AS LEVEL CHEMISTRY

ALKANES

ASSESSED HOMEWORK

Answer all questions
Max 86 marks

	
Name		……………………………………………………………..

Mark		……../86	……....%	Grade ………

1.	The graph below shows the boiling points of some alkanes.

(a)	Draw a smooth curve through the points on the graph and estimate the boiling points of
	octane C8H18, ………………	hexadecane, C16H34 .………………
[2]
(b)	State how decane, C10H22, can be separated from a mixture of the alkanes.
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[1]

(c)	Isomerisation of hexane, C6H14, produces a mixture of structural isomers, three of which are shown in the boxes below.
(i)	Draw, using skeletal formulae, two other structural isomers of hexane.

[2]
(ii)	Name isomer B. ....................................................................................
[1]
(iii)	Isomers A, B and C have different boiling points. In the boxes below, list the isomers A, B and C in order of their boiling points.
lowest boiling point				highest boiling point	
[1]
(iv)	Explain the order given in (c) (iii).
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[2]
(d)	Hexane can be reformed to produce cyclohexane as one of the products.
(i)	Draw the structural formula of cyclohexane.

[1]
(ii)	Write a balanced equation for the reforming of hexane into cyclohexane.

[1]
(iii)	Suggest one reason why oil companies reform alkanes such as hexane.
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[1]
 (e)	Oxygen-containing compounds can be added to improve the efficiency and performance of fuels.
	In Formula One racing cars, it is common practice to add oxygen-containing compounds, such as 2-methylpropan-2-ol, (CH3)3COH. The amount of oxygen-containing compounds added is strictly controlled by the Federation Internationale de l’Automobile, FIA.
(i)	Calculate the percentage by mass of oxygen in (CH3)3COH. Give your answer to three significant figures.

answer ……………………
[2]
(ii)	Write a balanced equation for the complete combustion of (CH3)3COH.

[2]
[Total 16 marks]

2.	The table below lists the boiling points of some alkanes.
alkane	number of carbon atoms	molecular formula	boiling point / °C	
butane	4	C4H10	0	
pentane	5	C5H12	36	
hexane	6		69	
heptane	7	C7H16	99	
octane	8	C8H18		
nonane	9	C9H20	152	
decane	10	C10H22	175	
(a)	What is the molecular formula of hexane?......................................................
[1]
(b)	(i)	State the trend in the boiling points of the alkanes.
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[1]
(ii)	Explain the trend in the boiling points of the alkanes.
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[1]
(iii)	Predict the boiling point of octane.	.................. °C
[1]
[Total 4 marks]

3.	(a)	Heptane can be isomerised to produce branched chain alkanes such as
2-methylhexane or 2,3-dimethylpentane.
	The equation below shows the isomerisation of heptane into 2-methylhexane.

(i)	Using skeletal formulae, complete the balanced equation for the isomerisation of heptane into 2,3-dimethylpentane.

[1]
(ii)	The boiling point of 2,3-dimethylpentane is 84 °C.
	Predict the boiling point of 2-methylhexane.	................. °C
[1]
(b)	Heptane can be reformed to produce methylcyclohexane which is a cycloalkane. Write a balanced equation to show the reforming of heptane to obtain methylcyclohexane

[2]
(c)	State why branched chain alkanes and cycloalkanes are more useful than straight chain alkanes.
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[1]
[Total 5 marks]

4.	Straight chain alkanes such as heptane, C7H16, are processed into branched-chain alkanes and cyclic compounds. These products are required to make petrol burn better in car engines than when using unbranched alkanes.
(i)	Draw the skeletal formula of a branched structural isomer of heptane and state its name.
	skeletal formula:

name: ...........................................................
[2]
(ii)	Write a balanced equation to show the formation of the cyclic compound methylcyclohexane from heptane.

[2]
[Total 4 marks]

5.	Crude oil is first separated by fractional distillation. The fractions can then be refined further by cracking, reforming and isomerisation.
The reaction sequence below shows the production of heptane, C7H16, from fractional distillation of crude oil, followed by cracking, reforming and isomerisation.

(a)	What is meant by the term fractional distillation?
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		[1]
(d)	The isomerisation of heptane produces seven branched alkanes, five of which are shown below.

		

	

	
(i)	Name compound C.
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[1]
(ii)	In the boxes above, draw skeletal formulae for the other two branched alkanes formed by isomerisation of heptane.
[2]

(iii)	Predict which of 2-methylhexane, 2,3-dimethylpentane and
2,2,3-trimethylbutane has the lowest boiling point.
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[1]
(iv)	Explain why 2-methylhexane, 2,3-dimethylpentane and
2,2,3-trimethylbutane have different boiling points.
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[2]
[Total 10 marks]

6.	In this question, one mark is available for the quality of written communication.
Alkanes can be separated from crude oil because they have different boiling points. The table below shows the boiling points of some alkanes.
alkane	boiling point/°C	Mr	
ethane	–89	30	
propane	–42	44	
butane	0	58	
pentane	36	72	
2-methylbutane	28	72	
2,2-dimethylpropane	10	72	
Explain the variation in boiling points of the alkanes shown.

[5]
Explain why, in industry, alkanes such as octane are processed by isomerisation.

[3]
	Illustrate your answers by referring to suitable examples. Write equations where appropriate.
Quality of Written Communication [1]
[Total 9 marks]

7.	Crude oil is a complex mixture of hydrocarbons. Initial separation is achieved by fractional distillation. The separate fractions are then further refined to produce hydrocarbons such as decane.
(a)	(i)	State what is meant by the term hydrocarbon.
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[1]
(ii)	A molecule of decane contains ten carbon atoms. State the molecular formula of decane.
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[1]
(iii)	Deduce the empirical formula of decane. ...............................................
[1]

(b)	Dodecane, C12H26, is a straight chain alkane that reacts with chlorine to produce a compound with molecular formula C12H25Cl.
	C12H26 + Cl2 ® C12H25Cl + HCl
The reaction is initiated by the formation of chlorine free radicals from chlorine.
(i)	What is meant by the term free radical?
................................................................................................................
[1]
(ii)	State the conditions necessary to bring about the formation of the chlorine free radicals from Cl2.
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[1]
(iii)	State the type of bond fission involved in the formation of the chlorine free radicals.
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[1]
(iv)	The chlorine free radicals react with dodecane to produce C12H25Cl. Write equations for the two propagation steps involved.
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[2]
(v)	How many different structural isomers can be formed when chlorine reacts with dodecane to form C12H25Cl?
answer ........................................................
[1]
(c)	Dodecane, C12H26, can be cracked into ethene and a straight chain alkane such that the molar ratio ethene: straight chain alkane is 2 : 1.
(i)	Write a balanced equation for this reaction.
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[2]
(ii)	Name the straight chain alkane formed.
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[1]

(d)	Straight chain alkanes such as heptane, C7H16, can be isomerised into branched chain alkanes and reformed into cyclic compounds.
(i)	Using skeletal formulae, write an equation to show the isomerisation of heptane into 2,2,3-trimethylbutane.

[2]
(ii)	Write a balanced equation to show the reforming of heptane into methylcyclohexane.

[2]
[Total 16 marks]

8.	Butane, C4H10, under certain conditions, reacts with Cl2 to form a mixture of chlorinated products. One possible product is C4H9Cl.
C4H10 + Cl2 ® C4H9Cl + HCl
	(a)	(i)	State the conditions.
................................................................................................................
[1]
(ii)	Write equations to show the mechanism of this reaction.
initiation ..................................................................................................
propagation ............................................................................................
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[3]
(iii)	Write one equation for a reaction that would terminate this mechanism.
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[1]
(iv)	State the type of bond fission involved in the initiation step. ..................
[1]

(b)	One other possible product of the reaction between butane and chlorine is compound J, C4H8Cll2, shown below.
	
(i)	Name compound J.
................................................................................................................
[1]
(ii)	Draw the skeletal formula of compound J.
[1]
(iii)	In addition to compound J, suggest one other possible structural isomer of C4H8Cl2 that could have been formed in this reaction.
[1]
[Total 9 marks]
 
9.	Butane, C4H10, reacts with chlorine to produce a chloroalkane with molecular formula C4H9Cl.
	The reaction is initiated by the formation of chlorine radicals from chlorine.
(i)	What is meant by the term radical?
.........................................................................................................................
[1]
(ii)	State the conditions necessary to bring about the formation of the chlorine free radicals from Cl2.
.........................................................................................................................
[1]
(iii)	State the type of bond fission involved in the formation of the chlorine radicals.
.........................................................................................................................
[1]

(iv)	The chlorine radicals react with butane in several steps to produce C4H9Cl.
	Write equations for the two propagation steps.
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[2]
[Total 5 marks]

 10.	Hexane reacts with Br2 in the presence of ultraviolet light.
	C6H14 + Br2 ® C6H13Br + HBr
(i)	State the type of reaction.
.........................................................................................................................
[1]
(ii)	Identify the three possible structural isomers of the product, C6H13Br, that could be formed from this reaction with hexane.

	
[3]
[Total 4 marks]

11.	Isomer L, C5H10, reacts with Cl2 in the presence of UV light to produce the organic product C5HgCl. The reaction takes place in three stages: initiation, propagation and termination.
(i)	The reaction is initiated by the fission of Cl2. State the type of fission involved.
.........................................................................................................................
[1]
(ii)	Write an equation to illustrate the fission of Cl2 in (i).
.........................................................................................................................
[1]
(iii)	The fission of Cl2 leads to a chain reaction involving two propagation steps. Complete the equations for the two propagation steps.
	C5H10	+	............	®	·C5H9	+	.............
[1]
	·C5H9	+	............	®	........	+	.............
[1]
[Total 4 marks]