Organic 1 Section 3 - Unit 12: Alkanes
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Explain why the sulfur compounds found in crude oil should be removed from the fractions before they are used for central heating fuel (2 marks)
• With combsution, SO2 would be produced - And this causes acid rain
Explain how oxides of nitrogen are formed (2 marks)
• Nitrogen and oxygen from the air react - At very high temperatures
Explain why the melting point of dodecane is higher than the melting point of the straight-chain alkane produced by cracking dodecane (2 marks)
• Longer carbon chain so larger surface area - So stronger Van der Waals forces between molecules
Outline the essential features of the fractional distillation of crude oil that enable the crude oil to be separated into fractions (4 marks)
• Fractions/hydrocarbons have different boiling points • Boiling point depends on chain length • Temperature gradient in column (cooler at the the top) • Larger molecules at the bottom of the column
Describe briefly how fractional distillation can be used to convert crude oil into a small number of fractions (4 marks)
• Crude oil is heated and vaporised • It is passed into column • The hydrocarbons condense at different heights • Similar molecules (in terms of size and boiling point) condense together
Describe what pollutants are in petrol engine cars, how they arise and how they can be removed by a catalytic converter (6 marks)
• CO • From 𝗶𝗻𝗰𝗼𝗺𝗽𝗹𝗲𝘁𝗲 combustion • NO/NO2 • When nitrogen reacts with oxygen from air at very high temperatures • Unburned hydrocarbons • Removal via the reaction: 2NO + 2CO → N2 + 2CO2 • To form harmless products with one of the catalysts: 𝗽𝗹𝗮𝘁𝗶𝗻𝘂𝗺, 𝗽𝗮𝗹𝗹𝗮𝗱𝗶𝘂𝗺 or 𝗿𝗵𝗼𝗱𝗶𝘂𝗺
Explain why hydrocarbons from heavy fractions are cracked and explain why these hydrocarbons are less easy to ignite than those in petrol (4 marks)
• Higher demand for petrol • The supply of heavy fractions is high • Larger Mr so less volatile • Due to stronger intermolecular forces
Explain why a catalytic converter is coated on a ceramic honeycomb (2 marks)
• Provides a larger surface area - And thus a faster reaction
Explain what is meant by the term fraction as applied to fractional distillation (1 mark)
• A mixture of alkanes with a similar boiling point
Explain the effect of a change of temperature on both the position of equilibrium and the rate of reaction, and justify why a compromise temperature is used industrially (6 marks)
• Le Chatelier’s principle predicts that equilibrium shifts to oppose any increase in temperature • Exothermic reaction, so equilibrium shifts in endothermic direction • So a higher temperature will reduce yield • At higher temperature, more high energy molecules • More collisions have energy higher than the activation energy • So rate of reaction increases
State and explain the 3 physical properties of long hydrocarbon chains (3 marks)
• Less volatile: higher mr so higher melting point is needed to convert it into a gas • Less flammable: stronger van der waals forces so higher melting point is needed to burn it • Less viscous: not very runny since there are stronger van der waals forces
State the economic reasons for cracking (3 marks)
• Shorter C chains are more in demand than larger fractions • To make use of excess larger hydrocarbons • Products of cracking are more valuable than the starting materials
State the conditions for thermal cracking and state what is produced (3 marks)
• 7000 kPa • 400°C to 900°C • Mainly alkenes and sometimes hydrogen
State the conditions for catalytic cracking (3 marks)
• Moderate Pressure • High Temperature - 450°C • Zeolite Catalyst
State what is usually produced in catalytic cracking and what this type of cracking is mainly used for (3 marks)
• Produces branched and cyclic alkanes • And aromatic hydrocarbons • Used for making motor fuels
State the pollutants formed in incomplete combustion and explain their consequences (5 marks)
• Nitrogen Oxides: Toxic and can form acidic gases or acid rain • Carbon Monoxide: Toxic • Carbon Dioxide: Global Warming • Unburnt Hydrocarbons: Contributes towards formation of smog • Soot: Global dimming and respiratory problems
Explain how SO2 can be removed from flue gases (5 marks)
• Flue Gas Desulfurisation • Gases pass through a scrubber • Which contains basic CaO • This reacts with SO2 in a neutralisation reaction • SO2 + CaO ➡ 2CaSO3