alkanes (organic) Flashcards
what are sigma bonds
single bonds that are formed by direct overlap of orbitals between the bonding atoms
shape of alkanes
each carbon is bonded to 4 other atoms via bonds. the bonding pair of electrons repel eachother therefore the atom will take shape that maximises the distance between each bonding pair of electrons
why are C-H bonds non polar
they have similar electronegativities
what forces are between molecules
weak van der waals forces which can be overcome
whats the trend in boiling point of alkanes
as chain length increases, boiling point also increases
whats the boiling point of branched alkanes
they have lower boiling points because theres less surface area in between molecules which lessens the van der waals forces
whats the reactivity like in alkanes
unreactive because the C-H and C-C bonds are strong and the bonds are non polar
what are some reactions alkanes can undergo
combustion and free radical substitution
whats the equations for combustion of alkanes
fuel+oxygen - CO2 + H2O
fuel+limited oxygen - CO + H2O
fuel+very limited oxygen - C + H2O
whats the equation for molar volume
volume of gas (dm3) / molar volume (24) = mole of particle
OR
mole of particle x molar volume (24) = volume of gas (dm3)
whats crude oil
mixture of around 150 hydrocarbons.
main source of organic chemicals.
a fossil fuel formed by plant and animal remains at high temp and pressure deep below the surface.
non-renewable because it forms very slowly.
mixture of straight and branched chain alkanes
recall all the steps of fractional distillation
- crude oil is heated in a furnace
- mixture of gases and liquids pass into the tower
- tower is cool at the top and hot at the bottom
- vapours rise up the tower passing trays with bubble caps
- when they reach a tray lower than their bpt, they condense
- liquid is piped off
- the shorter the chain, the higher up the column
- longest chain and highest bpt is at the bottom (tar) and is used on roads but can be processed more for more valuable products
what is fracking
natural gas trapped in shale rock underwater and can be extracted by drilling into it and forcing a pressurised water/sand mixture into it.
this causes shale rock to fracture which releases gas into the surface.
chemicals are added to the water to prevent corrosion (HCl and methanol)
5 advantages to fracking
provides energy security so we rely less on other countries.
high quality, paid well jobs in the UK.
reliability of supply of gas and also therefore electricity.
lower energy bills for household.
reduced reliance on coal.
5 disadvantages to fracking
drilling infrastructure in ur ‘backyard’.
concern about the amount of water used.
concern about the amount of chemicals used.
occasionally, fracking causes tremors that the public think are mini earthquakes.
burning natural gases produces CO2
why do we use cracking
longer chain alkanes are not as economically valuable as shorter chained alkanes so cracking helps break up long chains into shorter chains. also helps meet the demand for shorter chained products like petrol
what are the 2 most useful results of cracking
alkanes and shorter chain hydrocarbons
what is petrol
a mixture of alkanes with 4-12 carbon atoms
what are alkanes used for
chemical livestock (raw material to make other things) used to make things such as polymers, paints or drugs
what is thermal cracking
involves heating the alkanes to a high temperature under a high pressure (700-1200K and up to 7000kPa)
the C-C bonds break homolytically so that 1 electrons from the pair in a covalent bond can go to each carbon. this initially produces 2 shorter chains free radicals which are highly reactive and react in different ways to form a variety of shorter chained hydrocarbons
how do you avoid decomposition in thermal cracking
time kept at these conditions need to be minimised (1 sec)
what is catalytic cracking
takes place at a lower temp and pressure to thermal cracking (720-820K and 70-140kPa)
a zeolite catalyst is made from SO2 and aluminium oxide (honeycomb structure and high surface area and acidic)
catalytic cracking is mainly used to produce vehicle fuel
main products: branched alkanes, cycloalkanes, aromatics)
what are the products of catalytic cracking
cycloalkanes, aromatic hydrocarbons, branched alkanes
are alkanes soluble in water? why?
they are insoluble because the hydrogen bonds in water are stronger than the van der Waals forces in alkanes
write an equation for the combustion of propane
C3H8 + 5O2 > 3CO2 + 4H2O
what is a fuel
something that releases heat energy when combusted
what is incomplete combustion and what are the products formed
combustion in a limited supply of oxygen. either forms CO (carbon monoxide - poisonous) or C (carbon - global dimming)
which type of hydrocarbons are likely to undergo incomplete combustion
longer chains
environmental impacts of nitrogen oxide
form nitric acid - acid rain, photochemical smog
environmental impact of sulfur dioxide
formes sulfuric acid - acid rain
environmental impact of carbon (soot)
asthma, cancer, global dimming
environmental impact of carbon dioxide and water vapour
greenhouse gases - global warming - increases global temperatures and increases climate change
what are flue gases
gases given out by power stations
2 equations to desulfurise flue gases
CaO(s) + 2H₂O(l) + SO₂(g) + 1/2O₂(g) > CaSO₄.H₂O(s)
CaCO₃(s) + SO₂(g) + 1/2O₂(g) > CaSO₄ + CO₂(g)
what are catalytic converters made from
ceramic honeycomb coated with platinum, palladium and rhodium metals
what are greenhouse gases and how do they lead to global warming ?
gases which trap infrared radiation in the atmosphere - atmosphere heats up - global warming
what are carbon neutral activities
things that produce no overall carbon dioxide emissions
how are halogenoalkanes formed from alkanes
free radical substitution reaction
what are the 3 stages of free radical substitution
initiation - breaking halogen bond to form free radicals
propagation - chain part of the reaction where products are formed but the free radical remains
termination - free radical removed, stable products formed
what properties are needed for the formation of a free radical chlorine atom
UV light
what is the earths ozone layer
protects the earth from too many harmful uv rays
how do CFCs break down the ozone layer
free radical substitution
write free radical substitution reactions to show how Cl free radicals catalyse the decomposition of O3
Cl2 > 2Cl. (UV light)
Cl. + O3 > ClO. + O2
ClO. + O3 > 2O2 + Cl.
overall: 2O3 + 3O2
