alkanes

Question 1

what are alkanes

Answer 1

a homologous series of saturated hydrocarbons

Question 2

what is the general formula of an alkane

Answer 2

CnH2n+2

Question 3

why are alkanes insoluble

Answer 3

they are not polar and cannot overcome / disrupt the hydrogen bonding between water molecules.

Question 4

what is the trend in boiling points

Answer 4

As the alkane gets longer, the boiling point increases. This is because a longer alkane chain has increased intermolecular forces which require more energy to overcome and cause the state change from liquid to gas.

Question 5

why are the boiling points of longer alkanes higher

Answer 5

stronger van der waals forces bwt the molecules which require greater amounts of energy to overcome- as there are more electrons

Question 6

why are the boiling points of shorter alkanes lower

Answer 6

weaker VDW

Question 7

what is the process in which alkanes are fractioned into different sections

Answer 7

fractional distillation

Question 8

what is crude oil

Answer 8

Crude oil is a fossil fuel, formed from the decay of sea creatures over millions of years under high pressure and lack of oxygen

Question 9

how does fractional distillation work

Answer 9

Fractional distillation is carried out in a fractionating column which is very hot at the bottom and cool at the top
Crude oil enters the fractionating column and is heated so vapours rise
Vapours of hydrocarbons with very high boiling points will immediately condense into liquid at the higher temperatures lower down and are tapped off at the bottom of the column
Vapours of hydrocarbons with low boiling points will rise up the column and condense at the top to be tapped off
The different fractions condense at different heights according to their boiling points and are tapped off as liquids
The fractions containing smaller hydrocarbons are collected at the top of the fractionating column as gases
The fractions containing bigger hydrocarbons are collected at the lower sections of the fractionating column
The crude oil contains small amounts of other compounds
Some of these compounds may contain sulfur, and when burned sulfur dioxide can be produced
This is one of the causes of acid rain

Question 10

what are the fractions of hydrocarbons then used for after FD

Answer 10

they are not useful as heavy alkanes so they are cracked into shorter alkanes and alkenes which are higher in demand

Question 11

what is cracking

Answer 11

the thermal decomposition of alkanes

Question 12

what is thermal cracking

Answer 12

temp = 900
pressure = 70atm
catalyst= none
products= alkanes

Question 13

what is catalytic cracking

Answer 13

temp=450
pressure = 1-2 atm
cataylst= zeolites
products= alkenes + motor fuels

Question 14

why do alkanes undergo combustion

Answer 14

Alkanes are combusted (burnt) on a large scale for their use as fuels

Question 15

what is complete combustion

Answer 15

When alkanes are burnt in excess (plenty of) oxygen, complete combustion will take place and all carbon and hydrogen will be oxidised to carbon dioxide and water respectively

Question 15

what are the products of complete combustion

Answer 15

CO2 and H2O

Question 15

what is incomplete combustion

Answer 15

When alkanes are burnt in only a limited supply of oxygen, incomplete combustion will take place and not all the carbon is fully oxidised
Some carbon is only partially oxidised to form carbon monoxide
For example, the incomplete combustion of octane to form carbon monoxide

Question 16

what are the products of incomplete combsution

Answer 16

H2O
CO and/or C

Question 17

when will carbon be produced

Answer 17

Incomplete combustion often takes place inside a car engine due to a limited amount of oxygen present

Question 18

why is CO harmful

Answer 18

CO is a toxic and odourless gas which can cause dizziness, loss of consciousness and eventually death
The CO binds well to haemoglobin which therefore cannot bind oxygen and carbon dioxide
Oxygen is transported to organs
Carbon dioxide is removed as waste material from organs

Question 19

why are oxides of nitrogen and sulfur harmful

Answer 19

Normally, nitrogen is too unreactive to react with oxygen in air
However, in a car engine, high temperatures and pressures are reached causing the oxidation of nitrogen to take place:
N2(g) + O2(g) → 2NO(g)

N2(g) + 2O2(g) → 2NO2(g)

The oxides of nitrogen are then released in the exhaust fumes into the atmosphere
Car exhaust fumes also contain unburnt hydrocarbons from fuels and their oxides (VOCs)

Nitrogen oxides and sulfur dioxides can also dissolve and react in water with oxygen to form nitric acid which is a cause of acid rain
Acid rain can cause corrosion of buildings, endangers plant and aquatic life (as lakes and rivers become too acidic) as well as directly damaging human health

Question 20

how is carbon harmful

Answer 20

balckens buildings
can cause respiratory problems
global dimming

Question 21

how is CO2 harmful

Answer 21

greenhouse gas

Question 22

how does sulphur dioxide form

Answer 22

the sulphur in impurties in petroleum can react with oxygen in atmosphere or air producing SO2

Question 23

how is SO2 removed from waste gases in furnaces

Answer 23

flue gas desulfuristation

Question 24

how does flue gas desulfurisation work

Answer 24

sulpher reacts with either calcium oxide or calcium carbonate in a scrubber and undergoes an acid-base reaction

Question 25

what does the reaction of SO2 + CaO/ CaCO3 form

Answer 25

gypsum which is used to make plasterboard

Question 26

what are catalytic convertors

Answer 26

To reduce the amount of pollutants released in car exhaust fumes, many cars are now fitted with catalytic converters
Precious metals (such as platinum) are coated on a honeycomb to provide a large surface area and used as catalysts
The reactions that take place in the catalytic converter include:
Oxidation of CO to CO2:
2CO + O2 → 2CO2

or

2CO + 2NO → 2CO2 + N2

Reduction of NO to N2:
2CO + 2NO → 2CO2 + N2

Oxidation of unburnt hydrocarbons:
CnH2n+2 + (3n+1)[O] → nCO2 + (n+1)H2O

Question 27

what are the catalyst involved in catalytic conervtors

Answer 27

platinum
palladium
rubidium

Question 28

what is global warming

Answer 28

Climate change refers to long-term shifts in temperatures and weather patterns. -The Earth is getting warmer and many scientists believe it is due to increasing amounts of greenhouse gases in the
atmosphere.

Question 29

how do greenhouses gases cause global warming

Answer 29

Greenhouse gases are gases that absorbs the IR radiation given off by the Earth, but do not absorb the higher
frequency UV/visible radiation given off by the sun.
* Molecules that contain polar bonds absorb IR radiation to make bonds vibrate.
* therefore, molecules such as CO2 (the C=O bonds are polar), H2O (the O-H bonds are polar) and CH4 (the C-H bonds
are slightly polar).
* The burning of fossil fuels (including alkanes) releases carbon dioxide into the atmosphere. Carbon dioxide levels have
risen significantly in recent years due to increasing burning of fossil fuels.
* Carbon dioxide is a particularly effective greenhouse gas and its increase is thought to be largely responsible for global
warming.
* Methane levels have also increased significantly largely due to agriculture (e.g. rice production) and farming (e.g. cows).

Question 30

why do some molecules not absorb IR

Answer 30

• molecules such as O2 and N2 do not absorb IR radiation and are not greenhouse gases as they do not have polar
  bonds.