Alkanes Flashcards
What are alkanes?
Organic molecules which contain no double bonds so are saturated. General formula of Cn H2n+2. Made of hydrogen and carbon only so are hydrocarbons.
How are the fractions of crude oil separated?
By fractional distillation. The crude oil is heated until it vaporises and then sent into the fractionating tower which is cooler at the top than at the bottom. Longer chains have higher boiling points so condense and are collected at the bottom of the column. The shorter chains have lower boiling points so rise up the tower condensing nearer the top.
What is cracking
The breaking of C-C bonds in alkanes
How does thermal cracking work?
In a steam cracker the alkanes are heated to between 700-1200K under a high pressure up to 7000kPa. The C-C bonds break so that they each get one electron from the covalent bond forming two free radicals. There are not enough H atoms to form two alkanes so an alkene is formed along with an alkane.
How does catalytic cracking work?
Takes place at a lower temperature to thermal cracking (about 720K) and under slight pressure (but more than 1atm). The alkane is brought into contact with a zeolite catalyst (silicon dioxide and aluminium oxide) which breaks the C-C bond. The products are mostly motor fuels and aromatic hydrocarbons and cycloalkanes. It produces a high percentage of branched alkanes. The mechanism is heterolytic fission.
Why is a zeolite catalyst used in cracking?
Because it has a honeycomb structure so an enormous surface area which speeds up the reaction.
What happens in complete combustion?
A substance is burned in plenty of oxygen, CO2 and water are produced, lots of energy is released
What happens in incomplete combustion?
There is not enough oxygen for complete combustion, it produces CO / soot, and water
What is CO
A poisonous gas produced by incomplete combustion of alkanes that is potentially lethal
How do nitrogen compounds form from car engines?
NO2 (brown) and NO (colourless) are produced when the car engine is hot enough for the N2 and O2 in the air to combine.
The problem of nitrogen compounds from car engines and the equation
NO2 contributes to acid rain
Nitrogen dioxide + water + Oxygen -> nitric acid
Why are catalytic converters fitted to cars?
To reduce emissions of CO and oxides of Nitrogen.
Describe a catalytic converter and the reactions that happen
They are honeycomb structures made of ceramic material coated in platinum, rhodium and palladium as catalysts. As the pollutants pass over the catalyst, they react with each other to form less harmful products.
e.g 2NO + 2CO -> N2 + 2CO2
How does flue gas desulfurisation work?
Calcium oxide and water can be sprayed into the flue gas to react with the SO2 to form calcium sulfite which can be further oxidised to make calcium sulfate (gypsum) which is saleable as it is used to make plaster. You can use calcium carbonate instead of calcium oxide.
Define flue gas
Gases given out by power stations.
