Chapter 12- Alkanes Flashcards
What are alkanes?
Saturated hydrocarbons containing only C-C and C-H single bonds.
What is crude oil?
A mixture of hydrocarbons, mostly alkanes, both unbranched and branched. Its fractions can be separated by fractional distillation.
What is the process of fractional distillation?
-Crude oil heated
-Mixture of liquid and vapour passes into fractionating column with temp. gradient
-Vapours rise up the column through trays until they reach a cool enough tray where they condense
-Shorter chain hydrocarbons condense further up due to their lower boiling points
-Thick residue collects at the base (tar/bitumen) which can be processed further or used for road surfacing
What is cracking?
The breaking of longer chain hydrocarbons into shorter chain hydrocarbons through the breaking of C-C bonds
Why are longer chain alkanes cracked?
Longer chain alkanes are in lower demand while shorter chain alkanes and alkenes are in higher demand as they are more useful
Alkenes are more reactive than alkanes and can be used as chemical feedstock
What are the conditions and products of thermal cracking?
-High temperature: 700-1200K
-High pressure: up to 7000 kPa
-Produces mostly alkenes that can be used as chemical feedstock
What are the conditions and products of catalytic cracking?
-Moderately high temperature: 700K
-Moderately high pressure: higher than atmospheric pressure
-Zeolite catalyst (al. oxide and sil. dioxide) in honeycomb structure for high SA
-Produces motor fuels, aromatic compounds (branched alkanes, cycloalkanes) which are separated using fractional distillation
How are alkanes often used?
As fuels, eg methane, propane, petrol, etc.
How can combustion reactions of alkanes differ?
Depending on the amount of oxygen available, combustion can be complete or incomplete and produce different products
Plentiful: carbon dioxide and water
Limited: carbon monoxide and water
Very little: carbon (soot) and water
What are some pollutants that burning crude oil can produce?
CO2
CO
C
NOx
SO2
Unburnt hydrocarbons
H2O
What is carbon monoxide?
CO, a poisonous gas produced by incomplete combustion
What are nitrogen oxides?
NOx, contribute to acid rain and photochemical smog
Produced in petroleum engines at high temp. causing nitrogen and oxygen in the air to combine
What is sulfur dioxide?
SO2, contributes to acid rain
Produced from sulfur-containing impurities in crude oil and reacts with water and oxygen to form sulfuric acid
What are carbon particles/particulates?
C, can exacerbate asthma and cause cancer
Produced by incomplete combustion with very little oxygen available.
What are unburnt hydrocarbons and what do they do?
Hydrocarbons
Bad greenhouse gases, photochemical smog
What is carbon dioxide?
CO2, a greenhouse gas
Produced by complete combustion and contributes to climate change
What is water vapour?
H2O (g), a greenhouse gas
Always produced in combustion
What is flue gas desulfurisation?
The process in which sulfur dioxide is removed from flue gases (the gases given out by power stations).
How can sulfur dioxide be removed using calcium oxide (lime) ?
Slurry of CaO and water sprayed into flue gas.
CaO+2H2O+SO2+0.5O2 —> CaSO4.2H2O
Gypsum is produced which is saleable as it is used to make plaster.
How can sulfur dioxide be removed using calcium carbonate (limestone) ?
CaCO3 + 0.5O2 + SO2 —> CaSO4 + CO2
What do catalytic converters do?
Reduce output of CO, NOx and unburnt hydrocarbons in the exhaust gas mixture.
What is a catalytic converter made of?
Ceramic material in a honeycombic structure coated with platinum and rhodium (catalysts).
As the pollutants pass over the catalyst, they react with each other to form less harmful products.
How is the output of CO and NO reduced?
2CO and 2NO reacts to form N2 and 2CO2
How is the output of unburnt hydrocarbons and nitrogen oxide decreased?
Hydrocarbons and NO react to form N2, CO2 and H2O
