3.3.2 Alkanes Flashcards
Thermal cracking
900°C, 70 atmospheres. 1 electron from the pair in covalently bonded carbon atoms goes to each carbon atom forming 2 shorter chains (free radicals) with unpaired electrons. These are highly reactive + react in number of ways to form shorter chain molecules. It produces lots of alkenes
How does cracking involve breaking C-C bonds
Forms two or more shorter shorter chains (free radicals) which are highly reactive and form shorter chain molecules
Catalytic cracking
450°C, 1-2 atmospheres, zeolite catalyst, zeolites, products; aromatic hydrocarbons, motor fuels, cyclical alkanes, branched alkanes.
Zeolite catalyst
Silicon dioxide, aluminium oxide, honeycomb structure (enormous surface area + acidic)
Petroleum
Is crude oil. It’s a mixture made mostly of alkanes that can be separated by fractional distillation
Why is cracking needed economically
The demand for shorter alkanes is higher that the amount produced in fractional distillation so longer chain molecules are broken down into shorter chains
Why are alkanes used as fuels?
Burning alkanes release a large amount of energy
Shorter to longer chain trend
Boiling point increase + less flammable + more viscous (stronger intermolecular forces)
More smokey + darker (more carbon + chains tangled)
How does internal combustion of engines cause pollutants?
NOx - high temp + pressure make oxygen + nitrogen in air to react together (NO). Secondary reaction with O2 forms NO2
CO + C - formed when not enough oxygen is present during combustion (incomplete combustion)
Unburnt hydrocarbons - from engines reaction with NOx in presence of sunlight to form ground level ozone (O3)
Catalytic converters
Ceramic material coated with platinum + Rhodium. Has honeycomb structure with large surface area. Polluting gases react with each other to form less harmful products Carbon monoxide + nitrogen oxide 2CO(g) + 2NO(g) -> N2(g) + 2CO2(g) Hydrocarbon + oxide C3H8(g) + 5O2 (g) -> 3CO2(g) + 4H2O(g) Nitrogen oxide 2NO(g) -> N2(g) + 2CO2(g) Hydrocarbon + nitrogen oxide -> nitrogen + carbon dioxide + water
How does the combustion of hydrocarbons containing sulfur lead to sulfur dioxide ?
When fossil fuels containing sulfur are burnt, the sulfur reacts with oxygen to form sulfur dioxide which dissolves in moisture in the atmosphere which forms sulfuric acid and causes acid rain
Why can sulfur dioxide be removed from flue gases using calcium carbonate or oxide ?
Powdered calcium carbonate or oxide is mixed with water to form alkaline slurry. When the flue gases mix with the alkaline slurry, the acidic sulfur dioxide reacts with the calcium compounds to form calcium sulfite (harmless salt)
Reactive intermediate of catalytic cracking
Carbocation
Reactive intermediate + how its formed in thermal cracking
Free radical, homolytic fission
Why do hydrocarbons of different chain lengths have different boiling points ?
Different Van der Waals forces
