topic 8 - fuels

Question 1

hydrocarbons

Answer 1

compounds that only contain carbon and hydrogen

Question 2

what is crude oil (4)

Answer 2

• a complex mixture of hydrocarbons, which generally have their carbon atoms arrange in chains or rings
• our main source of hydrocarbons
• an important raw material that is used to create lots of useful substances used in the petrochemical industry
• is a finite resource, formed underground over millions of years from the buried remains of plants and animals being exposed to high temperatures and pressures

Question 3

fractions

Answer 3

simpler, more useful mixtures containing groups of hydrocarbons of similar lengths

Question 4

describe the process of fractional distillation of crude oil (6)

Answer 4

1) the oil is heated until most of it has turned to gas
2) gases enter a fractionating column
3) the liquid bitumen is drained off at the bottom
4) inside the fractionating column, there is a temperature gradient ; hotter at the bottom and cooler as you go up
5) longer hydrocarbons have higher boiling points so turn back into liquids sooner and drain out of the column early on
6) shorter hydrocarbons have lower boiling points so don’t turn back into liquids until later and then drain out near to the top of the column when they are cooler

Question 5

what are the 6 fractions of crude oil

Answer 5

gases
petrol
kerosene/ paraffin
diesel oil
fuel oil
bitumen

Question 6

what are is the gases fraction used for

Answer 6

domestic heating

cooking

Question 7

what is petrol used for

Answer 7

fuel for cars

Question 8

what is kerosene (paraffin) used for

Answer 8

fuel for aircraft

Question 9

what is diesel oil used for

Answer 9

fuel for some cars and trains

Question 10

what is fuel oil used for

Answer 10

fuel for large ships and in some large power stations

Question 11

what is bitumen used for

Answer 11

used to surface roads and roofs

Question 12

how do intermolecular forces vary between bigger and smaller molecules

Answer 12

intermolecular forces in smaller molecules break more easily as they are much weaker than in bigger molecules
in big molecule, even if forces break at one point, there are still lots of other places where the force is strong enough to hold it together

Question 13

how does boiling point vary across fractions

Answer 13

longer molecules/hydrocarbons have higher boiling points

as the intermolecular forces are harder to break

Question 14

how does ease of ignition vary across fractions

Answer 14

shorter hydrocarbons tend to be easier to ignite as they tend to have lower boiling points
they are then gases at room temperature and mix with oxygen in the air to give a GAS MIXTURE that bursts into flames if it comes into contact with a spark

Question 15

viscosity

Answer 15

measure of how easily a substance flows

Question 16

how does viscosity vary across fractions

Answer 16

the stronger the intermolecular forces, the harder it is for the liquid to flow
SO
long hydrocarbon -> high viscosity
short hydrocarbon -> low viscosity

Question 17

homologous series

Answer 17

a family of molecules which have the same general formula and chemical properties

Question 18

what do members of a homologous series vary by

Answer 18

molecular formulas of neighbouring compounds vary by a CH2 unit (methylene unit)

Question 19

what happens to physical properties in a homologous series

Answer 19

there is a gradual variation, for example in boiling points

Question 20

why can hydrocarbons be used as fuels

Answer 20

when hydrocarbons are burnt, their combustion reactions are exothermic and therefore release energy

Question 21

products of complete combustion of hydrocarbons

Answer 21

CO2 (carbon dioxide) and H2O (water)

Question 22

when does incomplete combustion happen

Answer 22

when there is not enough oxygen for complete combustion

Question 23

what are the products of incomplete combustion

Answer 23

CO (carbon monoxide) and carbon as soot particles

Question 24

why is carbon monoxide a toxic gas

Answer 24

it reacts with the haemoglobin in the blood to form carboxyhaemoglobin ; this reduces the blood’s ability to carry oxygen
lack of oxygen supply can lead to fainting, coma, tissue death or loss of life

Question 25

what problems do soot particles cause (3)

Answer 25

• deposit themselves on buildings making them look dirty
• decrease air quality
• cause/worse respiratory problems

Question 26

how is sulfur dioxide produced from fossil fuel combustion

Answer 26

SO2 comes from sulfur impurities in fossil fuels that are released when burned

Question 27

how does acid rain form

Answer 27

sulfur dioxide mixes with rainwater in the clouds, it dissolves to form dilute sulfuric acid which then falls as acid rain

Question 28

problems with acid rain (4)

Answer 28

• causes lakes to become acidic so many plants and animals die as a result
• kills trees
• damages limestone buildings and stone statues
• can corrode metal

Question 29

how are nitrogen oxides produced

Answer 29

created from reaction between nitrogen and oxygen caused by energy released from combustion reactions ; such as in the engine of a car

Question 30

problems with nitrogen oxides as pollutants (2)

Answer 30

• can contribute to acid rain

- can cause photochemical smog -> a type of air pollution that can cause breathing difficulties, headaches and tiredness

Question 31

2 pros of hydrogen as a fuel

Answer 31

• very clean - H2 + O2 produces energy and the only waste product is water
• obtained from water, a renewable resource

Question 32

4 cons of hydrogen as a fuel

Answer 32

• special, expensive engine needed
• hydrogen gas has to be manufactured which is expensive and needs energy from another source
• hard to store
• not widely available

Question 33

cracking

Answer 33

turning long saturated (alkane) molecules into shorter, unsaturated (alkene) molecules

produces lots of alkene molecules that can be used to make polymers

Question 34

thermal decomposition

Answer 34

when one substance breaks down into at least 2 new ones as you heat it

Question 35

cracking process

Answer 35

vaporised hydrocarbons are passed over powdered aluminium oxide catalyst (at 400°-700°C at 70 atm (conditions not necessary))
the long chain molecules are split apart over the catalyst

Question 36

role of cracking in supply and demand

Answer 36

longer molecules are cracked to give shorter molecules such as petrol which we have greater demand for