Alkenes

Question 1

What do all alkenes contain?

Answer 1

One double covalent bond between two carbon atoms

Question 2

What is the general formula for alkenes?

Answer 2

CₙH₂ₙ

Question 3

Why are alkenes considered unsaturated hydrocarbons?

Answer 3

• They have a double bond, which can open up to form single bonds to additional atoms meaning they are not maximally bonded
• They are comprised of hydrogen and carbon only

This means alkenes are much more reactive than alkanes

Question 4

What are the first four alkenes and their molecular formulae?

Answer 4

• Ethene - C₂H₄
• Propene - C₃H₆
• Butene - C₄H₈
• Pentene - C₅H₁₀

Make sure you know how to draw their displayed formulae

Question 5

How do you name isomers of alkenes?

Just flip over

Answer 5

As shown:

Make sure you understand

Question 6

What type of reaction is the reaction between alkenes and bromine?

Answer 6

Addition

Question 7

What is the word and symbol equation for alkenes reacting with bromine?

(Using ethene as an example)

Answer 7

• Ethene + Bromine –> 1,2-Dibromoethane
• C₂H₄ + Br₂ –> C₂H₄Br₂

• The double bond opens up, allowing the two carbon atoms to bond to another atom each
• The 1,2 indicates which of the carbon atoms along the chain has bromine bonded to it

Question 8

What is the test to distinguish between an alkane and an alkene and why are the results the way they are?

Answer 8

• Add bromine water, an orange solution to the sample of either an alkene or an alkane
• If it is added to an alkene, the solution will go colourless, if it is added to an alkane, nothing will happen and the solution will remain orange
• This is because alkenes are unsaturated, so can accept bromine in an addition reaction forming a colourless solution, however alkanes are saturated, so there is no reaction and it stays orange

• Both alkanes and alkenes are colourless to begin with
• A flame test can also be used where alkenes burn with a smoky flame and alkanes with a less smoky one