4.1.2 - alkanes Flashcards
what is the general formula of alkanes?
C(n)H(2n+1)
why are alkanes non-polar?
there’s no significant difference between the electronegativities of carbon and hydrogen
give 1 use of alkanes
fuels
explain the shape and bond angles in an alkane
there are 4 bonding pairs and 0 lone pairs around each carbon. all electron pairs repel. lone pairs repel more than bonding pairs. bong angle is 109.5.
explain how sigma bonds are formed in alkanes
the end on end overlap of bonding orbitals directly between atoms. how well they overlap determines the strength of the bond.
bonds can __________ in an alkane
rotate
explain the relative melting and boiling points of 2 alkanes with different chain lengths.
as chain length increases, boiling/melting point increases, as they have a greater surface area so have more points of contact between molecules. this means strength of induced dipole-dipole interactions increases, which require more energy to overcome.
explain the relative melting and boiling points of branched isomers
the more branches there are in an isomer molecule, the lower the boiling point. the branches cause fewer points of surface contact between molecules so induced dipole-dipole interactions are weaker and require less energy to overcome.
explain why production of carbon monoxide in the combustion of alkanes is bad.
carbon monoxide is poisonous. leads to oxygen deprivation, which causes fatigues, headaches and nausea.
are straight chain or branched chain isomers more efficient in combustion reactions?
branched
what is a radical?
highly reactive species with an unpaired electron.
what is homolytic fission?
covalent bond breaks and one electron goes to each atom forming 2 radicals.
what is heterolytic fission?
covalent bond breaks and both electrons go to the same atom forming 2 ions
what does a curly arrow represent?
movement of an electron pair
what are the 3 steps for free radical substitution and what conditions are required
1- initiation (radicals are made)
2 - propagation (radicals are used up and created)
3 - termination (radicals combine to make a neutral molecule)
conditions - UV and high temperature
what are the drawbacks of free radical substitution?
multi-substitution occurs (can overcome this by using excess alkane). substitution can occur anywhere along the chain, so a mixture of isomers forms
explain the importance of ozone
absorbs UV radiation. reduced the amount of UV at the earths surface. UV radiation causes sunburn and skin cancer.
explain the natural formation of ozone
ozone absorbs UV radiation
UV at Earth’s surface is reduced
(UV) O2 –> 2O.
O2 + O. –> O3
what is a chlorofluorocarbon?
contain chlorine, fluorine and carbon only - all hydrogens have been substituted.
give 3 properties of chlorofluorocarbons
volatile, non-flammable and non-toxic
give 4 (past) uses of chlorofluorocarbons
aerosol cans, dry cleaning, air conditioning and fridges
why are chlorofluorocarbons no longer used?
they were destroying the ozone layer. they catalyse the breakdown of ozone
what is the overall equation for the breakdown of ozone?
O3 + O –> 202 (with Cl. or NO. catalyst)
NO. radical destroy the ozone layer. how are nitrogen oxides formed?
produces by lighting in thunderstorms and in car/aircraft engine
what are hydrochlorofluorocarbons/hydrofluorocarbons and why are they more widely used than chlorofluorocarbons?
HCFCs and HFCs are used as an alternative to CFCs until safer products are developed. HCFCs are broken down in the atmosphere much more quickly (10-20 years whereas CFCs take 90 years). HFCs done contain Cl so done affect the oxone layer. HCFCs and HFCs are greenhouse gases
give 3 alternatives to CFCs (not inc HCFCs or HFCs)
aerosols with pump sprays/nitrogen propellants
fridges/freezers using ammonia as the coolant gas
CO2 is used to make expanded polymers.
