StudyPacks
STUDY.
KS4
SCIENCE
Crude Oil
Fractional distillation and Cracking
This Study Pack aims to cover:
1. Fractional distillation.
2. Properties of Crude Oil
3. Using Crude oil as fuel
4. Environmental problems of using Crude Oil.
SC6
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compiled resources based on the UK National Curriculum covering the Primary and Secondary Frameworks
including SATs and GCSE examinations.
Student Name
Contents
Crude oil and Fractional distillation
2-6
Practice questions
7-11
Exam questions
12-26
Mark scheme
27-33
1
2
3
4
5
6
Fractional distillation
Aims
Fractional distillation is used to separate the mixture of hydrocarbons in crude oil into different
fractions. In this activity you will have the chance to organise your understanding of how fractional
distillation works.
Questions
1 The fractionating column gets gradually cooler as you go upwards. Molecules in the diesel fraction
are medium-sized hydrocarbons. Explain why molecules in the diesel fraction emerge halfway up
and not higher or lower. You should refer to the boiling points in your answer.
..............................................................................................................................
..............................................................................................................................
..............................................................................................................................
..............................................................................................................................
..............................................................................................................................
2 What is the trend in the size of the molecules as you go from the top of the fractionating column to
the bottom?
..............................................................................................................................
..............................................................................................................................
..............................................................................................................................
Figure 1
All molecules attract each other with weak forces between the atoms on nearby molecules.
Describe how the molecules will change as they are boiled (turn from liquid to gas).
7
..............................................................................................................................
..............................................................................................................................
3 Do the short-chain hydrocarbons have a high or low boiling point? Explain why, using Figure 1 to
help you.
..............................................................................................................................
..............................................................................................................................
..............................................................................................................................
..............................................................................................................................
..............................................................................................................................
..............................................................................................................................
4 Do the long-chain hydrocarbons have a higher or lower boiling point than short-chain hydrocarbons?
Explain your answer.
..............................................................................................................................
..............................................................................................................................
..............................................................................................................................
..............................................................................................................................
..............................................................................................................................
..............................................................................................................................
..............................................................................................................................
5 What is the relationship between length of hydrocarbon chain and boiling point?
..............................................................................................................................
..............................................................................................................................
8
Crude oil and fuels Test yourself
1
The table shows some information about the first four alkanes.
Name of alkane
Formula
Boiling point in °C
Methane
CH4
–162
C2H6
–88
Propane
C3H8
Butane
a
i
0
Name the alkane missing from the table.
...................................................................................................................................
ii
(1)
What is the formula of butane?
...................................................................................................................................
(1)
iii Estimate the boiling point of propane.
...................................................................................................................................
b
Which one of the following is the formula of the alkane with 6 carbon atoms?
C6H6
C6H10
C6H14
C6H16
...................................................................................................................................
c
(1)
Explain why alkanes are hydrocarbons.
...................................................................................................................................
d
(1)
(1)
A molecule of methane can be represented as:
Draw a molecule of propane in the same way.
9
(2)
2
Some crude oil was distilled in a fractionating column. The table shows the boiling ranges of
three of the fractions that were collected.
a
Fraction
Boiling range in °C
A
60–120
B
160–230
C
240–320
Which of these fractions is the most flammable?
...................................................................................................................................
b
Which of these fractions is the most viscous?
...................................................................................................................................
c
(1)
Why do the fractions have boiling ranges and not boiling points?
...................................................................................................................................
3
(1)
Which of these fractions has the smallest hydrocarbon molecules?
...................................................................................................................................
d
(1)
(1)
Some land-fill sites produce a gas that can be collected and burned as a fuel. The gas is
mainly methane.
a
Choose the word from the list to complete the sentence.
condensed
distilled
oxidised
During the combustion of methane the elements in the fuel are ........................
b
(1)
Write a word equation for the complete combustion of methane, CH4.
...................................................................................................................................
...................................................................................................................................
c
Under what conditions could methane burn to produce carbon monoxide?
...................................................................................................................................
d
(2)
(1)
A sample of land-fill gas was burned. The waste gases contained sulfur dioxide. Explain
why.
...................................................................................................................................
(1)
10
4
Crude oil is a resource from which fuels can be separated.
a
The name of the main fuel fractions and one of the hydrocarbons in each fraction are
shown in the table.
i
How does the number of carbon atoms in a hydrocarbon affect its boiling point?
...................................................................................................................................
ii
(1)
Suggest the lowest temperature to which crude oil needs to be heated to vaporise all
the hydrocarbons in the table.
Temperature = ........... °C?
(1)
iii Dodecane boils at 216 °C. At what temperature will dodecane gas condense to liquid?
Temperature = ........... °C?
(1)
b In this question you will be assessed on using good English, organising information clearly
and using scientific terms where appropriate.
Describe and explain how the fractions are separated in a fractionating
column.
...................................................................................................................................
...................................................................................................................................
...................................................................................................................................
...................................................................................................................................
...................................................................................................................................
(6)
11
Exam Questions
Q1.
The table shows some information about alkanes.
Name
Formula
Relative formula mass
Boiling point in °C
methane
CH
16
–160
ethane
CH
30
–90
44
–40
4
2
6
propane
butane
CH
58
pentane
CH
72
36
hexane
CH
86
68
(a)
4
5
6
10
12
14
Give the formula of propane.
...................................................................................................................................
(1)
(b) (i)
increases?
What happens to the boiling points of the alkanes as the relative formula mass
.........................................................................................................................
(1)
(ii)
Draw a graph. Plot the points and draw a best fit line.
(3)
12
(iii)
What is the boiling point of butane?
.........................................................................................................................
(1)
(iv)
Show clearly on the graph how you found the boiling point of butane.
(1)
(c)
Circle which of the following is not an alkane.
C71116
C9H18
C11H24
C24H50
(1)
(Total 8 marks)
Q2.
(a) Crude oil is a mixture of many compounds. Most of the compounds consist of
molecules made only of carbon and hydrogen. Choose one word from the list below to complete the
sentence.
carbohydrates
carbonates
hydrocarbons
hydrogencarbonates
Compounds made only of carbon and hydrogen are called .......................................
(1)
(b) The fractions contain molecules with similar numbers of carbon atoms. The main fractions are
shown in the table below.
NAME OF FRACTION
NUMBER OF CARBON
ATOMS IN MOLECULES
petroleum gases
1 to 4
gasoline
4 to 12
naphtha
7 to 14
kerosene
11 to 15
diesel oil
14 to 19
lubricating oil
18 to 30
residue
more than 30
Naphtha burns more easily than diesel oil.
Explain why.
.....................................................................................................................................
.....................................................................................................................................
.....................................................................................................................................
(1)
(Total 2 marks)
Q3.
Crude oil is a mixture of many saturated hydrocarbons. They can be separated into
fractions by the process of fractional distillation.
State what is meant by:
(i)
hydrocarbon. ............................................................................................................
....................................................................................................................................(2)
13
(ii)
saturated. ..................................................................................................................
....................................................................................................................................
(1)
(iii)
fraction. ....................................................................................................................
....................................................................................................................................
(1)
(Total 4 marks)
Q4.
Crude oil is a complex mixture of hydrocarbons, mainly alkanes. The number of carbon
atoms in the molecules ranges from 1 to over 100.
(a)
How does the boiling point change as the number of carbon atoms in the molecules increases?
.....................................................................................................................................
.....................................................................................................................................
(1)
(b)
Name the method used to separate petroleum into fractions.
.....................................................................................................................................
(1)
(c)
The simplest hydrocarbon is methane, CH4. Its structure can be represented:
Draw the structure of ethane, C2H6.
(1)
(Total 3 marks)
Q5.
This question is about hydrocarbons.
(a)
Use two of the words in the box to complete the sentence.
air
finite
organic
renewable
sediment
water
Crude oil is a mixture of hydrocarbons. It was formed from
............................................................ materials that were trapped in
............................................................ over a very long period of time.(2)
(b)
Petrol is separated from crude oil by fractional distillation.
14
(i)
Which fraction has the lowest boiling point?
...........................................................................................................................
(1)
(ii)
Which fraction has the highest density?
...........................................................................................................................
(1)
(c) Some of the fractions containing larger hydrocarbon molecules are used to make plastics, such
as poly(ethene).
(i)
What type of chemical change is Reaction A?
...........................................................................................................................
(1)
(ii)
Explain what happens in Reaction B.
...........................................................................................................................
...........................................................................................................................
...........................................................................................................................
...........................................................................................................................
(2)
(d) Natural gas contains the hydrocarbon called methane. Some water heaters use methane as a
fuel. People could die from breathing the fumes produced by heaters that have not been checked and
serviced. Explain how these fumes are produced and why they are dangerous.
.....................................................................................................................................
.....................................................................................................................................
.....................................................................................................................................
.....................................................................................................................................
.....................................................................................................................................
.....................................................................................................................................
.....................................................................................................................................
(3)
(Total 10 marks)
15
Q6.
Hot air balloons are used mainly for pleasure trips.
(a) Air is a mixture of gases. Complete the
table. (Carbon dioxide has been done for you.)
Gas
Chemical formula
% in air
nitrogen
78
oxygen
O2
argon
0.9
carbon dioxide
CO2
0.03
(3)
(b) The air in the balloon is heated using a propane burner. Propane, C3H8, is a hydrocarbon that
burns in air forming carbon dioxide, CO2, and water, H2O.
(i)
What does hydrocarbon mean?
...........................................................................................................................
...........................................................................................................................
(1)
(ii)
Which gas, in the air, reacts with propane when it burns?
...........................................................................................................................
(1)
(iii)
What type of chemical reaction happens when a hydrocarbon burns?
...........................................................................................................................
(1)
(iv)
The formation of more carbon dioxide causes global problems. Explain why.
...........................................................................................................................
...........................................................................................................................
...........................................................................................................................
...........................................................................................................................
(2)
(Total 8 marks)
16
Q7.
(a)
Complete this sentence about crude oil.
Crude oil is mainly a mixture of compounds called ...................................... which
contain carbon and hydrogen only.
(1)
(b)
The diagram shows a laboratory experiment used to separate crude oil.
Complete each sentence by choosing the correct words
from the box.
condensation
distillation
melting
evaporation
sublimation
The main process taking place at A is .......................................................................
The main process taking place at B is .......................................................................
This method of separating crude oil is called ............................................................
(3)
(c) Complete this sentence by crossing out the word in each box that is wrong. The first one has
been done for you.
This method of separating crude oil works because the
the
their boiling point and the
the molecules are,
volatile they are.
(1)
(Total 5 marks)
17
Q8.
The hydrocarbons in crude oil can be separated into useful fractions.
Fraction
Boiling point
in °C
Naphtha
20–180
5–9
10
20
Gasoline (petrol)
20–200
5–10
10
20
Kerosene (paraffin)
180–260
10–16
15
23
Diesel
260–340
14–20
20
25
Fuel oil
370–600
20–70
45
12
(a)
Carbon
Relative %
chain length in crude oil
Relative %
demand
Why does gasoline (petrol) have a lower boiling point than fuel oil?
....................................................................................................................................
....................................................................................................................................
(1)
(b)
Suggest why gasoline (petrol) costs more than fuel oil.
....................................................................................................................................
....................................................................................................................................
....................................................................................................................................
....................................................................................................................................
(2)
(c)
Describe how fuel oil can be changed into gasoline (petrol).
....................................................................................................................................
....................................................................................................................................
....................................................................................................................................
....................................................................................................................................
(2)
(Total 5 marks)
Q9.
Most electricity in the UK is generated in power stations that burn fossil fuels. The diagram
lists some of the substances released into the air when fossil fuels are burned.
18
(a)
(i)
Which one of the substances released into the air causes acid rain?
.........................................................................................................................
(1)
(ii)
In the sentence below, draw a ring around the correct answer.
The type of environmental pollution caused by
global dimming
smoke particle is global warming
rising sea levels
(1)
(iii)
Suggest how the burning of fossil fuels may cause climate change.
.........................................................................................................................
.........................................................................................................................
.........................................................................................................................
.........................................................................................................................
(2)
(b)
The table shows the percentage of electricity generated by different energy sources.
Energy
sources
Percentage
(%)
Renewables
Nuclear
Coal
Gas and Oil
8
20
32
40
Complete the bar chart to show the percentage of electricity generated by coal and by gas and oil.
(2)
(Total 6 marks)
19
Q10.
(a)
Crude oil is a natural resource from which useful fuels can be separated.
Crude oil is a mixture of hydrocarbons.
Complete the sentence about a hydrocarbon molecule.
A hydrocarbon molecule is made up of ....................................... and carbon atoms only.
(1)
(b)
Many fuels come from crude oil. Some of these fuels are shown in the diagram.
Suggest two properties of these fuels that allow them to be separated from crude oil.
....................................................................................................................................
....................................................................................................................................
....................................................................................................................................
....................................................................................................................................
(2)
(c)
Fuels from crude oil burn to provide heat energy.
When a fuel burns, it combines with oxygen in the air and produces carbon dioxide and water.
When there is not enough oxygen, the fuel burns and also produces carbon monoxide and carbon
particles.
Draw a straight line from each substance that links it to a possible environmental problem.
One has been done for you.
(3)(Total 6 marks)
20
Q11.
Crude oil contains many different hydrocarbons.
(i)
Which formula in the list represents a hydrocarbon?
Draw a ring around the correct formula.
CO2
C6H12O6
C8H18
H2O
(1)
(ii)
Which word from the list below best describes crude oil?
Draw a ring around the correct word.
alloy compound element
mixture
(1)
(iii) Choose, from the list below, words to complete the passage about the separation of the
hydrocarbons in crude oil by fractional distillation.
atoms
fractions
burned
ions
condensed
evaporated
molecules
filtered
neutralised
During fractional distillation the many hydrocarbons in crude oil are separated
into ................................. each of which contains ................................ with a
similar number of carbon .................................... .
To do this the oil is first .................................... and then ....................................
at a number of different temperatures.
(5)
(Total 7 marks)
Q12.
Crude oil is a mixture of mostly alkanes.
(a)
Crude oil is separated into useful fractions by fractional distillation.
(i)
Describe and explain how the mixture of alkanes is separated by fractional distillation.
..........................................................................................................................
..........................................................................................................................
..........................................................................................................................
..........................................................................................................................
..........................................................................................................................
..........................................................................................................................
..........................................................................................................................
(3)
21
(ii)
The table gives the name and formula for each of the first three alkanes.
Complete the table to show the formula of butane.
Name of alkane
Formula
Methane
CH4
Ethane
C2H6
Propane
C3H8
Butane
(1)
(b)
The structural formula of methane, CH4, is:
H
│
│
H
Draw the structural formula of propane, C3H8
(1)
(c) The relative amounts of and the market demand for some hydrocarbons from the fractional
distillation of crude oil are shown in the graph.
22
(i)
Why is the market demand for the C5 – C8 fraction higher than the market demand for the C21 –
C24 fraction?
..........................................................................................................................
..........................................................................................................................
(1)
(ii)
Cracking is used to break down large hydrocarbon molecules into smaller hydrocarbon molecules.
Complete the symbol equation by writing in the formula of the other hydrocarbon.
C20H42 C16H34 + 2 ..........................................
(1)
(iii)
The C5 – C8 fraction has low supply and high market demand.
Suggest three ways in which the oil industry could overcome this problem.
1 .........................................................................................................................
............................................................................................................................
2 .........................................................................................................................
..........................................................................................................................
3 .......................................................................................................................
..........................................................................................................................
(3)
(Total 10 marks)
Q13.
Crude oil is a resource from which fuels can be separated.
(a) The name of the main fuel fractions and one of the hydrocarbons in each fraction are shown in
the table.
(i)
How does the number of carbon atoms in a hydrocarbon affect its boiling point?
...........................................................................................................................
...........................................................................................................................
(1)
23
(ii) Suggest the lowest temperature to which crude oil needs to be heated to vaporize all the
hydrocarbons in the table.
Temperature = ................................................ °C
(1)
(iii)
Dodecane boils at 216 °C. At what temperature will dodecane gas condense to liquid?
Temperature = ................................................ °C
(1)
(b)
The bar chart shows the relative supply and demand for the petrol and diesel fractions.
(i)
How does the relative supply and demand for petrol and diesel fractions cause problems for an
oil company?
...........................................................................................................................
...........................................................................................................................
...........................................................................................................................
...........................................................................................................................
(2)
(ii)
Suggest one way an oil company could solve these problems.
...........................................................................................................................
...........................................................................................................................
(1)
(Total 6 marks)
Q14.
A large reservoir is surrounded by trees. Planners need to protect the environment. The
distance around the reservoir is many kilometres. There will be only one road access to a car park a
few kilometres from the reservoir. From the car park people would be transported to accommodation,
activities or places of interest by steam train.
24
(a) Coal contains carbon and small amounts of sulfur. The steam train would cause environmental
problems if coal were used as the fuel.
Explain why.
....................................................................................................................................
....................................................................................................................................
....................................................................................................................................
....................................................................................................................................
....................................................................................................................................
....................................................................................................................................
....................................................................................................................................
....................................................................................................................................
(4)
(b) The planners have stated that, as a result of using the steam train, there must be no overall
increase of carbon dioxide added to the atmosphere. The steam train would be considered as ‘carbon
neutral’ if wood, from the surrounding forest, were used as the fuel.
Suggest why.
....................................................................................................................................
....................................................................................................................................
....................................................................................................................................
....................................................................................................................................
....................................................................................................................................
....................................................................................................................................
(3)
0(Total 7 marks)
Q15.
Crude oil is obtained by drilling into the Earth’s crust. The diagram shows a section
through the Earth’s crust to show how this is done.
25
(a)
Crude oil contains many hydrocarbons. Which elements do hydrocarbons contain?
....................................................................................................................................
(1)
(b)
The crude oil is separated by fractional distillation. The diagram shows a column used for this.
(i)
Explain, as fully as you can, how fractional distillation works.
............................................................................................................................
............................................................................................................................
............................................................................................................................
............................................................................................................................
............................................................................................................................
(3)
(ii)
Naphtha burns more easily than diesel oil. Explain why.
............................................................................................................................
............................................................................................................................
............................................................................................................................
(1)
(iii) Naphtha contains a saturated hydrocarbon with the formula C7H16 .
Draw the structural formula of this compound.
(2)
(Total 7 marks)
26
Mark scheme
M1.
(a)
C3H8
1
(b)
(i)
increases / gets larger
1
ii)
all 5 points plotted correctly
deduct 1 mark for each incorrectly plotted point
but ignore –90, 30
allow error of one square in any direction
2
smooth line graph
1
(iii)
allow
boiling point estimate from their graph
2 °C
1
(iv)
shown clearly on graph
allow just one construction line drawn
1
(c)
C9H18
1
[8]
27
M2.
(a)
hydrocarbons
M4.
(a) the more C atoms the higher the b.pt./temperature
Allow just higher. Not answer based on melting point
for 1 mark
for 1 mark
1
1
(b)
less carbon atoms / smaller molecule
(b)
for 1 mark
(fractional) distillation/fractionation
for 1 mark
1
[2]
M3.
(i)
carbon and hydrogen
only or compound of
1
(c)
for 1 mark each
2
(ii)
single bonds only
or no double bonds etc
or contains maximum number of hydrogen atoms
for 1 mark
1
must include H atoms and lines not CH3 – CH3
for 1 mark
(iii) molecules of similar chain length
similar boiling points
limited range of boiling points etc
1
[3]
any 1 for 1 mark
1
[4]
M5.
(a)
M6.
(a)
N2
1
organic
1
20–21%
accept an answer in this range
sediment
1
1
(b)
(i)
Ar
gases
1
1
(ii)
(b)
(i)
compound of carbon and hydrogen only
do not accept ‘mixture’
bitumen
1
1
(c)
(i)
(ii)
cracking
Oxygen or O2
1
accept thermal decomposition
do not accept endothermic
(iii)
1
exothermic
accept combustion or oxidation
1
(ii)
many or short or small (ethene) molecules
accept monomer
(iv)
increases greenhouse effect
1
accept double bonds open up or break
1
global warming or example
1
join to make larger molecules
[8]
accept polymer
accept polymerisation
M7.
accept short chain to long chain (or molecules)
(a)
hydrocarbons
1
do not accept unsaturated to saturated
1
(b)
evaporation
1
(d)
poor ventilation
accept limited air supply
condensation
1
accept insufficient oxygen
1
distillation
causes incomplete combustion
allow fractional distillation
1
accept produces CO
1
(c)
lower and more
1
[5]
(fumes contain) carbon monoxide which
dangerous toxic is not awarded a mark
do not accept harmful or poisonous
1
[10]
28
M8.
(a)
any one from:
M9.
(a)
(i)
sulfur dioxide / SO2
1
•
gasoline / petrol / it contains short(er) chains / hydrocarbons
or small(er) molecules or contains few(er) carbons
(ii)
accept fuel oil contains long(er) chain length / large(r)
molecules or contains many carbons
1
ignore particles
•
(iii)
carbon dioxide / CO2
ignore ozone
gasoline / petrol / it has weak(er) / small(er) intermolecular forces
1
accept fuel oil has strong(er) / great(er) intermolecular
forces
increases the levels (of carbon dioxide)
1
(b)
global dimming
accept it is a greenhouse gas or causes global
warming / greenhouse effect
only accept figures if used in a comparative statement
1
(b)
gas / oil bar correct length
any two from:
•
1
gasoline / petrol / it is in high demand
coal bar correct length1 [ 6
accept fuel oil is in low demand
•
gasoline / petrol / it is in short supply
accept fuel oil is plentiful
M10.
accept answers such as ‘gasoline / petrol / its supply is
less than demand for 2 marks
ignore formulae
(a)
hydrogen
1
or gasoline / petrol / its percentage in crude oil is less
than demand for 2 marks
(b)
any two from:
•
(high) tax / duty
•
different sized molecules / more or less (carbon) atoms (in
molecules)
•
cracking costs in terms of money / energy
ignore different densities
accept cracking expensive
2
(c)
•
fuels have different boiling points
•
fuels condense at different temperatures
any two from:
2
ignore particles
•
(fuel oil / it) heated / vaporised
•
with catalyst
(c)
accept a named catalyst
if first two bullet points are not awarded ‘cracking’ gains
1 mark
•
(to give / form / produce) short(er) chains / hydrocarbons or
small(er)
molecules or contains few(er) carbons
if wrong process named max 1 mark
2
[5]
M11.
(i)
C8H18
for one mark
1
(ii)
mixture
all three correct = 3 marks
for one mark
two correct = 2 marks
1
one correct = 1 mark
3
(iii) fractions molecules
atoms
evaporated condensed
[6]
in this order for 1 mark each
5
[7]
29
M12.
vapour
(a)
(i)
heat / evaporate the crude oil / change to gas or
M13.
(a) (i)
the greater the number (of carbon atoms), the
higher its boiling point
do not accept heat with catalyst
do not accept hydrocarbons for carbon atoms
1
allow converse
allow melting point
cool / condense (hydrocarbons)
1
allow small molecules at top and / or large molecules
at bottom
1
(ii)
accept answers in the range 344 to 350
1
at different temperatures / boiling points
if the answer describes cracking ‘ no marks
(iii)
216
1
(ii)
1
(b)
C4H10
(i)
EITHER
1
shortage of petrol or demand for petrol is higher than supply
(b)
H
H
H
│
│
│
H ─C ─ C ─ C ─ H
│
│
│
H
H
H
diesel is in excess or supply of diesel is higher than demand
1
OR
1
(c)
(i)
petrol low supply and diesel high supply (1)
C5 to C8 fraction are fuels or easier to burn or petrol (fraction)
accept C21 to C24 fraction not useful as fuels
petrol high demand and diesel low demand (1)
do not accept produce more energy
1
petrol / diesel not specified = max 1
1
(ii)
C2H4
do not accept C4H8
(ii)
any one from:
•
use diesel to make petrol
1
(iii)
any three from:
accept crack diesel or description of cracking
•
use different / lighter crude oils
•
make diesel cheap(er) (than petrol)
or make petrol more expensive
•
develop markets for low demand fractions
accept lobby the government to reduce the tax
on diesel / increase tax on petrol
•
develop new techniques / equipment to use low demand fractions
as fuels
•
mix ethanol with petrol
ignore biodiesel
•
1
cracking
[6]
•
convert low demand fractions to high demand fractions or bigger
molecules to smaller molecules
•
develop alternative / bio fuels
do not accept price
3
[10]
30
M15.
(a)
hydrogen and carbon
for 1 mark
M14.
(a)
explanations
•
1
any two environmental problems with linked
(b) (i)
the oil is evaporated / boiled / liquid converted to gas /
vaporised
global warming (1)
accept effects of global warming
oil is condensed/changed back to liquid/cooled below boiling point
(not just cooled)
caused by (formation of) carbon dioxide / greenhouse gas (1)
ignore greenhouse effect
•
liquids of different boiling points condense at different levels /
fractions with lower boiling points form near the top /
boiling point linked to chain length or Mr
acid rain (1)
accept effects of acid rain
ignore respiratory problems
each for 1 mark
3
caused by ( formation of) sulfur dioxide (1)
ignore respiratory problems
(ii) Assume they mean naphtha unless they say otherwise.
smaller molecules
/contains less atoms
/lower boiling point
/more volatile
/less bonds to break
/lower activation energy
caused by ( formation of) particles / particulates / fires /
smoke / carbon / pm 10 (1)
If the answer is given the opposite way around then diesel must
be specified.
accept sulfur oxide
ignore sulfuric acid
•
global dimming (1)
any one for 1 mark
•
1
scarring of landscape (1)
(iii)
caused by mining / quarrying of coal (1)
ignore ozone layer
max 4
(b)
any three from:
•
replant the trees / renewable / sustainable
ignore reusable
•
carbon dioxide is used by the trees / photosynthesis
accept trees absorb carbon dioxide as they grow
correct number of atoms = 1
correct number of bonds (attached to correct atoms) = 1
Accept diagrams which show electrons correctly.
CH3CH2CH2CH2CH2CH2CH3 = 1
do not allow respiration
•
it’s a (continuous carbon) cycle
accept ‘carbon dioxide goes back into the air’
for 2 marks
2
accept trees use CO2 which is released when trees are
burnt
[7]
•
no ‘new’ carbon (dioxide) is produced or
no locked up carbon (dioxide) is released
accept no carbon (dioxide) from fossil fuels is produced
3
[7]
]
31
Fractional distillation answers
Answers to questions
1 Key marking points:
Molecules enter the column as a gas.
Gaseous molecules diffuse up the fractionating column.
When molecules reach a height in the column where the temperature is just below their boiling point
they condense into a liquid.
They then fall back onto the column plate below the position they condensed and flow out of the
column.
Lower in the column, molecules in the diesel fraction would still be a gas and diffuse upwards.
Molecules cannot continue to move up the column once they have condensed.
2 The molecules get larger (longer chains) as you go down the fractionating column.
3 The molecules become separated from each other; they move far apart.
(It is important that students recognise that the molecules themselves are not broken up.)
4 Low boiling point.
Small molecules have less mass and need less energy to vaporise.
As the molecules are small there a few forces of attraction between them.
Small molecules get tangled less easily than large molecules.
5 Higher boiling point.
Large molecules are heavier and need more energy to vaporise.
Large molecules have many points which are attracted to other molecules. All these tiny forces of
attraction add up to a larger force.
Large molecules can get tangled.
6 The larger the molecule is the higher its boiling point (or the reverse argument).
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Answers to TEST YOURSELF
1a i ethane
ii C4H10
iii any value from –50 to –30 (oC)
b C6H14
c contain only carbon and hydrogen
d correct structure showing 3 C joined with single bonds (1) and 8 C – H
bonds (1)
1
1
1
1
1
2
2a
b
c
d
A
C
A
They are mixtures or not single compounds/ alkanes.
1
1
1
1
3a
b
c
d
oxidised
methane + oxygen → carbon dioxide + water reactants, products
limited supply of air
(land-fill gas) contains sulfur (compounds)
1
2
1
1
4a i the greater the number (of carbon atoms) the higher its boiling point
ii accept numbers in the range 344 to 350
iii 216 (ºC)
b
There is a clear and detailed scientific description of the processes that take place
during fractional distillation in fractionating column, including vaporation,
condensation and collection of fractions at different temperatures. The answer
shows almost faultless spelling, punctuation and grammar. It is coherent and in an
organised, logical sequence. It contains a range of appropriate or relevant specialist
terms used accurately.
There is a scientific description that includes condensation and collection of
fractions at different temperatures. There are some errors in spelling, punctuation
and grammar. The answer has some structure and organisation. The use of
specialist terms has been attempted, but not always accurately.
There is a brief description of evaporation or condensation and collection of
fractions. The spelling, punctuation and grammar are very weak. The answer is
poorly organised with almost no specialist terms and/or their use demonstrating a
general lack of understanding of their meaning.
1
1
1
5-6
3-4
1-2
No relevant content.
0
Examples of scientific points that may contribute to a candidate’s response:
• crude oil is heated (to above 350 ºC)
• crude oil is vaporized
• vapours/gases go into (fractionating) column
• column is hot at bottom, cool at top or correct temperature
• gradient indicated
• substances/vapours condense at their boiling points
• different substances/vapours condense at different levels in column
• low boiling points at the top of column or high boiling points at the bottom
• collected as liquids
• liquids/fractions contain a mixture of substances
• fractions have substances with a range of boiling points continuous
process
33