unit 3 section 2 alkanes and halogenalkanes

Question 1

how does fractional distillation work

Answer 1

• crude oil is vaporised at 350 degrees
  -crude oil goes in the bottom and rises up
  -largest hydrocarbons don’t vaporise, they stay at the bottom as their boiling points are too high
  -temperature gradient inside of fractionating column
  -each fraction of hydrocarbons condenses at different levels due to the length of the hydrocarbons and their boiling points ( bigger ones condense first as they have higher boiling points )
• the hydrocarbons with the lowest boiling points do not condense and are drawn off as gasess.

Question 2

what is cracking of hydrocarbons

Answer 2

it is the breaking of longer chain hydrocarbons into smaller chain hydrocarbons.

Question 3

what are the two types of cracking and explain them

Answer 3

thermal cracking - takes place at high temp (up to 1000 degrees) & pressure (up to 70 atm). produces lots of alkenes

catalytic cracking - zeolite catalyst, slight pressure and high temp, produces aromatic hydrocarbons, cycloalkanes and the alkanes needed to produce motor fuel.

Question 4

what are the benefits of using a catalyst in catalytic cracking

Answer 4

• reduces the cost - reaction can be done at lower temperatures and pressure
• speeds up the rate of reaction, so it takes less time and time is money.

Question 5

what is produced when you completely combust a hydrocarbon

Answer 5

it produces carbon dioxide and water

Question 6

what is produced when you incompletely combust and alkane

Answer 6

carbon monoxide & particulate carbon (soot) and maybe some carbon dioxide

Question 7

how is nitrogen monoxide produced in a car engine

Answer 7

nitrogen monoxide is produced when the high temperature and pressure in a car engine causes the nitrogen and oxygen to react to form nitrogen monoxide, this can then further react with the oxygen to from nitrogen dioxide

Question 8

what are the hazards of nitrogen monoxide

Answer 8

they are poisonous, they bind to the hemoglobin in red blood cell better than oxygen which stops oxygen from being carried around the body

Question 9

what is the equations for nitrogen reacting with oxygen to from nitrogen monoxide and the equation for nitrogen dioxide

Answer 9

N2(g) + O2(g) = 2NO(g)
2NO(g) + O2 = 2NO2(g)

Question 10

how does smog form

Answer 10

the fuel used in the engines is not all burn. the unburnt hydrocarbons react with nitrogen in the presence of sunlight to form ground-level ozone (O3), which is a major component of smog.

Question 11

what are the three main pollutants from car exhausts

Answer 11

nitrogen oxides , unburnt hydrocarbons and carbon monoxide

Question 12

explain how acid rain can be formed from nitrogen and sulfur

Answer 12

some fossil fuels contain sulfur. when they are burnt in car engines or power stations it forms sulfur dioxide and nitrogen dioxide . when they enter the atmosphere, they dissolve in the moisture and is converted into sulfuric acid and nitric acid and when it rains it forms acid rain

Question 13

how is sulfur dioxide removed in power stations and give the equation.

Answer 13

powdered calcium carbonate or calcium carbonate is mixed with water to make an alkaline slurry. this is then mixed with sulfur dioxide which forms a harmless salt (calcium sulfite).

CaO(s) + SO2(g) = CaSO3(s)

Question 14

explain global warming from fossil fuels

Answer 14

• fossil fuels burnt - CO2 released - enters the atmosphere - bad at absorbing infrared -reflects some energy back towards earth warming the planet

Question 15

what is a photochemical reaction

Answer 15

it is a reaction started by uv(ultra violet) light.
halogens react with alkanes in photochemical reactions

Question 16

what is a free radical substitution reaction

Answer 16

this is when a hydrogen atom is replaced by a chlorine atom or a bromine atom

Question 17

what is a free radical

Answer 17

it is a particle with an unpaired electron

Question 18

how is a free radical formed

Answer 18

it forms when a covalent bond splits equally, giving one electron to each species. this makes the particle very reactive.

Question 19

what is the initiation step of a photochemical reaction

Answer 19

it is the stage where the free radical is formed.

Question 20

what is the propagation step in a photochemical reaction

Answer 20

this is the stage where the free radicals are used up and created in a chain reaction

Question 21

what is the substitution step in a photochemical reaction

Answer 21

if the halogen is in excess, the eventually all the hydrogen will be replaced by the halogen.

Question 22

what is the termination step in photochemical reactions

Answer 22

this is the stage where free radicals react with each other to form stable molecules.

Question 23

what is a CFC molecule

Answer 23

it is a halogenalkane molecule that all the hydrogen’s have been replaced by chlorine and a fluorine atom.

Question 24

what are the equations for the formation of ozone

Answer 24

the oxygen is split into two radicals by uv light
O2 = *O + *O
O2 + *O = O3

Question 25

give the equations and explain how CFCs break down ozone into oxygen

Answer 25

the CFCs are split into two radicals by the uv light
CCl3F = *Cl2F + *Cl

Cl* + O3 = ClO* + O2
ClO* + O3 = 2O2 + *Cl

the chlorine radical is regenerated so the reaction continues.

Question 26

what was CFCs used in

Answer 26

they were used as coolants in fridges, propellants in aerosols and in fire extinguishers

Question 27

what molecules are now used instead of CFCs

Answer 27

they now use HCFCs & HFCs. these are alternatives that do not contain chlorine.

Question 28

what is a nucleophile

Answer 28

it is an electron-pair donor

Question 29

what is nucleophilic substitution

Answer 29

it is a reaction where one functional group is substituted for another

Question 30

what is an elimination reaction

Answer 30

it is a reaction where a small group of atoms breaks away from the larger molecule.

Question 31

what does an elimination reaction form

Answer 31

it forms an alkene

Question 32

how do you control whether a elimination reaction or nucleophilic substitution reaction takes place

Answer 32

you can influence what type of reaction takes place by controlling the conditions.
- reacting an halogenalkane with water under reflux and the molecule will predominantly undrego nucleophilic substitution and form an alcohol.
- by reacting an halogenalkane with ethanol under reflux, the molecule will prodominetely undergo elimination reaction to form an alkene.

Question 33

are alkanes bonds polar

Answer 33

no they are no polar, this is because they have similar electronegativies

Question 34

are alkanes soluble in water

Answer 34

they are not soluble in water. this is because the hydrogen bond in water is stronger than the van der waals forces of attraction

Question 35

what is crude oil

Answer 35

it is a mixture of fractions ( hydrocarbons with similar boiling points and properties )

Question 36

what is fracking and how is it done

Answer 36

-natural gas is held within shale rock
- drill into shale rock, force pressurised water and sand into rock to fracture it, collect gas
- HCL and Methanol are added to break up shale and prevent erosion

Question 37

what is the intermediate for thermal cracking

Answer 37

free radicals

Question 38

what is fuel

Answer 38

something that releases heat energy when combusted

Question 39

what is incomplete combustion

Answer 39

combustion in limited supply of oxygen

Question 40

which types of hydrocarbons are more likely to undergo incomplete combustion

Answer 40

longer chain hydrocarbons

Question 41

what is the environmental impact of unburnt hydrocarbons

Answer 41

photochemical smog