alkanes

Question 1

why are alkane molecules non-polar?

Answer 1

C and H have very similar electronegativities so form non-polar bonds

Question 2

how are neighbouring alkane molecules attached to each other?

Answer 2

by Van Der Waals’ forces- weak intermolecular forces

Question 3

describe alkane solubility in water?

Answer 3

hydrogen bonds between water molecules are much stronger than the Van Der Waals’ in alkanes so they are insoluble in water

Question 4

the longer the carbon chain the…..

Answer 4

higher the bp

higher the viscosity

lower the flammability

Question 5

why does the bp increase for longer chain alkanes?

Answer 5

-more SA (only talk about this for straight chains not branched)
-larger molecule
-greater number of electrons

SO more and stronger VDW forces between molecules = more energy to overcome

Question 6

what is the general formula for alkanes?

Answer 6

Cn H2n+2

Question 7

what is a hydrocarbon?

Answer 7

a compound containing hydrogen and carbon atoms only

Question 8

are alkanes saturated or unsaturated, why?

Answer 8

saturated
only single bonds between carbon atoms

Question 9

what is the general formula for cycloalkanes?

Answer 9

Cn H2n

Question 10

what is crude oil/ petroleum?

Answer 10

a mixture consisting mainly of alkane hydrocarbons and it is our main source of fuels and petrochemicals

Question 11

what is a mixture?

Answer 11

more than one different compounds or elements not chemically bonded together

Question 12

what is fractional distillation?

Answer 12

the separation of the components of a liquid into fractions which differ in boiling points

Question 13

what is a fraction?

Answer 13

a group of components that have similar boiling points and are removed at the same level in the fractionating column

Question 14

explain why branched alkanes have a lower boiling point?

Answer 14

more branched means less SA which means there are less points of contact where VDW’s can form

less VDW formed and weaker VDW = less energy to overcome

Question 15

when can we only use the assumption that branched alkanes have a lower boiling point?

Answer 15

when they have a similar Mr and so are similar in size

Question 16

describe the process of fractional distillation?

Answer 16

-vaporise crude oil in the furnace so the vapor can rise up the column
-the top of the column is cooler than the bottom
-larger, longer hydrocarbons condense at the bottom of the column, shorter, smaller hydrocarbons condense at the top of the column

Question 17

which two fractions don’t vaporise or condense?

Answer 17

bitumen has too high of a boiling point to vaporise so leaves at the bottom as a liquid

petroleum gas doesn’t condense so leaves the top of the column as a gas

Question 18

what are the two types of cracking?

Answer 18

thermal and catalytic

Question 19

how is cracking achieved?

Answer 19

by breaking C-C bonds, producing shorter chain hydrocarbons from longer chain hydrocarbons

Question 20

why do we do cracking?

Answer 20

we have a lower supply and higher demand for shorter hydrocarbons

we have a higher supply and lower demand for longer hydrocarbons

Question 21

what are the conditions for thermal cracking?

Answer 21

very high pressures and very high temperatures

Question 22

what does thermal cracking produce?

Answer 22

alkanes and a high percentage of alkenes

Question 23

how are mixtures of products produced in thermal cracking?

Answer 23

the C-C bonds can break at different positions in the carbon chain

Question 24

what are the products of thermal cracking used to make?

Answer 24

polymers (mainly alkenes)

Question 25

what conditions are needed for catalytic cracking?

Answer 25

high temperature, slight pressure and in the presence of a zeolite catalyst

Question 26

what does catalytic cracking produce?

Answer 26

cycloalkanes and branched alkanes as well as aromatic hydrocarbons such as benzene

Question 27

what are the products of catalytic cracking used as?

Answer 27

motor fuels

Question 28

how is energy obtained from alkane fuels?

Answer 28

when they are combusted products formed in the reaction depend on the supply of oxygen

Question 29

what is the equation for complete combustion?

Answer 29

alkane + oxygen ------> carbon dioxide + water

Question 30

when does incomplete combustion occur?

Answer 30

limited supply of oxygen

Question 31

what is the equation for incomplete combustion?

Answer 31

alkane + oxygen ---------> carbon monoxide + water

Question 32

what is the equation for further incomplete combustion?

Answer 32

alkane + oxygen -------> solid carbon (soot) + water

Question 33

when does further incomplete combustion occur?

Answer 33

very limited supply of oxygen

Question 34

name pollutants we need to know about?

Answer 34

unburnt hydrocarbons carbon dioxide carbon monoxide carbon nitrogen oxides sulfur dioxide

Question 35

how do unburnt hydrocarbons produce low level ozone?

Answer 35

reacts with NOx gases to form low level ozone

Question 36

what are the effects of the pollutants unburned hydrocarbons?

Answer 36

leads to low level ozone which causes respiratory problems global dimming

Question 37

what is the effect of carbon dioxide and how is it produced?

Answer 37

global warming combustion of fuels

Question 38

what is the effect of carbon monoxide and how is it produced?

Answer 38

toxic gas incomplete combustion of fuels in a limited supply of oxygen

Question 39

what can carbon (soot) lead to, and how is it produced?

Answer 39

particles lead to asthma incomplete combustion of fuels in a limited supply of oxygen

Question 40

what is the effect of nitrogen oxides and how are they produced?

Answer 40

acid rain and photochemical smog N2 and O2 from the air react at high temps in engines

Question 41

what is the effect of sulfur dioxide and how is it produced?

Answer 41

acid rain S from fuel impurities reacts with O2 in the air

Question 42

what has been fitted in most cars to remove unburnt CO and NO hydrocarbons?

Answer 42

catalytic converters

Question 43

write an equation for how a catalytic converter removes unburnt hydrocarbons?

Answer 43

2CO + 2NO -------> 2CO2 + N2

Question 44

what structure do catalytic converters contain?

Answer 44

honeycombed structure coated with a thin layer of Pt, Pd, Rh metals

Question 45

why is a thin layer of metals used in catalytic converters?

Answer 45

to reduce the amount needed (reduce cost)

Question 46

what does the honeycomb structure of a catalytic converter provide?

Answer 46

a large surface area

Question 47

what reacts to produce less polluting products?

Answer 47

NO and CO 2NO + 2CO --------> 2CO2 +N2

Question 48

what are the two ways of removing unburnt hydrocarbons?

Answer 48

reaction with oxygen reaction with NO

Question 49

what is produced which requires flue gas desulfurisation?

Answer 49

power stations burn coal or natural gas to produce electricity. Sulfur dioxide is also produced and this needs to be removed

Question 50

what are chimneys or flues coated with to react with sulfur dioxide produced?

Answer 50

calcium oxide or calcium carbonate which absorb and react with it SO2 + CaO --------> CaSO3 SO2 + CaCO3 --------> CaSO3 + CO2

Question 51

what does vacuum distillation allow for?

Answer 51

heavier fractions can be further separated without high temperatures which could break them down.

Question 52

what does vacuum distillation involve?

Answer 52

heavy residues from the fractionating column are distilled again under a vacuum lowering a pressure over the liquid will lower its boiling point