Alkanes Flashcards
alkane properties
what are the general properties of alkanes
- saturated hydrocarbons
- CnH2n+2
- produced from crude oil
- used for fuels
- covalent bonds between the atoms however weak VDWs between molecules
- around each C atom there is a tetrahedral shape with bond angle 109.5
alkane properties
what happens when you boil a liquid alkane
energy is needed to break the induced dipole-dipole attraction (VDWs) forces between the molecules in order to seperate them
alkane properties
what happens to the boiling point of alkanes as carbon chain length increases
- as the C chain length increases the boiling point increases
- because there is more surface contact between the molecules
- so there are more Van der Waal forces between molecules
- which need more energy to overcome
alkane properties
what happens to the boiling point of alkanes as branching occurs
- a branches isomer has a lower boiling point than the unbranched isomer
- in a more branched alkane, there is less surface contact between molecules leading to fewer VDW forces
- which need less energy to break the weaker VDW forces between molecules
alkane properties
what is the solubility of alkanes in water
alkanes are immiscible (insoluble) in water - because they are non-polar, they only contain VDW and so cannot form hydrogen bonds with water
fractional distillation
how is crude oil seperated
- by fractional distillation
- crude oil mixture is vapourised and passed into a fractioning column with a temperature gradient ( up = cooler)
- the gradient allows the mixture to seperate into fractions
- as the vapours condense at levels which match their boiling points
cracking
what is cracking
process that splits long chain alkanes into shorter more valuable alkanes, alkenes and hydrogen
- increasing branching in chains
- increases amount of gasoline
cracking
what is thermal cracking
more alkenes produced this method
- long-chain alkane used
- 400-900°
- high pressure needed
- mainly alkenes produced and sometimes hydrogen
- alkenes used for making polymers
cracking
what is catalytic cracking
- long-chain alkane used
- 450°
- slight pressure
- silica and aluminium oxide or zeolite catalyst
- produces branched and cyclic alkanes and aromatic compounds
- products used for motor fuels (short chains burn more volatilely)
alkenes
what is the test for alkenes
reagent: bromine water (brown liquid)
observation: orange to colourless
combustion
what is complete combustion
complete combustion releases CO2 and H2O because the fuel is fully oxidised
this occurs when there is an excess of oxygen
alkane + oxygen = carbon dioxide + water
C3H8 + 5O2 = 3CO2 + 4H2O
combustion
what is incomplete combustion
incomplete combustion can produce either carbon monoxide or carbon
this occurs when there is a limited supply of oxygen and produces less energy per mole than complete combustion
alkane + oxygen = carbon monoxide + water
alkane + oxygen = carbon + water
C3H8 + 3.5O2 = 3CO +4H2O
C3H8 + 2O2 = 3C + 4H2O
combustion
what is the effect and solution of the production of carbon dioxide
produced by complete combustion of fuels
effect - greenhouse gas that contributes to global warming
solution - burn less fuel or use carbon offsetting
combustion
what is the effect and solution of the production of carbon monoxide
produced by incomplete combustion of fuels
effect - a toxic gas that bonds to haemoglobin, reducing the capacity of oxygen
solution - ensure a plentiful supply of oxygen or use a catalytic converter
combustion
what is the effect and solution of the production of carbon particulates
produced by incomplete combustion of fuels
effect - black solid causes respiratory irritation, lung disease and global dimming
solution - ensure a plentiful supply of oxygen or use a catalyctic converter
combustion
what is the effect and solution of the production of sulphur dioxide
produced by burning fuels that contain sulfur (mainly coal)
effect - it is an acidic gas that can dissolve in water to produce acid rain
SO2 + 1/2 O2 = H2O + H2SO4
solution - flue-gas desulfurization
combustion
what is the effect and solution of the production of oxides of nitrogen
NOx are produced by nitrogen and oxygen reacting
occurs at high temperatures e.g inside a car engine
N2 + O2 = 2NO
effect - NOx are acidic gases that can cause acid rain, photochemical smog and respiratory irritation
solution - catalytic converters
free radical substitution
what is free radical subtitution
*reagents: *halogen (Cl2 or Br2 or I2) and excess alkane
*conditions: *UV radiation or 300°
- it is a substitiution reaction as H atom in the alkane molecule is replaced
- the Cl-Cl and C-H bonds break by homolytic fission to form radicals
- a radical is a specied with an unpaired electron
free radical substitution
what are the 3 steps involved in FRS
- initiation
- propagation
- termination
free radical substitution
what happens during initiation
breaking of the Cl-Cl bond by homolytic fission to make 2 chlorine radicals
the UV radiation provides the energy needed to break the Cl-Cl bond
free radical substitution
what happens during propagation
a very reactive chlorine radical attacks a methane molecule in a chain reaction
chlorine radical is used up in the first step, but is regenerated in the second
it is now able to attack a new methane molecule
free radical substitution
what happens during termination
reaction ends when any two radicals combine forming a new molecule
free radical substitution
what are the limitations of FRS
- leads to the formation of a mixture of products
- further substitution produces a mixture of halogenoalkanes
- reactions can occur at different positions in the carbon chain
free radical substitution
what are the disadvantages of FRS
- low % yield of desired halogenoalkane
- seperation by fractional distillation which is costly
the minimise further substitution an excess of methane is used
