Alkenes (Edexcel IGCSE Chemistry): Exam Questions

This question is about alkenes and alkanes.

Complete the table by giving the missing information about the alkene with the molecular formula C₃H₆

Alkenes are unsaturated compounds.

i) State what is meant by the term unsaturated.

(1)

ii) Describe a test to show that a compound is unsaturated.

(2)

When the alkane methane reacts with chlorine, the products are chloromethane (CH₃Cl) and hydrogen chloride gas.

i) Give a chemical equation for this reaction.

(1)

ii) What is the name of this type of reaction?

(1)

iii) State the condition needed for this reaction to occur.

(1)

When ethane reacts with chlorine, one of the products of the reaction has the formula C₂H₄Cl₂ There are two isomers with this formula.

i) State what is meant by the term isomers.

(2)

ii) Draw the displayed formulae of the two isomers with the formula C₂H₄Cl₂

(2)

Ocimene is an organic compound that gives some plants their particular smell.

The molecular formula of ocimene is C₁₀H₁₆

Calculate the relative formula mass (M_r) of ocimene.

 M_r = ..............................

Using ocimene as an example, explain what is meant by the term empirical formula.

The displayed formula of ocimene is

Explain why ocimene is described as an unsaturated hydrocarbon.

Ocimene is an alkene.

 i) Which of these types of reaction occurs between ocimene and bromine? 

(1)

 ii) Many alkenes have the general formula C_nH_2n

 Suggest why ocimene does not have this general formula.

 (1)

Ocimene can take part in combustion reactions. 

Complete the equation for the complete combustion of ocimene.

C₁₀H₁₆ + .......... O₂ → .............................. + ..............................

Two different products can form during the incomplete combustion of ocimene. 

One product is a solid and the other is a poisonous gas.

 i) Identify these two products.

 (2)

 ii) State why the gas produced is poisonous.

 (1)

The structures of some organic compounds are shown below. 

Compound R can be converted to compound T by reacting it with bromine. 

What kind of reaction is this?

This question is about the unsaturated hydrocarbon, ethene.

The displayed formula of ethene is

i) State the meaning of the term hydrocarbon. 

(2)

 ii) Give the reason why ethene is described as unsaturated.

 (1)

Ethene is bubbled through bromine water until there is no further colour change. 

Which of these is the appearance of the solution formed?

Ethanol is produced industrially by the reaction between ethene and steam. 

The equation for the reaction is

CH₂CH₂ (g) + H₂O (g) → CH₃CH₂OH (l)

 i) State the temperature and pressure used in this reaction.

 (2)

 temperature ..............................

 pressure ..............................

 ii) Give the molecular formula of ethanol.

 (1)

Name the product formed from the reaction between ethene and chlorine. 

The table shows the structures of six organic compounds.

i) Give the letter of a compound not shown as a displayed formula.

 (1)

 ii) Give the letter of a saturated compound with the general formula C_nH_2n

 (1)

 iii) Name compound E.

 (1)

 iv) Explain why compound A and compound D are isomers.

 (2)

Compound F reacts with bromine in the presence of ultraviolet radiation.

 i) Complete the equation for the reaction.

 (1)

   CH₄ + Br₂ →

 ii) Give the name of this type of reaction.

 (1)

i) Another compound, G, has this percentage composition by mass.

 C = 37.8%   H = 6.3%   Cl = 55.9%

 Show by calculation that the empirical formula of compound G is C₂H₄Cl

 (3)

 ii) The relative formula mass (M_r) of G is 127

 Determine the molecular formula of G.

 (2)

 molecular formula = ..............................

Compound E is used to make an addition polymer.

 i) Complete the equation to show part of the polymer formed from two molecules of compound E.

 (2)

 ii) Give one problem caused by the disposal of addition polymers.

  (1)

The organic compound butadiene is a colourless gas used in the manufacture of synthetic rubber for tyres. The displayed formula of butadiene is

Explain why butadiene is described as an unsaturated hydrocarbon.

i) Butadiene reacts with bromine water. State the colour change that occurs during this reaction.

(1)

from ..................................................... to ..............................................................

ii) The equation for the reaction between butadiene and bromine can be shown using displayed formulae.

The table gives some bond energies.

Use this information to calculate the enthalpy change, ΔH, for the reaction. Include a sign in your answer.

(4)

ΔH = ................................ kJ/mol

A scientist does an investigation to find out if butadiene would be a good fuel. He burns a sample of butadiene gas and  observes that carbon forms as black soot.

i) Complete the equation to explain the scientist’s observation.

(1)

2C₄H₆ + 7O₂ → ..........C + 4CO + 2CO₂ + ..........H₂O

ii) Explain how one of the products, other than carbon, may cause a problem.

(2)

iii) The equation for the combustion of butadiene in excess oxygen is

2C₄H₆ + 11O₂ → 8CO₂ + 6H₂O

   The enthalpy change for this reaction, ΔH, is – 3446 kJ/mol.

   Complete the energy profile diagram for the reaction.

   Label the enthalpy change for this reaction, ΔH, and the activation energy.

(4)

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)

i) Give the structural formula and name of the product of the reaction between ethene and bromine.

(2)

ii) State the type of reaction that occurs when ethene reacts with bromine. 

(1)

iii) Hydrogen will react in a similar way to bromine when it reacts with ethene.

Suggest the molecular formula of the product of the reaction between hydrogen and bromine.

(1)

Alkenes can be oxidised to carboxylic acids.

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 produce the following carboxylic acids when oxidised.

ethanoic acid and propanoic acid

only ethanoic acid

Alkenes polymerise to form addition polymers. Draw the displayed formula of poly(cyanoethene), include at least two monomer units. The displayed formula of the monomer, cyanoethene, is given below.

The alkenes are unsaturated hydrocarbons. They form a homologous series, the members of which undergo the following chemical reaction.

• addition reactions

• polymerization

• combustion.

Alkenes that have one carbon-carbon double bond have the same empirical formula.

i) State their empirical formula.

(1)

ii) Why is the empirical formula the same for these alkenes?

(1)

Complete the following equation for the addition reaction of propene.

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

Draw the repeat unit 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.

i) Determine the following mole ratio.

 moles of alkene : moles of O₂ : moles of CO₂

 From this ratio determine the formula of the alkene.

(3)

ii) Write an equation for the complete combustion of this alkene.

(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.

(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)

The structures of an alkane and an alkene are shown below. 

i) Compare the structures of propene with propane.

(4)

ii) Compare the reactions of propene with propane.

(4)

Write a balanced symbol equation for the incomplete combustion of propene to form carbon monoxide and water.