Topic: Alkenes & Polymers

Alkenes & Polymers quiz

1. What is a general formula?

Question 1 of 29

2. What is the molecular formula for propane?

Question 2 of 29

3. Recall a use of gasoline

Question 3 of 29

4. Explain why carbon monoxide is dangerous to humans

Question 4 of 29

5. This diagram shows one repeat unit of a polymer. Name the monomer used to make this polymer.

Question 5 of 29

6. What is a substitution reaction?

Question 6 of 29

7. State the general formula for alkenes

Question 7 of 29

8. What is crude oil?

Question 8 of 29

9. Of which homologous series is >C=C< (a carbon-to-carbon double bond) the functional group?

Question 9 of 29

10. What does the following diagram represent?

Question 10 of 29

11. State the temperature for the cracking of hydrocarbons

Question 11 of 29

12. What are isomers?

Question 12 of 29

13. What is a substance called if it, when burned, releases heat energy?

Question 13 of 29

14. Which fraction of crude oil is the most viscous?

Question 14 of 29

15. Bromine is added to a sample of an alkane and also to a sample of an alkene. Describe the result of each.

Question 15 of 29

16. Explain how crude oil is separated into fractions

Question 16 of 29

17. Give two reasons why addition polymers are hard to dispose of

Question 17 of 29

18.

A student has been asked to draw all the alkene isomers of C₄H₁₀ and draws the following. Which of the following statements about the student's diagram is correct?

Question 18 of 29

19. What is the name given to the type of reaction that occurs when ethane reacts with bromine under UV light

Question 19 of 29

20. Explain the term unsaturated

Question 20 of 29

21. State the general formula of alkanes

Question 21 of 29

22. What is the process of joining monomers together called?

Question 22 of 29

23. Recall the products of the complete combustion of alkanes

Question 23 of 29

24. Explain why cracking is an important process in the oil industry

Question 24 of 29

25. What is the molecular formula for propene?

Question 25 of 29

26. State the colour change in the reaction of an alkene with bromine water

Question 26 of 29

27. Explain the term hydrocarbon

Question 27 of 29

28. Why are alkanes classified as saturated hydrocarbons?

Question 28 of 29

29. Name this molecule

Question 29 of 29


 

2020-02-16T15:14:58+00:00Categories: Uncategorized|Tags: , |

Tricks for remembering the names of organic molecules

A good way to remember the names of organic molecules is to make up a silly mnemonic where the first letter of each word matches the first letter of the organic molecules. For example the first 10 alkanes in order are , Methane, Ethane, Propane, Butane, Pentane, Hexane, Heptane, Octane, Nonane and Decane. These can be memorised with “Many elephants prefer blue pinapples. However hungry orangutans never do.” This isn’t a very good mnemonic, but it is the one I made up for myself when I was at school, and you are best making up your own. The sillier, smellier and more colourful the better.

4:02a understand how to represent organic molecules using molecular formulae, general formulae, structural formulae and displayed formulae

The molecular formula shows the actual number of atoms of each element in a molecule.

The general formula shows the relationship between the number of atoms of one element to another within a molecule. Members of a homologous series share the same general formula. The general formula for alkanes is CnH2n+2 and the general formula for alkenes is CnH2n.

A structural formula shows how the atoms in a molecule are joined together.

The displayed formula is a full structural formula which shows all the bonds in a molecule as individual lines.

 

The terms above are demonstrated with the example of butane.

Image result for butane

  • Displayed formula:
  • Molecular formula: C₄H₁₀
  • General formula (alkanes): CnH2n+2
  • Structural formula: CH₃ – CH₂ – CH₂ – CH₃

 

The terms above are demonstrated with the example of ethene, which contains a double bond.

Image result for ethene

  • Displayed formula:
  • Molecular formula: C₂H₄
  • General formula (alkenes): CnH2n
  • Structural formula: CH₂ = CH₂

4:02 understand how to represent organic molecules using empirical formulae, molecular formulae, general formulae, structural formulae and displayed formulae

The molecular formula shows the actual number of atoms of each element in a molecule.

The empirical formula shows the simplest whole number ratio of atoms present in a compound. So the molecular formula is a multiple of the empirical formula.

The general formula shows the relationship between the number of atoms of one element to another within a molecule. Members of a homologous series share the same general formula. The general formula for alkanes is CnH2n+2 and the general formula for alkenes is CnH2n.

A structural formula shows how the atoms in a molecule are joined together.

The displayed formula is a full structural formula which shows all the bonds in a molecule as individual lines.

 

The terms above are demonstrated with the example of butane.

Image result for butane

  • Displayed formula:
  • Molecular formula: C₄H₁₀
  • Empirical formula: C₂H₅
  • General formula (alkanes): CnH2n+2
  • Structural formula: CH₃ – CH₂ – CH₂ – CH₃

 

The terms above are demonstrated with the example of ethene, which contains a double bond.

Image result for ethene

  • Displayed formula:
  • Molecular formula: C₂H₄
  • Empirical formula: CH₂
  • General formula (alkenes): CnH2n
  • Structural formula: CH₂ = CH₂

4:03 know what is meant by the terms homologous series, functional group and isomerism

A functional group is an atom or a group of atoms that determine the chemical properties of a compound.

For example the functional group of an alcohol is the -OH group and that of alkenes is the C=C carbon to carbon double bond.

 

A Homologous series is a group of substances with:

  • the same general formula
  • similar chemical properties because they have the same functional group
  • a trend (graduation) in physical properties

 

 

Isomers are molecules with the same molecular formula but with a different structure.

4:04 understand how to name compounds relevant to this specification using the rules of International Union of Pure and Applied Chemistry (IUPAC) nomenclature. Students will be expected to name compounds containing up to six carbon atoms

The names of organic molecules are based on the number of carbon atoms in the longest chain. This chain is the longest consecutive line of carbon atoms, even if this line bends. 

The name is based on the number of carbon atoms in the longest chain
1 Meth-
2 Eth-
3 Prop-
4 But-
5 Pent-
6 Hex-
7 Hept-
8 Oct-
9 Non-
10 Dec-

Hydrocarbons are molecules which contain only hydrogen and carbon.

 

Naming straight-chain alkanes

The simplest hydrocarbons are alkanes. They contain only single bonds, and have “-ane” in the name.

For example, the displayed formula of ethane (C₂H₆) is:

The name “ethane” contains “eth-” because there are 2 carbon atoms in the longest chain, and the name contains “-ane” because the molecule only has single bonds so is an alkane.

 

Another example is pentane (C₅H₁₂) which has the displayed formula:

The name “pentane” contains “pent-” because there are 5 carbon atoms in the longest chain, and the name contains “-ane” because the molecule only has single bonds, so is an alkane.

 

Remember, it does not matter if the longest consecutive line of carbons bends around. For example the displayed formula below still shows a very normal molecule of pentane (5 carbons in a row). Pentane is not normally drawn with the longest chain of carbons bent around because it could be confusing.

 

You might also see the bonds drawn at angles. Don’t worry, the displayed formula below is still pentane, as can be seen by the fact there are 5 carbon atoms in the longest chain, surrounded by hydrogen atoms bonded to the carbon atoms by single bonds.

Image result for displayed formula pentane

 

A shorter way to express the detailed structure of an organic molecule is the structural formula. The structural formula for pentane is CH₃-CH₂-CH₂-CH₂-CH₃, which tells us the same information about the molecule as does the displayed formula, without the hassle of having to draw all the bonds or all the hydrogen atoms.

 

Naming straight-chain alkenes

Another simple group of hydrocarbons is the alkenes. They contain a carbon-to-carbon double bond, which also means they have two fewer hydrogen atoms than their corresponding alkane. An alkene has “-ene” in its name.

For example, the displayed formula for ethene (C₂H₄) is:

ethene has 2 carbon atoms and 4 hydrogen atoms

 

and the displayed formula of propene (C₃H₆) is:

propene has 3 carbon atoms and 6 hydrogen atoms

 

With longer alkene molecules the double bond might appear in different locations of the carbon chain, so the name needs to be a little bit more complicated to be able to describe these differences clearly. A number is added in the middle of the name to indicate at which carbon the double bond starts.

So the displayed formula of pent-1-ene is:

 

and this is the displayed formula of pent-2-ene:

 

However, take care that when counting which carbon has the double bond. The numbers start from the end that produces the smallest numbers in the name. For example, this is the displayed formula for pent-1-ene again, but just drawn the other way round. It is still pent-1-ene (you can’t get pent-4-ene):

 

Naming straight-chain alcohols

We get the same pattern all over again with the group of organic molecules called alcohols, which are recognised by an -OH functional group. For example here is the displayed formula for ethanol, which has 2 carbon atoms in the longest chain:

Image result for displayed formula ethanol

 

and here is the displayed formula for butanol:

Image result for displayed formula butanol

 

Summary of naming simple straight-chain organic molecules

The following table summarises the naming of some of the straight-chain alkanes, alkenes and alcohols, giving a name and a molecular formula for each:

Carbons in longest chainAlkanesAlkenesAlcohols
1methane, CH₄-methanol, CH₄O
2ethane, C₂H₆ethene, C₂H₄ethanol, C₂H₆O
3propane, C₃H₈propene, C₃H₆propanol, C₃H₈O
4butane, C₄H₁₀butene, C₄H₈butanol, C₄H₁₀O

 

Naming branched alkanes and alkenes

The naming conventions for organic molecules cover more than the straight chain molecules. Branched molecules are named depending on the number of carbon atoms in the branch. A branch with 1 carbon is called “methyl” and a branch with 2 carbons is called “ethyl”. This is similar to the conventions covered above, plus the “-yl-” bit just says it is a branch.

For example, this is the displayed formula for 2-methyl hexane:

In the name 2-methyl hexane, the number 2 indicates that when counting along the longest carbon chain the methyl branch comes off the second carbon atom. The “methyl” bit of the name says there is one branch of 1 carbon. The “hex” bit of the name says the longest consecutive chain of carbon atoms is 6. The “ane” bit says the molecule has only single bonds.

 

When counting along the carbon atoms of the longest chain to work out the name, the numbering of carbon atoms starts from the end nearest to the branch. Another way to put this is that the name is given such that the numbers in the name are as low as possible. For example, here is the displayed formula for 4-ethyl octane:

 

Another example, this time with 2 methyl branches coming off the second and third carbons of the chain, is 2,3-dimethyl hexane. The “di” in the name indicates there are two methyl groups. This is the same way in which “di” indicates there are two oxygen atoms in carbon dioxide.

 

Another example of how the naming convention works for branches is 2,2-dimethyl hexane:

 

 

This naming of branches also applies to alkenes. Here is the displayed formula of 4-methylpent-1-ene:

4:05 understand how to write the possible structural and displayed formulae of an organic molecule given its molecular formula

The molecular formula describes the actual number of each type of each atom in a molecule.

For example, a molecular formula of C₆H₁₂ tells us that in each molecule there are 6 carbon atoms and 12 hydrogen atoms.

However, the molecular formula tells us nothing about how those atoms are arranged. For example, it does not tell us if there any branches of carbon atoms coming off the main carbon-carbon chain, nor how long or how many there might be.

On the other hand, the structural formula and displayed formula of a molecule tell us clearly how the atoms are arranged in that molecule.

This means that if we are given a molecular formula only, there may be several possible structural and displayed formulae all of which could apply for that molecule.

When trying to work out possible structural or displayed formulae from a molecular formula there are several clues:

  • If the molecular formula only has carbon and hydrogen in it, then of course the structural formula will only have atoms of these 2 elements.
  • If the molecular formula has twice as many carbons as hydrogens (CnH2n) then the molecule is an alkene and has a double bond somewhere between 2 of the carbon atoms.
  • If the molecular formula has two more than twice as many carbons as hydrogens (CnH2n+2) then the molecule is an alkane and only has single bonds.
  • If the molecular formula has two more than twice as many carbons as hydrogens and also has an oxygen atom (CnH2n+2O), then the molecule is an alcohol, and somewhere in the displayed formula will be a carbon single-bonded to an oxygen which itself is then single-bonded to a hydrogen.

4:06 understand how to classify reactions of organic compounds as substitution, addition and combustion. Knowledge of reaction mechanisms is not required

In a substitution reaction an atom or group of atoms is replaced by a different atom or group of atoms. For example when ethane reacts with bromine gas one of the hydrogen atoms in ethane is substituted by one of the atoms of bromine from within the bromine molecule:

CH₃-CH₃         +         Br-Br         →         CH₃-CH₂Br         +         H-Br

ethane         +         bromine         →         bromoethane         +         hydrogen bromide

 

 

An addition reaction occurs when an atom or group of atoms is added to a molecule without taking anything away. For example when ethene reacts with bromine gas, the product is simply the addition of the two molecules:

CH₂=CH₂         +         Br-Br         →         CH₂Br-CH₂Br

Image result for ethene + bromine displayed formula

 

 

A combustion reaction is another way to say ‘burning’ and is a reaction with oxygen. Combustion of hydrocarbons with excess oxygen gives the products water and carbon dioxide, and also releases heat energy (exothermic reaction). Two examples the combustion of propane and the combustion of butene:

C₃H₈         +         5O₂         →         3CO₂         +         4H₂O

C₄H₈         +         6O₂         →         4CO₂         +         4H₂O

4:08 describe how the industrial process of fractional distillation separates crude oil into fractions

  • Crude oil is separated by fractional distillation.
  • Crude oil is heated and the oil evaporates.
  • The gas goes into the fractional distillation tower. As the gas rises the temperature falls.
  • Fractions with higher boiling points condense and are collected nearer the bottom of the tower.

 

Fractional distillation – videos

This video does not quite use the right language for the various fractions as appropriate to the Edexcel iGCSE, but it is nevertheless a good description of the process. Make sure you use the notes on tutorMyself.com to get the exact language you will need for your exam.

And another somewhat older video showing the industrial process of fractional distillation:

4:09 know the names and uses of the main fractions obtained from crude oil: refinery gases, gasoline, kerosene, diesel, fuel oil and bitumen

Crude oil is separated into fractions by the process of fractional distillation.

FractionUse
Refinery gasesBottled gas
GasolineFuel for cars
KeroseneFuel for aeroplanes
Diesel OilFuel for lorries
Fuel OilFuel for ships
BitumenRoad Surfacing

4:10 know the trend in colour, boiling point and viscosity of the main fractions

The boiling point increases as the number of carbon atoms (chain length) increases.

The viscosity increases as the number of carbon atoms (chain length) increases.

The greater the number of carbon atoms (chain length), the darker in colour that fraction is.

The viscosity of a fluid describes how easily it flows. Water has a low viscosity, it flows very easily. Crude oil has a higher viscosity than water, it does not flow very easily.

Fractions (in order)Properties
Refinery gasesSmallest molecules. Lowest boiling point. Lowest viscosity. Lightest in colour.
Gasoline
Kerosene
Diesel
Fuel oil
BitumenLargest molecules. Highest boiling point. Highest viscosity. Darkest in colour.

4:12 know the possible products of complete and incomplete combustion of hydrocarbons with oxygen in the air

Complete Combustion happens when there is enough oxygen available, producing carbon dioxide (CO2) and water (H2O)

 

Incomplete Combustion happens when there is not enough oxygen available, with possible products being carbon monoxide (CO), carbon (C, soot), carbon dioxide (CO2) and water (H2O)

4:13 understand why carbon monoxide is poisonous, in terms of its effect on the capacity of blood to transport oxygen references to haemoglobin are not required

Carbon monoxide may be produced from the incomplete combustion of fuels:

Carbon monoxide is poisonous because it reduces the capacity of the blood to carry oxygen.

4:17 describe how long-chain alkanes are converted to alkenes and shorter-chain alkanes by catalytic cracking (using silica or alumina as the catalyst and a temperature in the range of 600–700⁰C)

Cracking involves the thermal decomposition of long-chain alkanes into shorter-chain alkanes and alkenes:

Conditions

Temperature: 600oC

Catalyst: aluminium oxide, Al2O3

4:18 explain why cracking is necessary, in terms of the balance between supply and demand for different fractions

Cracking converts long chain hydrocarbons into short chain hydrocarbons.

Long-chain alkanes are broken down into alkanes and alkenes of shorter length.

Crude oil contains a surplus long chains.

Shorter chain hydrocarbons are in greater demand, e.g. petrol.

Cracking also produces alkenes which are used in making polymers and ethanol.

4:20 explain why alkanes are classified as saturated hydrocarbons

 

Saturated:   A molecule containing only single bonds between carbon atoms. For example, alkanes as described as saturated molecules.

Unsaturated:   A molecule containing a carbon-carbon double or triple bond. For example, alkenes as described as unsaturated molecules.

 

4:21 understand how to draw the structural and displayed formulae for alkanes with up to five carbon atoms in the molecule, and to name the unbranched-chain isomers

The displayed formulae show all the atoms and bonds drawn out.

The molecular formulae just show the number of each type of atom in the molecule.

4:22 describe the reactions of alkanes with halogens in the presence of ultraviolet radiation, limited to mono-substitution knowledge of reaction mechanisms is not required

Alkanes react with bromine in the presence of UV light, e.g. sunlight.

A hydrogen atom in the alkane is replaced by a bromine atom.

This is known as substitution.

 

4:23 know that alkenes contain the functional group >C=C<

Alkenes are a homologous series of hydrocarbons which contain a carbon-carbon double bond. This double bond is shown in formulae as a double line.

The names of alkenes end with “ene”.

An example is ethene, the structural formula for which is CH₂ = CH₂

For a molecule with more than two carbon atoms, the position of the double bond within the molecule can vary as indicated by the name and the structural formula.

4:25 explain why alkenes are classified as unsaturated hydrocarbons

Saturated:   A molecule containing only single bonds between carbon atoms. For example, alkanes as described as saturated molecules.

Unsaturated:   A molecule containing a carbon-carbon double or triple bond. For example, alkenes as described as unsaturated molecules.

4:26 understand how to draw the structural and displayed formulae for alkenes with up to four carbon atoms in the molecule, and name the unbranched-chain isomers. Knowledge of cis/trans or E/Z notation is not required

The displayed formulae show all the atoms and bonds drawn out.

The molecular formulae just show the number of each type of atom in the molecule.

4:27 describe the reactions of alkenes with bromine, to produce dibromoalkanes

Alkenes react with bromine water. UV light is not required for this reaction.

The double bond is broken and the bromine atoms are added. This is an addition reaction.

During this reaction there is a colour change from orange to colourless.

For example:

This is how we can test for the presence of an alkene or another type of unsaturated molecule.

 

 

4:28 describe how bromine water can be used to distinguish between an alkane and an alkene

In the absence of UV light an alkane added to bromine water will not react: the bromine water will stay orange.

However, alkenes react with bromine water even without UV light. There will be a colour change of orange to colourless.

 

 

 

4:44 know that an addition polymer is formed by joining up many small molecules called monomers

One bond in the double bond breaks.

Monomers join together to form a long chain.

Polymer contains only single bonds.

4:45 understand how to draw the repeat unit of an addition polymer, including poly(ethene), poly(propene), poly(chloroethene) and (poly)tetrafluroethene

                              

 

 

                              

 

 

                              

 

 

                              

4:46 understand how to deduce the structure of a monomer from the repeat unit of an addition polymer and vice versa

To deduce the structure of the monomer from a repeat unit:

  1. Remove the extending single bonds
  2. Draw in a double bond

 

 

 

Here’s a more complicated example, going from the polymer to the structure of the monomer

4:47 explain problems in the disposal of addition polymers, including: their inertness and inability to biodegrade, the production of toxic gases when they are burned

Polymers are inert (unreactive) as they have strong C-C bonds.

This makes them non-biodegradeable.

Biodegradable: the breakdown of a substance by microorganisms.

if burnt the addition polymers could produce toxic gases such as carbon monoxide and hydrogen chloride.

Select a set of flashcards to study:

     Terminology

     Skills and equipment

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Section 1: Principles of chemistry

      a) States of matter

      b) Atoms

      c) Atomic structure

     d) Relative formula masses and molar volumes of gases

     e) Chemical formulae and chemical equations

     f) Ionic compounds

     g) Covalent substances

     h) Metallic crystals

     i) Electrolysis

 Section 2: Chemistry of the elements

     a) The Periodic Table

     b) Group 1 elements: lithium, sodium and potassium

     c) Group 7 elements: chlorine, bromine and iodine

     d) Oxygen and oxides

     e) Hydrogen and water

     f) Reactivity series

     g) Tests for ions and gases

Section 3: Organic chemistry

     a) Introduction

     b) Alkanes

     c) Alkenes

     d) Ethanol

Section 4: Physical chemistry

     a) Acids, alkalis and salts

     b) Energetics

     c) Rates of reaction

     d) Equilibria

Section 5: Chemistry in industry

     a) Extraction and uses of metals

     b) Crude oil

     c) Synthetic polymers

     d) The industrial manufacture of chemicals

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