alkanes [O1] PAPER 2

Question 1

what is the general formula of an alkane?

Answer 1

Cn H2n+2

Question 2

what are the conditions and main products of thermal cracking?

Answer 2

• high temperature
• high pressure
• C-C bonds are broken
• mainly alkenes

Question 3

what are the conditions and main products of catalytic cracking?

Answer 3

• high temperature
• slight pressure
• zeolite catalyst
• C-C bonds are broken
• mainly branched alkanes and aromatics

Question 4

why does boiling point increase as the chain length of an alkane increases?

Answer 4

• surface area increases
• molecules are packed closer together
• stronger van der Waals’ forces between molecules

Question 5

why are high temperatures needed for cracking?

Answer 5

break (C-C / C-H) bonds

Question 6

cracking equations - what must you ensure? (when filling blanks)

Answer 6

total number of Cs and Hs in products add up to the molecule on the left side of the arrow

Question 7

define hydrocarbon

Answer 7

contains ONLY hydrogen and carbon

Question 8

what is meant by a saturated hydrocarbon?

Answer 8

single bonds only
OR no double bonds

Question 9

what is meant by an unsaturated hydrocarbon?

Answer 9

C=C/carbon double bonds present

Question 10

why should sulfur compounds found in crude oil be removed from fractions before they are used for fuels?

Answer 10

• (when combustion occurs) sulfur dioxide is produced
• sulfur dioxide causes acid rain

Question 11

state the physical property that allows compounds to be separated by fractional distillation

Answer 11

different boiling points

Question 12

Outline the essential features of the fractional distillation of crude oil that enable the crude oil to be separated into fractions

Answer 12

• compounds have different boiling points and separate depending on their boiling points
• these boiling points depend on the size of the compounds
• temperature gradient in column (cooler at top of column/hotter at the bottom of the column)
• Larger molecules at bottom of column

Question 13

explain how oxides of nitrogen may be produced during the combustion of paraffin in air

Answer 13

nitrogen and oxygen from air react with each other at high temperatures

Question 14

write an equation to show how nitrogen monoxide in the air is converted to nitrogen dioxide

Answer 14

2NO + O2 -> 2NO2

Question 15

why is the melting point of a molecule made up of a long carbon chain higher than a molecule made up of a short carbon chain?

Answer 15

• larger molecule so has a larger surface area and molecules are packed closer together
• stronger van der Waal’s forces between molecules

Question 16

what are the hazards of soot (solid carbon)?

Answer 16

• causes global dimming
• causes breathing problems
• smog
• makes visibility poor
• exacerbates asthma

Question 17

Explain the main economic reason why alkanes are cracked

Answer 17

to produce substances (shorter chain molecules) that are in higher demand

[e.g. short chain hydrocarbons needed for fuel]

Question 18

identify a catalyst used in catalytic converters

Answer 18

iridium / platinum / palladium / rhodium

Question 19

why do the products of complete combustion result in environmental problems?

Answer 19

CO2 and H2O evolved are greenhouse gases / contribute to global warming

Question 20

equations for the combustion of alkanes to produce (1) incomplete combustion to produce a solid, (2) incomplete combustion to produce a gas and (3) complete combustion

Answer 20

(1) alkane + O2 (g) -> C (s) + H2O (g)

(2) alkane + O2 (g) -> CO (g) + H2O (g)

(3) alkane + O2 (g) -> CO2 (g) + H2O (g)

Question 21

why is carbon monoxide harmful?

Answer 21

it is toxic / poisonous

Question 22

Identify a compound that is used to react with the sulfur dioxide in the products of combustion before they enter the atmosphere.
Give one reason why this compound reacts with sulfur dioxide.
Suggest the name of the process that occurs when this reaction takes place.

Answer 22

• calcium oxide / calcium carbonate
• it is a base / neutralises the sulfur dioxide / acid base reaction
• neutralisation

Question 23

what raw material are hydrocarbons obtained from and how can they be separated out?

Answer 23

• crude oil
• fractional distillation

Question 24

A catalytic converter is used to remove carbon monoxide from the exhaust gases in a car.
Write an equation to show how carbon monoxide is removed in a catalytic converter.

Answer 24

2NO + 2CO → N2 + 2CO2

Question 25

suggest why water in exhaust gases may contribute to global warming

Answer 25

it is a greenhouse gas / absorbs infrared radiation

Question 26

When some petrol was accidentally contaminated in 2007, the sensors in the affected cars caused a decrease in the supply of petrol to the engine.
Suggest the effect that the contaminated fuel would have on the performance of the cars.
State how the oil company might have recognised the problem before the petrol was sold.

Answer 26

• car is less powerful
• test it before sale / quality control

Question 27

do alkanes dissolve in water?

Answer 27

no - they are non-polar
(water is polar - like dissolves in like)

Question 28

info about ozone

Answer 28

• its decomposition is catalysed by chlorine free-radicals
• it absorbs harmful UV radiation from the upper atmosphere
• decomposes to form oxygen
• ozone holes are regions of the upper atmosphere where there is a reduced conc. of ozone

Question 29

The combustion of petrol in car engines produces the pollutant nitrogen monoxide.

Give an equation for a reaction that removes nitrogen monoxide in a catalytic converter.

Answer 29

2NO + 2CO → N2 + 2CO2

Question 30

Platinum, palladium and rhodium are metals used inside catalytic converters. A very thin layer of the metals is used on a honeycomb ceramic support.

Explain why a thin layer is used in this way.

Answer 30

• reduces the amount of metal needed
• maximises the surface area

Question 31

The petrol fraction obtained from crude oil can be used as fuel in cars.
State the meaning of fraction, as used in the term petrol fraction.

Answer 31

group of hydrocarbons with similar boiling points

Question 32

Give a reason why carbon dioxide absorbs infrared radiation.

Answer 32

C=O bonds vibrate at the same frequency as infrared radiation

Question 33

Some carbon particulates are also formed in both diesel and petrol vehicles. Explain why carbon particulates are formed.

Answer 33

incomplete combustion