Organic 1 Section 3 - Unit 12: Alkanes Flashcards
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 ๐ถ๐ป๐ฐ๐ผ๐บ๐ฝ๐น๐ฒ๐๐ฒ combustion
- NO/NO2
- When nitrogen reacts with oxygen from air at very high temperatures
- Unburned hydrocarbons
- Removal via the reaction:
2NO + 2CO โ N2 + 2CO2 - To form harmless products with one of the catalysts: ๐ฝ๐น๐ฎ๐๐ถ๐ป๐๐บ, ๐ฝ๐ฎ๐น๐น๐ฎ๐ฑ๐ถ๐๐บ or ๐ฟ๐ต๐ผ๐ฑ๐ถ๐๐บ
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 Chatelierโs 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
- 400ยฐC to 900ยฐC
- Mainly alkenes and sometimes hydrogen
State the conditions for catalytic cracking (3 marks)
- Moderate Pressure
- High Temperature - 450ยฐC
- 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
