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O LevelChemistryCambridgeExam Questions11. Organic Chemistry11.5 Alkenes

Alkenes (Cambridge O Level Chemistry)

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11.5 Alkenes

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11.5 Alkenes

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1a
Sme Calculator4 marks

A compound X contains 82.76% of carbon by mass and 17.24% of hydrogen by mass.

i)
Calculate the empirical formula of compound X.

[2]

ii)
Compound X has a relative molecular mass of 58.

Deduce the molecular formula of compound X.

[2]

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1b2 marks

Alkenes are unsaturated hydrocarbons.

i)
State the general formula of alkenes.

[1]

ii)
State the empirical formula of alkenes.

[1]

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1c2 marks

What is meant by the term unsaturated hydrocarbon?

unsaturated

hydrocarbon 

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1d3 marks

Describe a test that would distinguish between saturated and unsaturated hydrocarbons.

Reagent: 

Observation (saturated hydrocarbon):

Observation (unsaturated hydrocarbon):

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1e4 marks

Addition polymers can be made from alkenes. The diagram shows part of an addition polymer.

polymer-polybutene

i)
Draw a circle on the diagram to show one repeat unit in this polymer.

[1]

ii)
Give the structure and the name of the monomer used to make this polymer.

[2]

iii)
Give the structure of an isomer of the alkene in (e)(ii).

[1]

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2a1 mark

Large hydrocarbons can be cracked to form smaller hydrocarbons.
Complete the chemical equation for cracking tridecane, C13H28, to form an alkene and one other hydrocarbon.

C13H28 → C3H6 + ..............

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2b1 mark

Ethene is an alkene.
Draw the structure of ethene showing all of the atoms and all of the bonds.

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3a1 mark

The properties of five alkenes at room temperature are shown in the table.

alkene number of carbon atoms in a molecule state at room temperature density
in g / cm3		 boiling point
/ °C
ethene 2 gas 0.0012 –104
propene 3 gas 0.0018 –47
butene 4 gas 0.0024  
pentene 5 liquid 0.64 30
hexene 6 liquid 0.67 63

Answer these questions using only the information in the table.

Describe the general trend in the density of the alkenes.
 

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3b1 mark
Predict the boiling point of butene.
 

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1a1 mark

The structures of five alkenes, A, B, C, D and E, are shown.

q5-0620_s19_qp_41

What is the general formula of alkenes?

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1b2 marks

Two different alcohols can be produced from alkene B by an addition reaction.

q5f-0620_s19_qp_41

Draw the structures of the two alcohols. Show all of the atoms and all of the bonds.

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2a
Sme Calculator2 marks

The alkenes are a series of unsaturated hydrocarbons. They have the general molecular formula CnH2n.

Deduce the molecular formula of an alkene which has a relative molecular mass of 126.

Show your working.

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2b
Sme Calculator8 marks

The structural formula of propene is drawn below. 

 
propene
 
i)
Draw a diagram showing the arrangement of the valency electrons in one molecule of this covalent compound.
 
Use x to represent an electron from an atom of carbon.
Use o to represent an electron from an atom of hydrogen.
 
[3]
 
ii)
Draw the structure of the polymer formed from propene
 
[2]
 
iii)
Bond energy is the amount of energy, in kJ, which must be supplied to break one mole of the bond.
 
bond bond energies in kJ / mol
H-H +436
C=C +610
C-C +346
C-H +415
 
Use the data in the table to show that the following reaction is exothermic.
 
cie-igcse-hydrogenation-of-propene
 
[3]

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2c4 marks

This question is concerned with some of the addition reactions of but-1-ene.

 
i)
Name the product formed when but-1-ene reacts with water.
 
[1]
 
ii)
Complete the equation.
 
CH3–CH2–CH=CH2 + Br2 → .....................................................................
 
[2]
 
iii)
Deduce the formula of the compound which reacts with but-1-ene to form 1-iodobutane.
 
[1] 

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3a3 marks

Alkanes and alkenes are both series of hydrocarbons.

i)
Explain the term hydrocarbon.

[1]
ii)
What is the difference between these two series of hydrocarbons?

[2]

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3b1 mark

Alkenes and simpler alkanes are made from long-chain alkanes by cracking.

Complete the following equation for the cracking of the alkane C20H42.

C20H42  → 2C4H8 + 2C2H4 + ...............

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3c7 marks

Alkenes such as butene and ethene are more reactive than alkanes.


Alkenes are used in the petrochemical industry to make a range of products, which includes polymers and alcohols.

i)
Dibromoethane is used as a pesticide. Complete the equation for its preparation from ethene.

ethene-bromination

[1]

ii)
The structural formula of a poly(alkene) is given below.
Deduce the structural formula of its monomer.
polybut-2-ene-q-image
[2]

iii)
How is butanol made from butene, CH3 –CH2 –CH=CH2?

Include an equation in your answer.

[2]
iv)
Cracking changes alkanes into alkenes. How could an alkene be converted into an alkane?

Include an equation in your answer.
[2]

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3d4 marks

20 cm3 of a hydrocarbon was burnt in 175 cm3 of oxygen. After cooling, the volume of the remaining gases was

125 cm3. The addition of aqueous sodium hydroxide removed carbon dioxide leaving 25 cm3 of unreacted oxygen.

Volume of oxygen used = .......... cm3
[1]

Volume of carbon dioxide formed = .......... cm3
[1]

Deduce the formula of the hydrocarbon and the balanced equation for the reaction.
[2]

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4a6 marks

But-1-ene is a typical alkene. It has the structural formula shown below.

CH3–CH2–CH=CH2

The structural formula of cyclobutane is given below.

cyclobutane-displayed-black

These two hydrocarbons are isomers.

 
i)
Define the term isomer.
 
[2]
 
ii)
Draw the structural formula of another isomer of but-1-ene.
 
[1]
 
iii)
Describe a test which would distinguish between but-1-ene and cyclobutane.
 
reagent ..................................................
 
result with but-1-ene ..................................................
 
result with cyclobutane ..................................................
 
[3]

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4b2 marks

Describe how alkenes, such as but-1-ene, can be made from alkanes.

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4c3 marks

Name the product formed when but-1-ene reacts with:

 
bromine, ......................................................................................................................
[1]
 
hydrogen, ....................................................................................................................
[1]
 
steam. .........................................................................................................................
[1]

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5a4 marks

The alkenes are a homologous series of unsaturated hydrocarbons.

The table below gives the names, formulae and boiling points of the first members of the series.

Name Formula Boiling point  / oC
 ethene C2H4 -103.7
 propene C3H6 -47.6
 butene C4H8 -6.3
 pentene C5H10 30
 hexene    

i)
Complete the table by giving the formula of hexene and by predicting its boiling point.

[2]

ii)
Deduce the formula of the alkene which has a relative molecular mass of 168.
Show your working.

[2]

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5b3 marks

Describe a test that will distinguish between the two isomers, but-2-ene and cyclobutane.

test ..............................................................................................................................................................................

result with but-2-ene  ..........................................................................................................................................

result with cyclobutane  ......................................................................................................................................

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5c3 marks

Alkenes undergo addition reactions.

i)
What class of organic compound is formed when an alkene reacts with water?
[1]

ii)
Draw the structure of the polymer formed from but-2-ene.

[2]

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6a7 marks

Two homologous series of hydrocarbons are the alkanes and the alkenes.

One general characteristic of a homologous series is that the physical properties vary in a predictable way.

i)
State three other general characteristics of a homologous series.

[3]

ii)
How can the molecular formula of a hydrocarbon show whether it is an alkane or an alkene?

[2]

iii)
How do alkanes and alkenes differ in their molecular structures?
[2]

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6b5 marks

Cracking is the thermal decomposition of alkanes into smaller hydrocarbons and possibly hydrogen.

i)
State two conditions required for the cracking of an alkane.

[2]

ii)
One type of cracking produces an alkane and an alkene.

Complete an equation for the cracking of heptane into an alkane and an alkene.

C7H16 → ................ + ................ 

[1]

iii)
Complete an equation for the cracking of heptane into hydrogen and two other products.

C7H16  →................ + ................ + H2

[1]

iv)
Suggest one reason why cracking is important.
 [1]

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6c2 marks

Hydrocarbons burn in excess oxygen to form carbon dioxide and water. 20 cm3 of a gaseous hydrocarbon burned in an excess of oxygen, 200 cm3. After cooling, the volume of the residual gas at r.t.p. was 150 cm3, 50 cm3 of which was
oxygen.

i)
Determine the volume of the oxygen used.
[1]

ii)
Determine the volume of the carbon dioxide formed.
[1]

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7a3 marks

The fractional distillation of crude oil usually produces large quantities of the heavier fractions. The market demand is for the lighter fractions and for the more reactive alkenes. The heavier fractions are cracked to form smaller alkanes and alkenes as in the following example.

C8H18  → C4H10  +      C4H8
octane    butane             butene

 

i)
Write a different equation for the cracking of octane.
 
C8H18 → .............................. + ..............................
 
[1]
 
ii)
The cracking of octane can produce isomers with the molecular formula C4H8.
 
Draw the structural formulae of two of these isomers.
 
[2]

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7b4 marks
i)
Give the essential condition for the reaction between chlorine and butane.
 
[1]
 
ii)
What type of reaction is this?
 
[1]
 
iii)
This reaction produces a mixture of products. Give the names of two products that contain four carbon atoms per molecule.
 
.................................................. and ..................................................
 
[2]

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7c2 marks

Alkenes are more reactive than alkanes and are used to make a range of organic chemicals. Propene, CH3–CH=CH2, is made by cracking. Give the structural formula of the addition product when propene reacts with the following.

 
i)
water
 
[1]
 
ii)
bromine
 
[1]

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7d
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Propene reacts with hydrogen iodide to form 2-iodopropane.

CH3−CH=CH2 + HI → CH3−CHI−CH3

1.4 g of propene produced 4.05 g of 2-iodopropane.

Calculate the percentage yield.

moles of CH3–CH=CH2 reacted = ..................................................

maximum moles of CH3–CHI–CH3 that could be formed = ..................................................

mass of one mole of CH3–CHI–CH3 = 170 g

maximum mass of 2- iodopropane that could be formed = ..................................................

percentage yield .................................................. %

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8a6 marks

A hydrocarbon has the following structural formula.

hydrocarbon

i)
State the molecular formula and the empirical formula of this hydrocarbon.
 
molecular formula ...............................................................................................................
empirical formula ................................................................................................................

[2]

ii)
Draw the structural formula of an isomer of the above hydrocarbon.
 
[1]

iii)
Explain why these two hydrocarbons are isomers.

[2]
iv)
Are these two hydrocarbons members of the same homologous series?
Give a reason for your choice.
 
[1]

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8b3 marks

Alkenes can be made from alkanes by cracking.

i)
Explain the term cracking.

[2]

ii)
One mole of an alkane, when cracked, produced one mole of hexane, C6H14, and two moles of ethene.

What is the molecular formula of the original alkane?

[1]

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8c5 marks

Alkenes are used in polymerisation reactions and addition reactions.

i)
Draw the structural formula of the product formed by the addition polymerisation of but-2-ene. Its formula is given below.

sfVPdcYZ_but-2-ene

[3]

ii)
Give the name and structural formula of the addition product formed from ethene and bromine.

name ...................................................................................................................................
structural formula

[2]

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9a3 marks

The fractional distillation of crude oil usually produces large quantities of the heavier fractions.

The market demand is for the lighter fractions and for the more reactive alkenes. The heavier fractions are cracked to form smaller alkanes and alkenes as in the following example.

C8H18 →   C4H10 + C4H8
octane     butane    butene

 

i)
Write a different equation for the cracking of octane.

C8H18  → ...................... + .....................

[1]

ii)
The cracking of octane can produce isomers with the molecular formula C4H8.

Draw the structural formulae of two of these isomers.
[2]

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9b4 marks
i)
Give the essential condition for the reaction between chlorine and butane.

[1]

ii)
What type of reaction is this?

[1]

iii)
This reaction produces a mixture of products. Give the names of two products that contain four carbon atoms per molecule.

[2]

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9c2 marks

Alkenes are more reactive than alkanes and are used to make a range of organic chemicals. Propene, CH3–CH=CH2, is made by cracking. Give the structural formula of the addition product when propene reacts with the following.

 

i)
water

[1]

 

ii)
bromine

[1]

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9d
Sme Calculator4 marks

Propene reacts with hydrogen iodide to form 2-iodopropane.

CH3−CH=CH2 + HI →  CH3−CHI−CH3

1.4 g of propene produced 4.0 g of 2-iodopropane.

Calculate the percentage yield.

moles of CH3–CH=CH2 reacted = .........................................

maximum moles of CH3–CHI–CH3 that could be formed = .........................................

mass of one mole of CH3–CHI–CH3 = 170 g

maximum mass of 2- iodopropane that could be formed = .........................................

percentage yield ......................................... %

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1a3 marks

The alkenes are unsaturated hydrocarbons. They form a homologous series, the members of which have the same chemical properties.

They undergo addition reactions and are easily oxidised.

The following hydrocarbons are isomers.

hydrocarbon-isomers

i)
Explain why these two hydrocarbons are isomers.

[2]



ii)
Give the structural formula of another hydrocarbon which is isomeric with the above.

[1]

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1b6 marks

Give the structural formula and name of each of the products of the following addition reactions.

i)
ethene and bromine
structural formula of product



name of product ................................................................................................... 
[2]
ii)
propene and hydrogen
structural formula of product



name of product ................................................................................................... 
[2]
iii)
but-1-ene and water
structural formula of product



name of product ................................................................................................... 
[2]

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1c4 marks

Alkenes can be oxidised to carboxylic acids.

i)
For example, propene, CH3 –CH=CH2, would produce ethanoic acid, CH3 –COOH, and methanoic acid, H–COOH.

Deduce the formulae of the alkenes which would form the following carboxylic acids when oxidised.

ethanoic acid and propanoic acid



only ethanoic acid
[2]


 

ii)
Describe the colour change you would observe when an alkene is oxidised with acidified potassium manganate(VII).
[2]

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1d2 marks

Alkenes polymerise to form addition polymers.

Draw the structural formula of poly(cyanoethene), include at least two monomer units.

The structural formula of the monomer, cyanoethene, is given below.

polycyanoethane

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2a2 marks

The alkenes are unsaturated hydrocarbons. They form a homologous series, the members of which have similar chemical properties

   ● easily oxidised
   ● addition reactions
   ● polymerisation
   ● combustion.

All the alkenes have the same empirical formula.

i)
State their empirical formula.

[1]
ii)
Why is the empirical formula the same for all alkenes?
[1]

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2b3 marks

Alkenes can be oxidised to carboxylic acids by boiling with aqueous potassium manganate(VII).

i)
Pent-2-ene, CH3–CH2–CH=CH–CH3, oxidises to CH3–CH2–COOH and CH3COOH.

Name these two acids.

CH3–CH2–COOH .......................................................................................................
CH3COOH ............................................................................................................ 
[2]


ii)
Most alkenes oxidise to two carboxylic acids. Deduce the formula of an alkene which forms only one carboxylic acid.
[1]

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2c2 marks

Complete the following equations for the addition reactions of propene.

CH3–CH=CH2 + Br2 → .................................... 

CH3–CH=CH2 + H2O → ....................................

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2d2 marks

Draw the structural formula of poly(propene).

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2e4 marks

0.01 moles of an alkene needed 2.4 g of oxygen for complete combustion. 2.2 g of carbon dioxide were formed.

Determine the following mole ratio.

moles of alkene : moles of O2 : moles of CO2

From this ratio determine the formula of the alkene.


[3]

   Write an equation for the complete combustion of this alkene.

[1]

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3a2 marks

Petroleum is a source of many important chemicals.

Name two industrial processes which must take place to produce alkenes from petroleum.

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3b4 marks

Ethene, CH2=CH2, and propene, CH2=CHCH3, can both be converted into polymers.

 

i)
What type of polymerisation takes place when ethene forms a polymer?

[1]
ii)
What is the empirical formula of the polymer formed from ethene?

[1]
iii)
Propene has the structural formula CH2=CHCH3.

Draw two repeat units of the polymer made from propene.
[2]

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3c2 marks

Ethene will react with steam to form ethanol.

Propene will react with steam to form two isomers, both of which are alcohols.

Suggest the structures of these alcohols.

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3d5 marks

Esters are organic chemicals noted for their characteristic smells. Ethanoic acid and methanol will react to form an ester.

i)
Name the catalyst needed to form an ester from ethanoic acid and methanol.

[1]
ii)
Name the ester formed when ethanoic acid reacts with methanol.

[1]
iii)
Draw the structure of the ester formed when ethanoic acid reacts with methanol. Show all bonds.

[2]
iv)
Give the name of a polyester.
[1]

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4a4 marks

Alkenes are unsaturated hydrocarbons. They undergo addition reactions.

Two of the methods of making alkenes are cracking and the thermal decomposition of chloroalkanes.

i)
Complete an equation for the cracking of the alkane, decane.

C10H22  →  ..................... + ......................
decane        

[2]

ii)
Propene can be made by the thermal decomposition of chloropropane.

Describe how chloropropane can be made from propane.

reagents:  propane and ..............................
conditions ...................................................

[2]

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4b4 marks

The following alkenes are isomers.

alkene-isomers

i)
Explain why they are isomers.
[2]
ii)
Give the name and structural formula of another hydrocarbon that is isomeric with the above.

name ......................................................
structural formula

[2]

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4c3 marks

Give the name of the product when but-1-ene reacts with each of the following.

steam .........................................................
hydrogen ....................................................
bromine ......................................................

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4d7 marks

Alkenes can polymerise.

i)
Deduce the name and structural formula of the monomer from the structure of the polymer.

polypropene-repeat-unit

   name of monomer .......................................................
    structural formula

[2]

ii)
Draw the structure of the polymer formed from the following monomer.

unknown-monomer

[2]

iii)
Describe the pollution problems caused by the disposal of polymers in landfill sites and by burning.

landfill sites ...............................................................................................................................

[2]
burning .......................................................................................................................................
[1]

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