alkanes (organic)

Question 1

what are sigma bonds

Answer 1

single bonds that are formed by direct overlap of orbitals between the bonding atoms

Question 2

shape of alkanes

Answer 2

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

Question 3

why are C-H bonds non polar

Answer 3

they have similar electronegativities

Question 4

what forces are between molecules

Answer 4

weak van der waals forces which can be overcome

Question 5

whats the trend in boiling point of alkanes

Answer 5

as chain length increases, boiling point also increases

Question 6

whats the boiling point of branched alkanes

Answer 6

they have lower boiling points because theres less surface area in between molecules which lessens the van der waals forces

Question 7

whats the reactivity like in alkanes

Answer 7

unreactive because the C-H and C-C bonds are strong and the bonds are non polar

Question 8

what are some reactions alkanes can undergo

Answer 8

combustion and free radical substitution

Question 9

whats the equations for combustion of alkanes

Answer 9

fuel+oxygen - CO2 + H2O
fuel+limited oxygen - CO + H2O
fuel+very limited oxygen - C + H2O

Question 10

whats the equation for molar volume

Answer 10

volume of gas (dm3) / molar volume (24) = mole of particle
OR
mole of particle x molar volume (24) = volume of gas (dm3)

Question 11

whats crude oil

Answer 11

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

Question 12

recall all the steps of fractional distillation

Answer 12

1. crude oil is heated in a furnace
2. mixture of gases and liquids pass into the tower
3. tower is cool at the top and hot at the bottom
4. vapours rise up the tower passing trays with bubble caps
5. when they reach a tray lower than their bpt, they condense
6. liquid is piped off
7. the shorter the chain, the higher up the column
8. longest chain and highest bpt is at the bottom (tar) and is used on roads but can be processed more for more valuable products

Question 13

what is fracking

Answer 13

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)

Question 14

5 advantages to fracking

Answer 14

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.

Question 15

5 disadvantages to fracking

Answer 15

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

Question 16

why do we use cracking

Answer 16

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

Question 17

what are the 2 most useful results of cracking

Answer 17

alkanes and shorter chain hydrocarbons

Question 18

what is petrol

Answer 18

a mixture of alkanes with 4-12 carbon atoms

Question 19

what are alkanes used for

Answer 19

chemical livestock (raw material to make other things) used to make things such as polymers, paints or drugs

Question 20

what is thermal cracking

Answer 20

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

Question 21

how do you avoid decomposition in thermal cracking

Answer 21

time kept at these conditions need to be minimised (1 sec)

Question 22

what is catalytic cracking

Answer 22

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)

Question 23

what are the products of catalytic cracking

Answer 23

cycloalkanes, aromatic hydrocarbons, branched alkanes

Question 24

are alkanes soluble in water? why?

Answer 24

they are insoluble because the hydrogen bonds in water are stronger than the van der Waals forces in alkanes

Question 25

write an equation for the combustion of propane

Answer 25

C3H8 + 5O2 > 3CO2 + 4H2O

Question 26

what is a fuel

Answer 26

something that releases heat energy when combusted

Question 27

what is incomplete combustion and what are the products formed

Answer 27

combustion in a limited supply of oxygen. either forms CO (carbon monoxide - poisonous) or C (carbon - global dimming)

Question 28

which type of hydrocarbons are likely to undergo incomplete combustion

Answer 28

longer chains

Question 29

environmental impacts of nitrogen oxide

Answer 29

form nitric acid - acid rain, photochemical smog

Question 30

environmental impact of sulfur dioxide

Answer 30

formes sulfuric acid - acid rain

Question 31

environmental impact of carbon (soot)

Answer 31

asthma, cancer, global dimming

Question 32

environmental impact of carbon dioxide and water vapour

Answer 32

greenhouse gases - global warming - increases global temperatures and increases climate change

Question 33

what are flue gases

Answer 33

gases given out by power stations

Question 34

2 equations to desulfurise flue gases

Answer 34

CaO(s) + 2H₂O(l) + SO₂(g) + 1/2O₂(g) > CaSO₄.H₂O(s) CaCO₃(s) + SO₂(g) + 1/2O₂(g) > CaSO₄ + CO₂(g)

Question 35

what are catalytic converters made from

Answer 35

ceramic honeycomb coated with platinum, palladium and rhodium metals

Question 36

what are greenhouse gases and how do they lead to global warming ?

Answer 36

gases which trap infrared radiation in the atmosphere - atmosphere heats up - global warming

Question 37

what are carbon neutral activities

Answer 37

things that produce no overall carbon dioxide emissions

Question 38

how are halogenoalkanes formed from alkanes

Answer 38

free radical substitution reaction

Question 39

what are the 3 stages of free radical substitution

Answer 39

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

Question 40

what properties are needed for the formation of a free radical chlorine atom

Answer 40

UV light

Question 41

what is the earths ozone layer

Answer 41

protects the earth from too many harmful uv rays

Question 42

how do CFCs break down the ozone layer

Answer 42

free radical substitution

Question 43

write free radical substitution reactions to show how Cl free radicals catalyse the decomposition of O3

Answer 43

Cl2 > 2Cl. (UV light) Cl. + O3 > ClO. + O2 ClO. + O3 > 2O2 + Cl. overall: 2O3 + 3O2