Investigating alkanes and alkenes

IGCSE Chemistry

section 3 

  

Investigating alkanes and alkenes

 

Part A: Comparing combustion of an alkane and an alkene - demonstration

1.    Put a few drops of hexane (an alkane) on a watch glass or in an evaporating basin.

2.    Make sure the bottle is well away, then, ignite the liquid (preferably in a fume cupboard).

3.    Repeat this using hexene (an alkene).

Questions

1.    Describe the colour of the flame when hexane was burning

2.    Was there any difference between the alkane and alkene?

3.    Write a word equation for the complete combustion of hexane

4.    What is the black substance visible at the top of each flame and why is it formed?

5.    Write a word equation for the reaction in question 5

6.   What is the environmental impact of burning alkanes?

Part B: Using bromine water

1.    Place two small clean dry test tubes into your test tube rack.

2.    Place approximately 2 ml of the hexane into the first test tube and one ml of hexene into the second.

3.    Add 5 drops of the bromine water into each of the two samples, taking care to not breath the bromine vapor or spill the bromine on your skin.

4.    Cover each test tube with a bung and, with your finger on the bung, shake each test tube. 

5.    Record any colour changes i.e. colour before and after the addition of the bromine water in a table.

Questions

1.    In which test tube did an immediate reaction take place?  Give a reason for your answer.

2.    What reaction took place in the test tube you answered in (1)?

3.    Write a word equation and balanced symbol equation for the reaction.

4.    Could we use bromine water to find out if a hydrocarbon is an alkane (saturated) or an alkane (unsaturated). Explain your answer.

Part C: Using potassium manganate

Repeat steps 2 to 5 with both hexane ands hexane but using potassium manganate. Record your observations in a table

Questions

1.    In which test tube did an immediate reaction take place?  Give a reason for your answer.

2.    Could we use potassium manganate to find out if a hydrocarbon is an alkane (saturated) or an alkane (unsaturated). Explain your answer.

Reactions of alkanes and alkenes

IGCSE chemistry 

section 3 

    

Reactions of alkanes and alkenes

Alkanes

As alkanes are saturated they are not reactive.  However, there are three types of reactions they carry out.

1.    Combustion

This is our luck as we use the shorter alkanes as fuels. Combustion is an exothermic reaction.  During complete combustion (which means in enough oxygen) alkanes produce carbon dioxide and water.  When there is not sufficient oxygen carbon monoxide or carbon and water are produced.

a.    Write a word equation for the complete combustion of methane.

b.    Write a balanced symbol equation for the complete combustion of methane.

c.    Write a word equation for the incomplete combustion of ethane to form carbon monoxide and water.

2.  Substitution

The balanced symbol equation below shows another type of reaction of alkanes. This reaction only happens in the presence of UV light (i.e. sunlight).

                          CH₄   +    Br₂   ®  CH₃Br  +   HBr

a.    Make the reactants with the ball and sticks – use green for bromine and “do” the reaction.

b.    Describe what has happened during the reaction.

c.    Rewrite the equation using displayed formula.

3. Cracking

The longer alkanes are not so good as fuels because they do not burn easily so they are used to make alkenes by a reaction called cracking.  More on this reaction in section 5c.

Alkenes

1.    Combustion

a.    Write a word equation for the complete combustion of ethene.

b.    Write a balanced symbol equation for the complete combustion of ethene.

We don’t use alkenes as fuels because they are more useful to us because of the reaction below.

2.  Addition

The balanced symbol equation below shows another type of reaction of alkenes. This reaction happens in any condition even in the dark!!

                          C₂H₄   +    Br₂   ®  C₂H₄Br₂

a.    Make the reactants with the ball and sticks – use green for bromine and “do” the reaction.

b.    Describe what has happened during the reaction.

c.    Rewrite the equation using displayed formula.

Reactions of halogens

IGCSE Chemistry 

Reactions of halogens   

 

Introduction

The Group 7 elements are called the halogens. This experiment involves some reactions of the halogens.

What to record

Complete the table by recording any colour changes or not.  Indicate the colours.

	Effect on indicator paper	Reaction with potassium chloride solution	Reaction with potassium bromide solution	Reaction with potassium iodide solution
Chlorine water
Bromine water
Iodine water

 

What to do

1.     Put a piece of Universal Indicator paper onto a white tile.

2.     Use a glass stirring rod to transfer a few drops of the first solution onto the indicator paper.

3.     Repeat this with a fresh piece of paper and the different solutions.

4.     In a test-tube, add some chlorine solution to a solution of potassium bromide.

5.     Add some chlorine solution to a solution of potassium iodide.

6.     Now try mixing solutions of bromine and potassium iodide. If there is time, mix the other combinations of solutions to complete the table.

Safety

Wear eye protection. Do not breathe chlorine gas.

 

Questions

1.     Which halogen solution is the strongest bleaching agent?

2.     Which halogen is the most reactive?

3.     Write a word and symbol equation for the reaction of chlorine with potassium bromide.

the migration of ions

IGCSE chemistry

section 1i 

the migration of ions

 

Introduction

In an electrolysis experiment, the ions migrate towards electrodes of opposite charge. In the first part of this experiment the migration of manganate ions is observed. In the second part of this experiment, silver and chromate ions meet as they migrate towards opposite electrodes. Silver chromate, an insoluble red compound is formed.

 

What to record

Draw a diagram to show the filter paper at the end of both experiments. Mark the ends of the paper with a + or – to show which terminal of the power supply each end is connected to.

 

What to do

 

Part 1

1.     Cut a piece of filter paper slightly smaller than a microscope slide. Draw a faint pencil line across the middle.

2.     Moisten the filter paper with tap water. Fasten the paper to the slide with crocodile clips.

3.     Use forceps to put a small crystal of potassium manganate(VII) in the centre of the paper.

4.     Connect the clips to a power supply set at not more than 20 V DC. Switch on and wait about ten minutes.

 

 

 

Part 2

1.     Cut a piece of filter paper slightly smaller than a microscope slide. Draw a faint pencil line across the middle.

2.     Moisten the paper with tap water. Fasten the paper to the slide with crocodile clips.

3.     At the end of the paper where the positive electrode will be placed, moisten the paper with silver nitrate solution. (Will stain fingers)

4.     At the end of the paper where the negative electrode will be placed, moisten the paper with potassium chromate solution. (Toxic).

5.     Connect the clips to a power supply set at no more than 20 V DC. Switch on and wait about 10 min, or until you see a change.

 

Safety

Wear eye protection.

 

 

Questions

1.     Potassium manganate (VII) consists of two ions ­– potassium ions and manganate (VII) ions. One of these ions is coloured. Which is it likely to be?

2.     From the direction of movement, what does this indicate about the charge on the manganate ion?

3.     What is happening in the second experiment?

4.      The formula of silver nitrate is AgNO₃ and potassium chromate is K₂CrO₄. Write the ionic formula equation for the reaction.