Alkenes (Cambridge (CIE) O Level Chemistry): Exam Questions

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

What is the general formula of alkenes?

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

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

The alkenes are a series of unsaturated hydrocarbons. They have the general molecular formula C_nH_2n.

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

Show your working.

The structural formula of propene is drawn below.  

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. 

Use the data in the table to show that the following reaction is exothermic. 

 [3]

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] 

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]

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 + ....................

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.

[1]

ii) The structural formula of a poly(alkene) is given below.

Deduce the structural formula of its monomer.

[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]

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]

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.

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]

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

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

bromine, ......................................................................................................................

[1]

hydrogen, ....................................................................................................................

[1]

steam. .........................................................................................................................

[1]

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.

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]

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

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

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

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

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]

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]

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]

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]

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.

octane → butane + butene

C₈H₁₈ → C₄H₁₀ + C₄H₈

 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]

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]

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]

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 .................................................. %

A hydrocarbon has the following structural formula.

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]

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]

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.

[3]

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

name ..........................................................................................................

structural formula

[2]

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]

A refinery obtains large quantities of the heavier fractions from crude oil, but the market demand is for the lighter fractions and for alkenes.

i) Explain how cracking octane helps the refinery meet this demand.

[2]

ii) Suggest why the refinery would not crack the lighter fractions.

[1]

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]

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 ......................................... %

Undecane, C₁₁H₂₄, is cracked.

Three products are formed, A, B and C.

Product A burns with a squeaky pop when a lighted splint is held near it.

Identify A.

State what its formation shows about cracking.

Product B has a relative molecular mass of 56.

Product B decolourises aqueous bromine.

Deduce the molecular formula of B.

Explain your reasoning.

Deduce the molecular formula of product C.

A catalyst is used in this reaction.

Suggest why.

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.

i) Explain why these two hydrocarbons are isomers.

[2]

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

[1]

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]

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.

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]

Alkenes can be oxidised to carboxylic acids.

Pent-2-ene, CH₃CH₂CH=CHCH₃, oxidises to CH₃CH₂COOH and CH₃COOH.

Name these two acids.

Complete the following equations for the addition reactions of propene.

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

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

Draw the structural formula of poly(propene).

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]

Petroleum is a source of many important chemicals.

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

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]

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.

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]

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  →  ..................... + ......................        

[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]

The following alkenes are 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]

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

steam .........................................................

hydrogen ....................................................

bromine ......................................................

Alkenes can polymerise.

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

name of monomer .......................................................    

structural formula

[2]

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

[2]

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

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

[2]

burning ..........................................................................................................

[1]

Cracking converts large alkane molecules into smaller molecules. Hydrogen is also produced.

Name the industrial process in which hydrogen and nitrogen react together to form ammonia.

The table shows the percentage of crude oil that separates into each group of fractions, and the percentage of each group that is in demand.

Use the data in the table to explain why cracking is economically important to an oil refinery.

When a large alkane is cracked, the products cannot all be alkanes.

Explain why.

Iodine(I) chloride, ICl, takes part in addition reactions with alkenes in the same way as bromine.

Iodine(I) chloride reacts with ethene.

Give the structural formula of the product.

Explain why the reaction between iodine(I) chloride and ethene is described as an addition reaction.

Iodine(I) chloride also reacts with propene, CH₃CH=CH₂, in an addition reaction.

Give a structural formula for the product.

Aqueous bromine is added to the product formed from ethene and iodine(I) chloride.

State and explain what is seen.

Compound Y has the structural formula shown.

CH₃CHBrCH₂Br

Y is made from decane, C₁₀H₂₂, in two stages.

Stage 1 is cracking. Stage 2 is an addition reaction with bromine.

Name compound Y.

Deduce the structural formula of the alkene used in stage 2.

Explain how you deduced it.

Write an equation for the cracking of decane, C₁₀H₂₂, that produces the alkene used in stage 2.

Some vegetable oils are described as polyunsaturated.

Compound P has the structural formula shown.

CH₂=CHCH₂CH=CH₂

P is also described as polyunsaturated.

Suggest what is meant by polyunsaturated.

One molecule of P reacts with excess hydrogen in the presence of a catalyst.

Deduce the molecular formula of the product.

Show your reasoning.

Name the homologous series to which the product belongs.