formula
Structural Formula
Ethene
Propene
But-1 ene
Pent -1-ene
Hex-1-ene
Hept-1-ene
Oct-1 ene
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Physical Properties •
Very similar to Alkanes
•
Gases up to Butene
•
Exhibit _______________ isomerism.
Chemical Properties a) Combustion Like all hydrocarbons alkenes burn but not as well producing lots of soot and carbon. !! !! ! + !! → 2!!! ! + 2!! !(!) b) Addition Reactions Alkenes undergo _________________________ reactions. One part of the ______________________ bond breaks to form a single bond and other atoms are added onto the two atoms.
i)
Bromine Bromine adds to an alkene without the need for heat or a catalyst. This results in the bromine water decolourises.
The product is a halogenoalkane 1,2 dibromoethane. The ____________________________ of bromine water is the definitive test for the presence of a double bond in a molecule. ii)
Hydrogen
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Ethene undergoes _______________________________to form ethane at 150˚C and using a Nickel catalyst. This process is the basis of margarine production.
The fact that we can add hydrogen to them means we called them ‘unsaturated’. Once no more hydrogen can be added, it becomes ‘saturated’. iii)
Steam Ethene is _______________________________ with steam under a phosphoric acid ______________________ to produce an alcohol ethanol.
iv)
Polymerization Alkenes can even under go addition reactions with each other. This is called _____________________________ when many thousands of small molecules all join together to form a long chain.
We present the long chain using a the repeat unit as shown below.
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35
Drawing the structures 1. Define the empirical formula. 2. What is the empirical formula of these molecules a) CH2=CH2
b)
c)
3. Define the molecular formula 4. What is the molecular formula of these molecules?
a)
b)
c) CH2=CHCH3
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5. Define the structural formula 6. Write the structural formula for these molecules
a)
b) C3H8 c) C2H5OH 7. Define the display formula 8. Draw the display formula for these molecules. a) C3H8 b) HOCH2CH2OH c) C2H4
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Oil & alkanes 1)
The alkanes are a homologous series of saturated hydrocarbons. a) What is a hydrocarbon?
(2)
b) What is a homologous series?
(2)
c) What is meant by the word saturated in this context?
(1)
d) Draw the structure of propane.
(1)
e) Octane is a straight chain alkane containing eight carbon atoms. i) Write its molecular formula.
(1)
ii) Draw its structural formula.
(1)
f) Write a balanced equation for the complete combustion of propane. (1) g) Write a balanced equation for the incomplete combustion of propane, where a toxic gas is formed. (1) 2)
Crude oil is a fossil fuel that is a complex mixture of hydrocarbons, mainly alkanes. a) Describe how crude oil was formed.
(4)
b) By what process are the hydrocarbons in crude oil separated? (1) c) Upon what physical property does this separation technique depend on? (1) d) Explain in simple terms how this process works.
(3)
e) Give a piece of evidence that shows that each fraction is a mixture of compounds and not a pure compound. (1)
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Alkanes Questions 1. Complete the complete combustion reactions for the following alkanes. a. Methane
b. Ethane
c.
Cyclohexane
2. Draw the chemical symbol equation for the reaction between bromine and methane under UV light.
3. What happens when alkanes and water are put together and why?
4. Describe an environmental issue associated with the combustion of alkanes across the globe. Use a diagram if necessary. Describe the process and give an example of something that is being done to tackle the problem.
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Alkanes and alkenes Task A name of alkane methane ethane propane butane pentane hexane heptane octane nonane decane undecane dodecane hexadecane eicosane
The table on the right shows the names of some alkanes, the number of carbon atoms in their molecules, and their boiling points. The boiling points are measured in kelvin (K) to avoid negative numbers. 1. On graph paper, plot a graph of boiling point against number of carbon atoms. •
Put the boiling points up the side. Go from 0K to 700K, and use 1 small square (2mm) for each 10K. Your axis will be 14cm long.
•
Put the number of carbon atoms along the bottom. Go from 0 to 20, and use 3 small squares (6mm) for each atom. Your axis will be 12cm long.
number of carbon atoms 1 2 3 4 5 6 7 8 9 10 11 12 16 20
approximate boiling point (K) 110 185 230 275 310 340 370 400 425 445 470 490 560 615
2. Look carefully at your graph. Write down what happens to the boiling point as the number of carbon atoms goes up. 3. a) Room temperature is about 300K. Any alkanes with a boiling point less than this will be gases. Which alkanes are gases? b) Alkanes with more than 14 carbon atoms are usually solids. Which alkanes are solids? c) Any alkanes that are not in your answers to a) or b) are liquids. Which alkanes are liquids? Task B Read the information in the box, then answer question 4. Long alkanes with lots of carbon atoms are not very good as fuels. If they are heated strongly on a catalyst, they can be broken into smaller molecules, which do make good fuels. This reaction is called cracking. Some of the smaller molecules are alkenes. Alkenes are like alkanes, but two of their carbon atoms are joined by two bonds (a double bond), not just one bond. 4. a) What is “cracking”? b) What is the difference between alkanes and alkenes (apart from their name!)? c) What is a “double bond”? 5. The structural formula of propene is
H
H
H
H
C
C
C
H
and its molecular formula is C3H6.
H
Draw the structural formulae and molecular formulae for the first seven alkenes (from ethene with 2 carbon atoms to octene with 8 carbon atoms – methene does not exist). Write down the names of the molecules (naming them is similar to alkanes, but the names end in ene, not ane). 6. Work out the molecular formula for eicosene (20 carbon atoms). N-m06-18 (© Nigel Saunders 2000) 55
Faulkner’s
Combustion Reactions
1. Write both the word and symbol equations for the combustion of: a) Methane b) Ethane c) Propane d) Butane 2. Write a symbol equation for the cracking of the following long chain alkanes a) C12H24 b) C15H32
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Fractional distillation 1. Hydrocarbons burn in excess oxygen to form carbon dioxide and water. Give the equation for the complete combustion of heptanes. [2]
2. What is the mixture that is fed into the fractionating column
[1]
3. What is the property that all the compounds are separated according to?
[1]
4. Here is a list of fractions in alphabetical order. Write them in order of increasing boiling point and give a use for each fraction: bitumen, diesel, fuel oil, gasoline, kerosene, refinery gases. [12]
5. Describe, using bullet points, the process of separating crude oil into its fractions.
[4]
6. What happens to all the unwanted fractions?
[1]
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58
Large hydrocarbon
Cracking product 1
Decane
Octane
Name of cracking product 1
Cracking product 2
Ethene
Name of cracking product 2
605WS
Alkenes
Number of C Atoms
ALKANES Name and Formula
ALKENES Name and Formula
Structure
1
Methane CH4
Structure
X
2
Ethane C2H6
3
Propane C 3H 8
4
Butane C4H10
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605WS
1. What is a saturated hydrocarbon (e.g. an Alkane)? ……………………………………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………………………………… 2. What is the difference between C2H6 and C2H4? ……………………………………………………………………………………………………………………………………………… 3. What is an unsaturated hydrocarbon (e.g. an Alkene)? ……………………………………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………………………………… 4. The general formula for an Alkane is CnH2n+2 deduce the general formula of an Alkene. …………………………………………………………………………………………………………………………………………….. 5. What is the colour change with an Alkene on the addition of bromine water? …………………………………………………………………………………………………………………………………………….. 6. Draw an equation for the reaction between ethane (C2H4) and Bromine (Br2). 7. Why is there no colour change when bromine water is added to an Alkane? ………………………………………………………………………………………………………………………………………………
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More about alkanes and alkenes
D13.1
The diagram shows two hydrocarbon molecules, A and B.
H
H
H
H
C
C
C
H
H
molecule A
H
H
H
H
H
H
C
C
C
C
C
H
H
H
H
H
H
molecule B
1 What is a hydrocarbon? 2 a) What is a saturated molecule? b) Which molecule, A or B, is saturated? 3 a) What is an unsaturated molecule? b) Which molecule, A or B, is unsaturated? 4 a) Which molecule, A or B, is an alkane? b) Which molecule, A or B, is an alkene? 5 a) What would you see if a few drops of bromine water were added to molecule A? b) What would you see if a few drops of bromine water were added to molecule B? 6 a) What is the formula of molecule A? b) What is the formula of molecule B? 7 Which molecule is called pentane? 8 Name the other molecule. 9 Give a use for molecule A. 10 Give a use for molecule B.
D
Š Pearson Education Limited 2003
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61 Longman Higher Science for GCSE 1
Thinking creatively
1. Personify each homologous series and write a story that involves characters from each of them. 2. Write a first hand account of fractional distillation from the point of view of a molecule going through it. 3. Create a poster which summaries all the key equations we need to know. Include the reagents and reaction conditions in there too. 4. Write a story about a person waking up one morning to find that there are not polymers. Use it to explain why life would be like without them. 5. Make a model using polystyrene balls of part of a simple protein.
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Taking it further Why not reach the section on Organic Chemistry in an A-level textbook? What it say about isomerism? What other homologous series are there? Pick any two molecules from different homologous series and try and work out how you can transform one into the other! Science is trying to answer the questions how? What is the mechanism? How does one compound form another? See if you can learn some reaction mechanisms! Books to read: Mauve: How One Man Invented a Colour that Changed the World, Simon Garfield Molecules at an Exhibition: Intriguing molecules from everyday life, John Emsley The Double Helix, James Watson Napoleon’s Buttons: How 17 Molecules Changed History, Penny Le Couteur There is plenty of useful information on line about the environmental affects of the use of crude oil as well as the political effects of the pursuit of it. Read up on it and come to your own conclusions. Tell them to your room mate in prep tonight! Why not watch some online tutorials or lectures from the world’s universities like MIT, Oxford or Cambridge that are available for free online? The Pharmaceutical industry is the fine end of organic chemistry. What is the process of bringing a drug to market and how much does it cost? What are the main health issues that need to be tackled? The Agrochemical industry is also extremely important to keep feeding the world. What are the big problems that they are facing and need to overcome in the future? Something light hearted? Listen to past episodes of ‘The Infinite Monkey Cage’ by Brian Cox and Robin Ince.
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Properties of Alkanes Wear safety specs and lab coats for the practical. Do not pour any residues down the sinks. Instead place them in the large bottle provided in the fume cupboard.
1.
Flammability
Take a crucible lid and place it on top of a clay-pipe triangle which is on a heat proof-mat. Using a dropper pipette, place a few drops of liquid hexane onto the lid. Now try to set light to the liquid carefully with a lighted splint. Note down the flammability + colour and smokiness of the flame. Repeat the experiment with a little octane.
2.
Solubility in water
Take a test tube and half fill it with water. Using a dropper pipette, add a few drops of hexane to the water. Bung up the tube and shake. Leave to settle and note down what has happened. Repeat with octane.
3.
Bromine water
Take a test tube and add a little hexane, using a dropper pipette. Now carefully add ten drops of bromine water(CARE- CORROSIVE). Bung up the tube and shake. Let it settle and note down what you observe. Repeat with octane.
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Chemical properties of Alkenes In this practical, we shall look at three liquid alkenes called hex-1-ene (C6H12), oct-1-ene (C8H16) and cyclohexene (C6H10). Method 1. Test with Bromine water Take a clean dry test tube and place a few drops of hex-1-ene into it. Now add a couple of drops of bromine water. Bung up the tube and shake for a few seconds. Repeat the test with oct-1-ene. Repeat the test with cyclohexene. Record your observations in the table below: Hex-1-ene
Oct-1-ene
Cyclohexene
Observations with bromine water
2. Combustion Take a crucible lid stand it on top of a clay pipe triangle which is standing on a heatproof mat. Add a few drops of hex-1-ene to the lid and ignite the alkene with a lighted splint. Examine the colour and appearance of the flame. Repeat the experiment using oct-1-ene and then cyclohexene. Hex-1-ene
Oct-1-ene
Cyclohexene
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Observations on igniting the alkene
Q.1 Write a balanced equation for the reaction of hex-1-ene and bromine.
[2]
Q.2 What does this bromine water test prove about all alkenes?
[2]
Q.3 What TYPE of reaction has occurred with the bromine water?
[1]
Q.4 Draw the displayed formula of oct-1-ene
[1]
Q.5
[2]
Write the balanced equation for the complete combustion of cyclohexene.
Q.6 Why do alkenes burn more dirtily than alkanes?
[2]
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Results TEST
HEXANE
OCTANE
Flammability
Solubility in water
Action with bromine water
Q.1
What are molecular formulae of the two alkanes being used?
__________________________________________________________________ [2] Q.2
Write a word equation with state symbols for the combustion of hexane.
__________________________________________________________________ [2] Q.3
Write a balanced equation for the complete combustion of octane.
_______________________________________________________________[2] Q.4
What is formed by the combusting alkane instead of carbon dioxide when a
limited supply of oxygen is used?
Why is this product an environmental hazard?
_____________________________________________________________________
__________________________________________________________________ [2]
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Q.5
Write a balanced equation for the incomplete combustion of methane.
__________________________________________________________________ [2] Q.6
What happens when alkanes are added to water?
__________________________________________________________________ [2]
Q.7
Are alkanes more or less dense than water?
__________________________________________________________________ [1] Q.8
What happens when bromine water is added to an alkane?
__________________________________________________________________ [2] Q.9
What does the bromine water test detect in an organic if a positive test is
seen?
__________________________________________________________________ [2] Q.10 What does this test prove about alkanes?
__________________________________________________________________ [1] Q.11 Draw out the displayed formulae of (i) hexane (ii) octane
__________________________________________________________________ [2] Q.12
From their formula, why are hexane and octane both alkanes?
__________________________________________________________________ [1] Q.13 Why are alkanes described as being ‘saturated’ in terms of their structure?
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__________________________________________________________________ [1]
Q.14 Give an important everyday or industrial use of
(i) methane __________________________________________________________
(ii) propane __________________________________________________________
(iii) butane___________________________________________________________
(iv) octane _________________________________________________________[4]
Q.15 What is the name of
(i)
C7H16_____________________________________________________________
(ii)
C5H12_______________________________________________________ [2]
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603P
Cracking Apparatus • Paraffin oil (alkane) • Porous Pot • Glass or ceramic wool • Boiling tube with delivery tube • Water bath • Gas jar and lid • Bromine water • Dropping pipette • Bunsen burner • Heat-proof mat • Test tubes and bungs
Procedure 1. Place some mineral wool in the bottom of the boiling tube and add the paraffin oil to a depth of 1 cm. 2. With the boiling tube nearly on its side, place a layer of porous pot pieces in the dry part of the tube. 3. Set up the apparatus as shown in the diagram 4. Using the hottest flame heat the porous pot strongly; it may be necessary to heat the paraffin also. 5. Collect the gas produced in test tubes and bung once all the water has been displaced. 6. Discard the first test tube collected. 7. When no more gases are produced remove the delivery tube from the water before removing the heat – this will avoid the chance of suck-back.
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603P Analysis: • • •
Remove the rubber bung and add bromine water to each of the tubes. Replace the bungs and shake. Observe to see if there are any colour changes.
Questions: 1. Why was the first test tube of gas discarded? ………………………………………………………………………………………………………… 2. What was the role of the porous pot? ………………………………………………………………………………………………………… 3. What colour changes were observed when Bromine water was added? ………………………………………………………………………………………………………… 4. What is Bromine water the test for? ………………………………………………………………………………………………………… 5. What is the definition of an unsaturated hydrocarbon? ………………………………………………………………………………………………………… ………………………………………………………………………………………………………… 6. Why are longer chain hydrocarbons cracked into smaller chains?
………………………………………………………………………………………………………… 7. Write a balanced chemical equation for the cracking reaction assuming that the paraffin is C10H22 and that hexane and ethene are the products.
…………………………………………………………………………………………………………………………………
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