Alkanes Flashcards
are alkanes saturated or unsaturated? why?
- saturated
- each carbon is bonded 4 times with the maximum number of hydrogens possible
- there are no double bonds
why do cycloalkanes differ from alkanes? how are they similar?
- they are a functional group isomer of alkenes
- general formula CnH2n
- still saturated
what is crude oil?
a mixture of different length hydrocarbons
describe the process of fraction distillation
- crude oil vaporised to 350c using a furnace
- vapour oil enters the column and rises through the trays, the longest hydrocarbons do not vaporise and run down to the bottom
- column has a negative temperature gradient - cooler at the top
- fractions condense at different heights due to the mixture of different length chains and therefore the differing boiling points (fractions separated by bp)
- when they reach a level at a lower temp than their boiling point, they condense and are removed
- small molecules condense at the top at lower temperatures and big molecules condense at the bottom at higher temperatures
what is petroleum?
mixture consisting mainly of alkane hydrocarbons that can be separated by fractional distillation
what is cracking? why is it useful?
- breaking of longer chain hydrocarbons to shorter chain hydrocarbons through the breaking of a C-C bond
- heavier fractions can be cracked to lighter fractions which are of a higher demand
- an alkene and a shorter chain hydrocarbon is produced
what are the 2 types of cracking?
- thermal cracking
- catalytic cracking
what is thermal cracking?
- high temperature and pressure used (700-1200K and 7000kPa)
- high percentage of alkenes are produced
what is catalytic cracking?
- higher temperature and slight pressure used (720-820K and 70-140kPa)
- zeolite catalyst used (lowers cost + speeds up process)
- produces mainly aromatic hydrocarbons used in fuels for vehicles
what are the products of complete combustion?
carbon dioxide and water
what is produced when alkanes burn with a limited oxygen supply? what will be produced if they burn with even less oxygen?
- carbon monoxide and water
- soot (carbon) and water
what are the problems associated with incomplete combustion?
- carbon monoxide is poisonous, it binds to haemoglobin in the blood and therefore prevents oxygen from binding
- soot causes breathing problems, makes buildings look dirty and can clog up buildings
what is the greenhouse effect?
carbon dioxide absorbs infrared radiation from the sun but emits some of it back into the earth causing a global temperature increase
what is photochemical smog? what are the problems associated with it?
- when solid carbon particulates and ozone mix a photochemical smog is created
- harms respiratory systems of animals and damages plants
what are oxides of nitrogen? what can be done to minimise the amount going into the atmosphere?
- oxides of nitrogen (NOx) are made when oxygen and nitrogen found in the air combine under high pressure and temperature
- car engines provide these conditions
- catalytic converters help to reduce the amount of unburnt hydrocarbons and NOx going into the atmosphere
What is acid rain? How does it form and why is it dangerous?
-burning fossil fuels can release sulfur dioxide
- some fossil fuels contain Sulfur based impurities
- when burned, the sulfur reacts with Oxygen producing sulfur dioxide (SO2)
-sulfur dioxide = acidic gas - reacts with Water in the atmosphere to form sulfuric acid which falls as acid rain (NOx do the same)
-acid rain causes damage to plants, kills fish + erodes buildings
How can SO2 be removed from flue gases?
- wet scrubbing
- an alkali is used to neutralise sulfur dioxide in flue gases
- involves dissolving calcium carbonate, oxide in water and spraying acidic SO2 gas
state the 3 steps of free radical chain reactions
- initiation
- propagation
- termination
what happens in the initiation stage of a free radical chain reaction ?
a radical is created using UV light (photochemical reaction)
what happens during the propagation stage of a free radical chain reaction?
when a radical reacts with a non-radical molecule, new radicals are created + then react further with more non-radicals, resulting in a chain reaction
what happens in the termination stage of a free radical chain reaction?
2 radicals react, forming a non-radical therefore ending the chain reaction
what happens in the initiation stage of the free radical substitution reaction with methane and chlorine? give any appropriate equations.
- the Cl-Cl bond is broken by UV light (photodissociation)
- 2 highly reactive radicals are produced
Cl2 -UV-> 2Cl○
what happens in the propagation stage of the free radical substitution reaction with methane and chlorine? give any appropriate equations.
- Cl radical reacts with a methane molecule to make a methyl radical (○CH3) and HCl
- the methyl radical reacts with a Cl2 molecule, forming a chloromethane and a Cl○ radical
- this Cl○ radical can react with more methane
Cl○ + CH4 –> HCl + ○CH3
○CH3 + Cl2 –> CH3Cl + Cl○
what happens in the termination stage of the free radical substitution reaction with methane and chlorine? give any appropriate equations.
- 2 radicals react to make a stable non-radical molecule
Cl○ + Cl○ –> Cl
what happens in the termination stage of the free radical substitution reaction with methane and chlorine? give any appropriate equations.
- 2 radicals react to make a stable non-radical molecule
Cl○ + Cl○ –> Cl2
○CH3 + ○CH3 –> CH3CH3 (C2H6)
Cl○ + ○CH3 –> CH3Cl - if many Cl○ present, will get a di/tri/tetra chloromethane (CH2Cl2/CHCl3/CCl4)
- if not, mainly just get single substitution (CH3Cl)
