3.2 Alkanes Flashcards
KEY DEFINITION - saturated hydrocarbons
- hydrocarbons that contain single C-C bonds
KEY DEFINITION - hydrocarbons
- compounds that contain carbon and hydrogen atoms only
KEY DEFINITION - structural isomers
- compounds with the same molecular formula but a different structural formula
What are the physical properties of the alkenes?
- non-polar (C and H have similar electronegativities)
- simple molecular structure - weak Van der Waals forces between molecules
- not soluble in water
- as the length of the carbo chain increases, the boiling point increases
Explain the trend in boiling points for straight chain alkanes:
- as the length of the carbon chain increases, the boiling point of the alkane increases
- because the number of electrons in the molecules increases, so the Van der Waals forces between molecules get stronger
- more energy is needed overcome forces
Explain the trend in boiling points for branched chain alkenes?
- as the number of branches increases, the boiling point of the alkane decreases
- because there are fewer points of contact molecules so the Van der Waals forces between molecules become weaker
- less energy needed to overcome forces
What are the conditions for thermal cracking?
- very high temperatures and pressures
Give 3 equations for the thermal cracking of 12 C atoms
1) C12H26»_space; C2H6 + 2C5H10
2) C12H26»_space; C3H8 + C2H4 + C7H14
3) C12H26»_space; C4H10 + 2C2H4 + C4H8
What are the conditions for catalytic cracking?
- high temperatures
- moderate pressure
- zeolite catalyst
What are the products of catalytic cracking and what are they used for?
- branched and cyclic alkanes
- aromatic hydrocarbons (e.g. benzene)
- used as motor fuels
Give 3 examples of products after cracking hexane
1) 2,3-dimethylbutane
2) cyclohexane + H2
3) benzene + 4H2
What are the products of thermal cracking and what are they used for?
- shorter alkanes and a high % of alkenes
- used to make polymers
Products of complete combustion:
- carbon dioxide and water
Products of initial incomplete combustion:
- carbon monoxide (CO) and water
Products of further incomplete combustion:
- solid carbon (soot) and water
Why are alkanes generally unreactive?
- non-polar, saturated molecules
- strong C-C bonds
KEY DEFINITION: free radical
- a species with an unpaired electron
What are the 3 stages of free-radical substitution?
1 - initiation
2- propagation
3 - termination
Effects of unburned hydrocarbons:
- low level ozone
- causes respiratory problems
Effect of carbon dioxide:
- global warming
Effect of carbon monoxide:
- toxic gas
Effect of carbon:
- particles exacerbate asthma
Effects of nitrogen oxides:
- causes acid rain
- photochemical smog
Effect of sulphur dioxide:
- causes acid rain
