AlkanesAQA A-Level Chemistry: Topic test

1. 1
   Kerosene, used as aviation fuel, is a fraction of crude oil that consists mainly of alkanes with 10 to 16 carbon atoms per molecule. It leaves the fractionating column at a lower level than the petrol fraction, which contains alkanes with 5 to 10 carbon atoms.

   (a)

   What is meant by saying that the alkanes in kerosene are saturated?

   [1 mark]

   • ATheir molecules contain only single bonds
   • BTheir molecules contain only carbon and hydrogen atoms
   • CTheir molecules contain no more than 16 carbon atoms
   • DTheir molecules have the maximum possible number of carbon atoms

   (b)

   Why does kerosene condense lower in the column than petrol?

   [1 mark]

   • AIts molecules are shorter, so it condenses at a higher level
   • BIts molecules have stronger covalent bonds, so they condense at a higher level
   • CIts molecules are longer, so they have stronger London forces and a higher boiling range
   • DIts molecules are longer, so they have weaker London forces and a lower boiling range

   (c)

   Explain how fractional distillation separates crude oil into fractions such as kerosene and petrol.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A refinery uses cracking to convert long-chain alkanes into more useful products. A sample of hexadecane, C₁₆H₃₄, is cracked in a thermal cracker and in a catalytic cracker.

   (a)

   Hexadecane cracks to give octane, C₈H₁₈, and one other product only. What is the other product?

   [1 mark]

   • AC₈H₁₈
   • BC₈H₁₆
   • CC₈H₁₄
   • DC₄H₈

   (b)

   Which conditions are used in catalytic cracking?

   [1 mark]

   • AA high pressure, a high temperature and no catalyst
   • BA slight pressure, a low temperature and a zeolite catalyst
   • CA high pressure, a high temperature and a zeolite catalyst
   • DA slight pressure, a high temperature and a zeolite catalyst

   (c)

   State one economic advantage of catalytic cracking over thermal cracking, and give one use of its main products.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A diesel lorry burns hexadecane, C₁₆H₃₄, as a representative fuel. In a road tunnel the air supply to the engine is limited and the engine temperature is very high.

   (a)

   Write balanced equations for the complete combustion of hexadecane and for its incomplete combustion to form carbon, and state one effect of the carbon particles released.

   [3 marks]

   (b)

   Explain how nitrogen monoxide forms in the engine although the fuel contains no nitrogen, and state one problem it causes.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Demand for kerosene, the jet fuel fraction, is rising at a refinery. Fractional distillation of its crude oil gives only about 10% kerosene (mainly C₁₀–C₁₆ alkanes) but about 30% of heavy fractions of C₂₀ and above, which are in low demand. The kerosene contains traces of sulfur compounds, and the refinery's furnaces burn a sulfur-containing fuel oil, with the flue gases passed through a scrubber.

   (a)

   Explain how the refinery could use cracking to meet the demand for kerosene, including the conditions and products of both types of cracking, an equation for the cracking of C₂₀H₄₂ into a C₁₂ alkane and an alkene, and why this is economically sensible.

   [6 marks]

   (b)

   Using dodecane, C₁₂H₂₆, as a representative component of kerosene, write equations for its complete combustion and for its incomplete combustion to carbon monoxide. Explain how nitrogen oxides form in a jet engine, and how calcium oxide in the scrubber removes the sulfur dioxide from the furnace flue gases, with an equation.

   [6 marks]

   Total for question 4: 12 marks

5. 5
   In the free-radical chlorination of methane, excess chlorine leads to further substitution, giving dichloromethane, CH₂Cl₂, trichloromethane, CHCl₃, and tetrachloromethane, CCl₄, as well as chloromethane, CH₃Cl.

   (a)

   Which equation represents a propagation step in the formation of dichloromethane?

   [1 mark]

   • ACH₃Cl + Cl₂ → CH₂Cl₂ + HCl
   • B•CH₂Cl + Cl• → CH₂Cl₂
   • CCl₂ → 2Cl•
   • DCH₃Cl + Cl• → •CH₂Cl + HCl

   (b)

   Which is the overall equation for the formation of tetrachloromethane?

   [1 mark]

   • ACH₄ + 4Cl₂ → CCl₄ + 4HCl
   • BCH₄ + 4Cl₂ → CCl₄ + 2HCl
   • CCH₄ + 3Cl₂ → CCl₄ + 4HCl
   • DCH₄ + 2Cl₂ → CCl₄ + 2H₂

   (c)

   Explain why a mixture of chloroalkanes forms, and how the proportion of chloromethane in the product can be increased.

   [2 marks]

   Total for question 5: 4 marks

6. 6
   A school Bunsen burner burns natural gas, which is mostly methane. With the air hole open the flame is blue. With the air hole closed the flame is yellow and a black deposit forms on a test tube held in it.

   (a)

   What causes the yellow colour of the flame?

   [1 mark]

   • ACarbon dioxide formed by complete combustion
   • BWater vapour condensing in the flame
   • CGlowing particles of carbon from incomplete combustion
   • DUnburnt methane glowing in the flame

   (b)

   Which equation represents incomplete combustion of methane forming carbon monoxide?

   [1 mark]

   • ACH₄ + 2O₂ → CO₂ + 2H₂O
   • B2CH₄ + 3O₂ → 2CO + 4H₂O
   • CCH₄ + O₂ → CO + 2H₂O
   • DCH₄ + O₂ → C + 2H₂O

   (c)

   Explain why closing the air hole gives a yellow flame and a black deposit.

   [2 marks]

   Total for question 6: 4 marks

7. 7
   Crude oil is separated in a fractionating column into fractions that include LPG (C₁–C₄), petrol (C₅–C₁₀), kerosene (C₁₀–C₁₆), diesel (C₁₄–C₂₀) and fuel oil (C₂₀ and above). The fuel oil fraction is in low demand.

   (a)

   State which of the five fractions leaves the column at the lowest level and explain why it does not reach the top of the column.

   [3 marks]

   (b)

   Fuel oil containing C₂₀H₄₂ is cracked to give octane, C₈H₁₈, and propene, C₃H₆, only. Write the equation, state the conditions for thermal cracking and for catalytic cracking, and state why cracking fuel oil is economically worthwhile.

   [4 marks]

   Total for question 7: 7 marks

8. 8
   2-Methylpropane, (CH₃)₃CH, is the main component of some camping-stove fuel (liquefied petroleum gas). It can also be converted into chloroalkanes by reaction with chlorine in ultraviolet light.

   (a)

   Explain, with equations, why using this stove inside a closed tent is hazardous.

   [6 marks]

   (b)

   Explain, with equations, how 2-chloro-2-methylpropane is formed from 2-methylpropane and chlorine, naming each type of step. Include the role of ultraviolet light.

   [6 marks]

   Total for question 8: 12 marks

End of questions