O LevelChemistryCambridgeTopic Questions11. Organic Chemistry11.4 Alkanes

Alkanes (Cambridge O Level Chemistry)

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1a2 marks

The alkanes are a family of saturated hydrocarbons. Their reactions include combustion, cracking and substitution.

 
i)
What is meant by the term hydrocarbon?
 
[1]
 
ii)
What is meant by the term saturated?
 
[1]

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1b3 marks
i)
What is the general formula for the homologous series of alkanes?
 
[1]
 
ii)
Calculate the mass of one mole of an alkane with 14 carbon atoms.
 
[2]

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1c4 marks

Cracking is used to obtain short-chain alkanes, alkenes and hydrogen from long-chain alkanes.

 
i)
Give a use for each of the three products listed above.
 
short-chain alkanes ..................................................
 
alkenes ..................................................
 
hydrogen ..................................................
 
[3]
 
ii)
Write an equation for the cracking of decane, C10H22, which produces two different alkenes and hydrogen as the only products.
 
[1]

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1d2 marks

Chlorine reacts with propane in a substitution reaction to form 1-chloropropane.

 
CH3–CH2–CH3 + Cl2 → CH3–CH2–CH2–Cl + HCl
 
i)
What is the essential condition for the above reaction?
 
[1]
 
ii)
There is more than one possible substitution reaction between chlorine and propane.
 
Suggest the structural formula of a different product.
 
[1]

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1a5 marks

Many organic compounds which contain a halogen have chloro, bromo or iodo in their name.

The following diagram shows the structure of 1-chloropropane.

cie-igcse-sq-11-3-1-chloropropane-3a
i)
Draw the structure of an isomer of this compound.

[1]

ii)
Describe how 1-chloropropane could be made from propane.

[2]

iii)
Suggest an explanation why the method you have described in (ii) does not produce a pure sample of 1-chloropropane.
[2]

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1b3 marks

Organic halides react with water to form an alcohol and a halide ion.


CH3–CH2–I + H2O → CH3–CH2–OH + I–

i)
Describe how you could show that the reaction mixture contained an iodide ion.

[2]

ii)
Name the alcohol formed when 1-chloropropane reacts with water.

[1]

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1c7 marks

The speed (rate) of reaction between an organic halide and water can be measured by the following method.
A mixture of 10 cm3 of aqueous silver nitrate and 10 cm3 of ethanol is warmed to 60 °C.
Drops of the organic halide are added and the time taken for a precipitate to form is measured.
Silver ions react with the halide ions to form a precipitate of the silver halide.


Ag+ (aq) + X- (aq) → AgX (s)

Typical results for four experiments, A, B, C and D, are given in the table.

cie-igcse-sq-11-3-table-3c

i)
Explain why it takes longer to produce a precipitate in experiment A than in B.

[2]

ii)
How does the order of reactivity of the organic halides compare with the order of reactivity of the halogens?

[2]

iii)
Explain why the time taken to produce a precipitate would increase if the experiments were repeated at 50 °C.

[3]

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2a
Sme Calculator4 marks

The alkanes are generally unreactive. Their reactions include combustion, substitution and cracking.

The complete combustion of an alkane gives carbon dioxide and water.

 
i)
10 cm3 of butane is mixed with 100 cm3 of oxygen, which is an excess. The mixture is ignited. What is the volume of unreacted oxygen left and what is the volume of carbon dioxide formed?
 
C4H10 (g) + 6.5O2 (g) → 4CO2 (g) + 5H2O (l)
 
Volume of oxygen left = .................................................. cm3
 
Volume of carbon dioxide formed = .................................................. cm3
 
[2]
 
ii)
Why is the incomplete combustion of any alkane dangerous, particularly in an enclosed space?
 
[2]

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2b3 marks

The equation for a substitution reaction of butane is given below.

 
CH3−CH2−CH2−CH3 + Cl2 → CH3−CH2−CH2−CH2−Cl + HCl
 
i)
Name the organic product.
 
[1]
ii)
This reaction does not need increased temperature or pressure.
 
What is the essential reaction condition?
 
[1]
 
iii)
Write a different equation for a substitution reaction between butane and chlorine.
 
[1]

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2c5 marks

Alkenes are more reactive and industrially more useful than alkanes.

They are made by cracking alkanes.

 
C7H16 → CH3−CH=CH2 +  CH3−CH2−CH=CH2 + H2
heptane propene but−1−ene

 

i)
Draw the structural formula of the polymer poly(propene).
 
[2]
 
ii)
Give the structural formula and name of the alcohol formed when but-1-ene reacts with steam.
 
name ..................................................
 
[1]
 
structural formula
 
[1]
 
iii)
Deduce the structural formula of the product formed when propene reacts with hydrogen chloride.
 
[1]

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