Alkenes (T3)

Question 1

What are alkenes?

Answer 1

• unsaturated hydrocarbons - they contain a carbon-carbon double bond - the presence of the double bonds means that they do not contain as many hydrogen atoms as the corresponding alkane

Question 2

What are the three smallest alkenes and their chemical formulas?

Answer 2

• ethene: C2 H4 - propene: C3 H6 - butene: C4 H8

Question 3

How are the physical properties of alkenes similar to alkanes?

Answer 3

• the small alkenes with up to four carbon atoms are gases - they are gases up to C4 H8

Question 4

How are the chemical reactions of alkenes similar to alkanes?

Answer 4

They also burn in air or oxygen to give carbon dioxide and water

Question 5

Explain what happens when bromine is added to an alkene

Answer 5

• when bromine is added to an alkene, the alkene undergo an addition reaction - part of the double bond breaks, freeing up two electrons for additional hydrogen atoms and adding bromine atoms in the place of one of the hydrogens - bromine adds to alkenes without any need for heat or a catalyst - this reaction is often carried out using bromine solution

Question 6

What is produced if bromine is added to ethene?

Answer 6

• 1,2-dibromoethane is produced - it is a colourless liquid

Question 7

Why is there no alkene with just one carbon atom?

Answer 7

Because all alkenes contains a carbon-carbon double bond

Question 8

Why would not every carbon in a alkene be linked by a double bond to the next one?

Answer 8

• all it take is one double bond to be present to be an ‘-ene’ - an ‘-ane’ will always be single bonds

Question 9

What is CH2=CH2 and why would it be written like that?

Answer 9

• ethene - its standard written formula is C2 H4 but by writing it as CH2=CH2 it highlights the double bond between the two carbon atoms

Question 10

Describe how propene only has one double bond…

Answer 10

• C3 H6 - there is only one double bond in the structure - the end carbon with the single bond has three hydrogen atoms bonded to it - if each carbon atom was double bonded to the hydrogen atoms next to it there would be no spare electrons to bind to hydrogen atoms

Question 11

Describe how butene only has one carbon-carbon double bond…

Answer 11

• C4 H8 - the end carbon has three hydrogen atoms bonded to it while the middle carbon only has one, balancing out the general formula