SC22

Question 1

SC22a
1) State the names, formulae and structures of the first four members of the alkane homologous series.
2) Distinguish between saturated hydrocarbons and unsaturated hydrocarbons

Answer 1

1) The general formula is: CnH2n+2
Methane: CH4
Ethane: C2H6
Propane: C3H8
Butane: C4H10
2) Hydrocarbons that are saturated contain single bonds of carbon, and contain only carbon and hydrogen atoms. Hydrocarbons that are unsaturated contain double bonds of carbon, and only contain carbon and hydrogen atoms.

Question 2

SC22a
1) State the names, formulae and structures of the first three members of the alkene homologous series.
2) Define the term ‘functional group’ and describe the functional group in alkenes.
3) What is an isomer, and describe the similarities and differences between butane, but-1-ene and but-2-ene.

Answer 2

1) The functional group for alkenes is C=C
Ethene: C2H4
Propene: C3H6
Butene: C4H8
2) A functions group is an atom or group of atoms that is mainly responsible for a molecule’s chemical properties. Define the term ‘functional group’ and describe the functional group in alkenes.
3) Isomers are molecules with the same molecular formula but a different arrangement of atoms. Butene has two isomers. But-1-ene has a carbon double bond at the right end, while but-2-ene has the double carbon bond in the middle.

Question 3

SC22b
1) Describe what an ‘addition reaction’ is.
2) Describe the reaction of bromine with ethene and other alkenes, and the equation
3) Recall how bromine water is used to distinguish between alkanes and alkenes.

Answer 3

1) An addition reaction is a reaction in which reactants combine to form one larger product molecule and no other products.
2) Alkenes react with bromine (Br2) to produce dibromoalkanes at room temperature. Bromine reacts with ethene to produce a colourless liquid called 1,2-dibromoethane. This is C2H4Br2.
3) Bromine water is a dilute solution of bromine in water. It has an orange-brown colour. When it is mixed with alkenes a chemical reaction occurs, leading to colourless product. Alkanes do not cause decolorisation and so bromine water is used to test between alkanes and alkenes.

Question 4

SC22b
1) Explain how the bromine water test distinguishes between alkanes and alkenes.
2) Recall the products of complete combustion of alkanes and alkenes.
3) Explain why the products of the complete combustion of a hydrocarbon are carbon dioxide and water.

Answer 4

1) The test works because the carbon double bond reacts with the bromine to form a colourless product. The bromine is therefore removed from the solution, which loses its colour. However, for alkanes the bromine water remains orange-brown.
2) The products of the complete combustion of alkanes and alkenes are water and carbon dioxide.
3) Carbon and hydrogen atoms in the hydrocarbon fuel react with oxygen in an exothermic reaction: carbon dioxide and water are produced, and energy is released.