Download Structured questions

Structured questions
1
The following diagram shows a simplified structure of a fractionating tower in petroleum
refinery.
a)
b)
How was petroleum formed in nature?
i) Name the process that takes place in the fractionating tower.
ii) Does the process involve a physical or chemical change? Explain your answer.
c)
i)
d)
e)
Explain, in terms of molecular size, why fraction A has a lower boiling point than
fraction D.
ii) State TWO other differences between properties of fraction A and those of fraction
D.
i) Suggest what fractions A and B may be.
ii) Fractions C and D are used as fuel for jets and lorries respectively.
Suggest what fractions C and D may be.
iii) Fraction E is bitumen. Suggest ONE use of bitumen.
Draw a labelled diagram to show how the above process can be carried out in the
laboratory.
(17 marks)
2
Crude oil is a fossil fuel. The following experimental set-up is used to separate a sample of
crude oil into different fractions.
a)
b)
Why is crude oil considered as a fossil fuel?
i) Name the process shown in the above diagram.
ii) Why can crude oil be separated into different fractions by this process?
c)
i)
ii)
d)
e)
What is the function of cold water in the experimental set-up?
Why should the thermometer be placed exactly next to the opening of the delivery
tube?
Two fractions, A and B, are collected at 100℃ and 250℃ respectively.
State how these two fractions are different in
i) sootiness of flame when burnt;
ii) viscosity; and
iii) molecular size.
In the world, the demand for lower boiling fractions of crude oil is greater than its supply
while the supply of higher boiling fractions of crude oil is in excess.
i)
ii)
3
Suggest a process that can produce lower boiling fractions from higher boiling
fractions.
Apart from lower boiling fractions, name another type of compounds that is
produced in the process mentioned in (i). Suggest TWO uses of that type of
compounds.
(12 marks)
A student tried to break down (crack) a sample of octane C8H18 with the set-up shown below.
a)
i)
ii)
What may solid X be?
What is the function of solid X?
b)
Why was the mineral wool NOT heated directly?
c)
After the experiment, the student found that there was a layer of oily liquid on the surface
of the water in the beaker. Suggest what this oily liquid might be.
One of the reactions involved in the cracking of octane gives two hydrocarbons, each
containing the same number of carbon atoms.
i) Write a chemical equation for this reaction.
ii) Suggest a chemical test to distinguish between the two hydrocarbons from each
other.
After the experiment, the student wanted to remove the heat source before the delivery
tube was disconnected. However, his teacher stopped him from doing that.
i) Explain the action of the teacher.
d)
e)
f)
ii) What should the student do instead?
Suggest TWO reasons to explain why cracking is an important process.
(12 marks)
4
The following table shows the annual supply and demand for different fractions of petroleum
in a certain city in 2005:
a)
b)
Oil fraction
Annual supply (million tonnes)
Annual demand (million tonnes)
Refinery gas
5
9
Petrol
12
42
Naphtha
8
8
Kerosene
30
30
Diesel oil
22
26
Heavy oil
64
14
Explain why the annual demand for petrol was greater than that for kerosene.
Cracking is used to convert excess oil fractions to oil fractions that are insufficient.
i) (1) What is the meaning of the term “cracking”?
(2) What are the conditions for cracking?
ii)
c)
d)
Draw a labelled diagram to show the experimental set-up for cracking in the
laboratory.
One of the major components in petrol is octane C8H18.
i) What is the structural formula of octane?
ii) Is octane a saturated or an unsaturated hydrocarbon? Explain briefly.
iii) Write an equation for the complete combustion of octane.
iv) Suggest how you would show that octane contains hydrogen and carbon.
Octane reacts with chlorine gas readily under a suitable condition.
i)
What is the condition for the reaction?
ii) Name the type of reaction that occurred.
iii) Is it possible to prepare 1-chlorooctane using this reaction? Explain briefly.
(18 marks)
5
a)
Consider the following structinal formulae of six compounds.
i)
ii)
iii)
iv)
b)
What is the meaning of a homologous series?
Which of the above compounds belong to the same homologous series?
Name compounds A and B.
A mixture of bromine vapour and compound B is exposed to diffused sunlight. State
the expected observation and write a chemical equation for the reaction involved.
Propene and but-2-ene belong to the same homologous series.
i) (1) Draw an electron diagram of propene, showing the outermost shell electrons
only.
(2) Write the structural formula of but-2-ene.
Propyne is an organic compound. Its structural formula is CH3−CCH. Explain
whether propyne or propene would produce a sootier flame when burnt.
iii) Apart from combustion and reaction with halogens, describe a chemical test to
distinguish between compound B and but-2-ene. Write down an equation for the
ii)
reaction involved.
(14 marks)
6
A student wanted to investigate some properties of propene and but-1-ene.
a) i) Write the structural formulae of propene and but-1-ene.
ii) Using propene and but-1-ene as examples, illustrate TWO characteristics of the
members of a homologous series.
iii) State ONE difference between the chemical properties of propene and but-1-ene.
b)
The student then tried to determine the molecular formula of a hydrocarbon W with the
following set-up:
A known mass of W was heated in excess oxygen. The following results were obtained.
Mass of U-tube A before the experiment = 70.435 g
Mass of U-tube B before the experiment = 67.544 g
Mass of U-tube A after the experiment = 84.493 g
Mass of U-tube B after the experiment = 101.908 g
i)
What were the functions of
(1) anhydrous calcium chloride and
(2) sodium hydroxide pellets?
ii) Determine the empirical formula of W.
iii) The relative molecular mass of W is 308.0. What is the molecular formula of W?
iv) State TWO assumptions for the above experiment.
(Relative atomic masses: H = 1.0, C = 12.0, O = 16.0)
(16 marks)
7
The following equation illustrates a possible reaction in cracking process.
C11H24 → saturated hydrocarbon A + unsaturated hydrocarbon B
a)
b)
c)
d)
e)
What is the meaning of the term “cracking”?
What is the meaning of the term “unsaturated hydrocarbon”?
Each molecule of A contains 8 carbon atoms.
i) What is the molecular formula of A?
ii) Write the structural formula of B.
iii) To which homologous series does compound B belong?
iv) Suggest ONE use of B.
Which of the hydrocarbon, A or B, gives an equal number of moles of carbon dioxide and
water upon complete combustion. Explain your answer.
A is used as a fuel for motor vehicles.
i) The carbon dioxide formed when A is burnt contributes to the greenhouse effect.
Explain why carbon dioxide can cause the greenhouse effect.
ii)
8
(1) Explain why car exhaust also contains carbon monoxide and oxides of nitrogen.
(2) State ONE undesirable effect of carbon monoxide.
(3) Explain, with the help of a chemical equation, the formation of acid rain from
nitrogen dioxide.
(17 marks)
Town gas is a common domestic fuel. Town gas contains a mixture of gases.
a) Name the most abundant component of town gas.
b)
c)
Suggest a test for the gas named in (i).
What will be observed when town gas is passed over hot copper(II) oxide in the absence
of air? Explain briefly with the help of equations.
d)
If there is a leakage of town gas in your home, explain why
i) you have to open all the windows and doors immediately;
ii) you should NOT switch off the electrical appliances.
Explain why an odorant is added to town gas.
Kerosene was used as a domestic fuel in a few decades ago but it is almost completely
replaced by LPG or town gas nowadays. State ONE disadvantage associated with the use
of kerosene.
e)
f)
(10 marks)
9
a)
b)
c)
Butane is one of the major components of liquefied petroleum gas (LPG). LPG is
obtained from petroleum.
i) How can LPG be obtained from petroleum?
ii) Suggest another way from which LPG can be obtained.
iii) Write the structural formula of butane.
iv) Write an equation of the complete combustion of butane.
v) Suggest a chemical test for ONE of the products in the complete combustion of
butane.
vi) Under room conditions, butane exists as a gas. State a condition under which butane
can be liquefied at room temperature.
Nowadays, many taxis switch from using diesel to using LPG. Suggest a reason for such
change.
LPG is also a common domestic fuel.
10
i)
State TWO safety precautions associated with the use of LPG as a domestic fuel.
ii)
State ONE potential hazard associated with the use of LPG as a domestic fuel.
(10 marks)
The tables below show the melting points and boiling points of the first four members of
alkanes and the first three members of alkenes.
Melting point (℃)
Boiling point (℃)
Alkane
Methane
-182
-162
Ethane
-183
-89
Propane
-190
-42
Butane
-138
-0.5
Alkene
Melting point (℃)
Boiling point (℃)
Ethene
-169
-104
Propene
-185
-47
But-1-ene
-185
-6
a)
b)
c)
d)
e)
f)
Answer:
Both alkanes and alkenes are hydrocarbons. What is the meaning of the term
“hydrocarbon”?
Which of the above hydrocarbons exist(s) as a liquid / liquids at -100℃ ?
i) Write the general formula of alkanes.
ii) State and explain the trend of the boiling points of alkanes.
iii) State TWO other physical properties of alkanes which show the same trend as
boiling points.
Natural gas is a mixture of hydrocarbons.
i) What is the origin of natural gas?
ii) State the major component of natural gas.
iii) Write an equation for the complete combustion of the major component of natural
gas.
Explain whether ethene and propene have similar chemical properties.
i) Both ethane and ethene can react with halogens. However, the conditions for the
reactions are different. State the difference.
ii) Suggest ONE reaction in which both ethane and ethene behave similarly. Write a
chemical equation for the reaction involving ethane or ethene.
(15 marks)
10
11
Installation of catalytic converters in car exhaust systems can reduce the emission of unburnt
hydrocarbon (such as C8H18) and two other air pollutants. The following diagram shows the
internal structure of a catalytic converter.
a)
Name a catalyst used in a catalytic converter.
b)
i)
c)
ii)
i)
d)
ii)
i)
e)
f)
12
Suggest how the emission of unburnt hydrocarbons (such as C8H18) can be reduced
by the installation of catalytic converters.
Write a chemical equation for the reaction involved.
Name TWO other air pollutants the emission of which a catalytic converter can
reduce.
Write chemical equations for the reactions involved.
Suggest ONE reason why unleaded petrol should be used in a motor vehicle fitted
with a catalytic converter.
ii) State another advantage of using unleaded petrol in motor cars.
Even if all the above air pollutants are removed, the car exhaust still causes
environmental problems. Explain why.
Apart from installing catalytic converters, suggest ONE method to reduce air pollution
caused by motor cars.
(12 marks)
The illustration below shows the exhaust from a motor car using unleaded petrol:
a)
b)
c)
i) Explain why the exhaust contains carbon monoxide.
ii) State ONE health hazard associated with carbon monoxide.
Explain why the exhaust contains oxides of nitrogen.
i) State ONE health hazard associated with particulates.
ii)
d)
e)
f)
A student wants to show the presence of particulates in car exhaust. Suggest a
method for him.
i) Suggest ONE other pollutant that may be found in the exhaust.
ii) State ONE health hazard associated with the pollutant suggested in (i).
Suggest ONE reason why car exhaust pipes rust rapidly.
Fuel cells have been developed as an alternative to petrol for cars.
i)
Complete and balance the following ionic half-equations for the processes occurring
at electrodes of a fuel cell.
H2(g) → H+(aq)
O2(g) + H+(aq) → H2O(l)
ii) Give the overall cell reaction.
iii) What is the direction of electron flow in the external circuit?
iv) State ONE advantage, and ONE disadvantage of using fuel cells to power cars
compared to petrol.
(15 marks)
13
Burning of fuels in factories and motor cars produces many air pollutants.
a) Sulphur dioxide is usually found in exhaust fumes from factories.
i) Explain, with the help of an equation, why sulphur dioxide is produced in factories.
ii)
b)
Explain how sulphur dioxide affects
(1) our health;
(2) the growth of plants.
iii) In Hong Kong, does the car exhaust contain a large amount of sulphur dioxide?
Explain briefly.
iv) Suggest a chemical test for sulphur dioxide.
Car exhaust usually contains pollutants, such as nitrogen monoxide, nitrogen dioxide,
unburnt hydrocarbons, etc.
i)
Explain, with the help of equations, how nitrogen monoxide and nitrogen dioxide
are formed in car engines.
ii) State ONE environmental problem associated with the emission of oxides of
nitrogen into the atmosphere.
iii) Name a pollutant which is formed from the reaction between oxides of nitrogen and
unburnt hydrocarbons. What is the effect of this pollutant on our health?
iv) State another pollutant that may be found in the car exhaust.
(16 marks)
14
The following bar chart shows the amount of some air pollutants in a certain city in 2005.
a)
b)
c)
i) Which air pollutant was produced in the greatest amount by motor vehicles?
ii) Explain why this pollutant can be found in exhaust of motor vehicles.
iii) State ONE harmful effect of this pollutant.
iv) Suggest a method to reduce the amount of this pollutant.
Emission from power stations contains a large amount of particulates.
i) State ONE health problem associated with particulates.
ii) State ONE way to remove particulates from the emission.
i) What can be deduced from the fact that less amount of sulphur dioxide was emitted
from power stations than from industries?
ii) Sulphur dioxide dissolves in rainwater to give acid rain.
(1) Write an equation for the formation of acid rain from sulphur dioxide.
(2) State TWO undesirable effects of acid rain.
iii) Suggest ONE measure to reduce the emission of sulphur dioxide.
(11 marks)
15
The following table shows the monthly mean pH values of rainwater collected from four
different districts (P, Q, R and S) in city X.
a)
District
Monthly mean pH value of rainwater
P
5.1
Q
4.6
R
3.9
S
5.2
b)
The normal pH value of rainwater is about 5.6. Explain, with the help of an equation,
why rainwater is slightly acidic.
When rainwater has a pH value lower than 5.0, it is considered as acid rain.
c)
d)
i) Name TWO pollutants that contribute to the formation of acid rain.
ii) Write equations for the formation of acid rain from the two pollutants named in (i).
Which district (P, Q, R or S) is most likely to be an industrial area? Explain your answer.
The problem of acid rain becomes more and more serious in city X in recent years.
e)
i) Suggest TWO possible reasons for this phenomenon.
ii) Suggest TWO methods to reduce the problem of acid rain.
It is found that the pH value of rainwater is lower in thunderstorm. Explain the
phenomenon briefly.
(14 marks)
16
A student tried to investigate the pH value of rainwater collected in an area with an incinerator
and a lot of factories.
a) Could he use methyl orange to measure the pH value of rainwater? Explain briefly.
b) Name a device he can use to measure the pH value of rainwater accurately.
c) He found that the pH value of rainwater was about 4.5.
i) Name TWO pollutants from factories that were responsible for the acidity of
rainwater. Explain why the pollutants were emitted from factories.
ii) Write an equation to show the formation of acid rain from ONE of the pollutants
named in (i).
iii) The rainwater led to faster corrosion of buildings made of marble.
(1) Name the major component of marble.
(2) Explain, with the help of an ionic equation, why the buildings corroded much
faster than normal.
iv) When 1,1,1-trichloroethane was burnt in an incinerator, several pollutants were
produced.
(1) Apart from the pollutants suggested in (c)(i), name ONE pollutant which was
also emitted from the incinerator.
(2) State ONE harmful effect of this pollutant.
(13 marks)
17
The following bar chart shows the estimated time when fossil fuels are used up.
a)
b)
Fossil fuel A is petroleum.
i) Why is petroleum known as a “fossil fuel”?
ii) Identify fossil fuels B and C.
iii) State TWO reasons why petroleum is more commonly used in industries than fossil
fuel C.
iv) Why is it important to conserve fossil fuels?
Nowadays, many countries have been developing alternative energy sources instead of
using fossil fuels.
i)
ii)
18
Apart from conserving fossil fuels, suggest ONE reason why it is necessary to
develop alternative energy sources.
There are several alternative sources of energy. For each of the following sources of
energy, state ONE advantage and ONE disadvantage of using each of them as an
alternative to fossil fuels.
(1) Solar energy
(2) Geothermal energy
(3) Hydroelectric power
(13 marks)
Fossil fuels are the world’s major sources of energy. In burning of fossil fuels, a large amount
of carbon dioxide is released into the atmosphere.
a) Suggest a natural process by which carbon dioxide in the atmosphere is used up.
b) Explain why carbon dioxide is important to living things on the Earth.
c) Suggest TWO consequences resulted from the excess emission of carbon dioxide.
d)
Fossil fuels are considered as non-renewable energy sources. Explain the meaning of the
term “non-renewable energy sources”?
e) Alternative energy sources such as nuclear energy become more and more important in
many countries.
i) Is nuclear power a renewable energy source?
ii) State ONE advantage and ONE disadvantage of using nuclear power as an
alternative to fossil fuels.
f) In some farms, animal wastes (manure) are collected and allowed to decompose in the
absence of air. A gas X is produced which can also be used as an alternative to fossil fuels.
i) Name gas X.
ii) Write an equation for the complete combustion of gas X.
iii) State ONE disadvantage of using animal wastes as fuel.
iv) State ONE environmental problem associated with the emission of gas X into the
atmosphere.
v) Suggest ONE reason why animal wastes are not yet widely used as an energy source.
(13 marks)
19
The following flow chart shows the industrial production of ethanol.
a)
b)
c)
d)
i)
ii)
i)
ii)
Name Process 1.
At which part of a petroleum refinery does Process 1 take place?
Name Process 2.
Draw a labelled diagram to show the experimental set-up for carrying out Process 2
in the laboratory.
i) What is compound B?
ii) A component of the heavy fraction of petroleum has a molecular formula C12H26. In
one of the reactions in Process 2, only a hydrocarbon and compound B are formed
from this component. Write a chemical equation for the reaction involved.
Catalytic hydration is employed to convert compound B into ethanol.
i) Write a chemical equation for the reaction involved.
ii) The ethanol produced is contaminated with a trace amount of water. Suggest a way
to obtain pure ethanol.
e)
20
iii) Can the ethanol produced in this way be used to make wine? Explain briefly.
In some agricultural countries, ethanol can also be produced by another process.
Moreover, “gasohol” (a mixture of petrol and ethanol) is often used as a fuel for motor
cars in such countries.
i) What is the process for producing ethanol in these countries?
ii) Burning gasohol causes less pollution than burning petrol. Explain why.
iii) Suggest ONE advantage and ONE disadvantage of using gasohol as fuel for motor
cars in such countries.
(16 marks)
A student tried to produce ethanol with the following set-up.
a)
b)
Name the process for the production of ethanol shown in the diagram.
What was mixture X?
c)
d)
Write a chemical equation for the reaction involved in the conical flask.
State any observable change(s) in the limewater after a few days. Explain briefly with the
help of an equation.
The student found that the concentration of ethanol produced remained at 9% by volume
after a week.
i) Explain this phenomenon.
ii) How can he obtain a higher concentration of ethanol from the mixture in the conical
flask?
After the process in (e)(ii), the student added the concentrated ethanol solution to
e)
f)
g)
acidified potassium dichromate solution.
i) Name an acid used to acidify the potassium dichromate solution.
ii) What would be observed when the concentrated ethanol solution was mixed with
acidified potassium dichromate solution?
iii) Write an equation for the reaction involved.
Suggest ONE reason for each of the following statements:
i) Drinking a small quantity of wine may be good for health.
ii) Excessive drinking of alcoholic beverages may cause health problems.
h)
Apart from making alcoholic beverages, state ONE use of ethanol in daily life.
(16 marks)
21
An alcoholic drink can be produced by the fermentation of apple. After fermentation, the
resulting solution contains 10% of ethanol.
a) i) Describe how the alcoholic drink can be produced by fermentation of apple in the
laboratory.
ii) Draw a labelled diagram to show how fermentation can be carried out in the
laboratory.
b)
c)
d)
e)
f)
22
Apart from ethanol, state ONE product which is also produced in fermentation.
i) Suggest a process which can increase the concentration of ethanol in the solution
obtained.
ii) Draw a labelled diagram of the laboratory set-up for the process suggested in (i).
Acidified potassium dichromate solution can be used to test for the presence of ethanol in
the breath of a drunk driver. Explain the principle behind.
Explain why alcoholic drink becomes sour after it is exposed to air for several days.
State ONE advantage and ONE social problem associated with alcoholic drinking.
(14 marks)
In some countries, a mixture of petrol and ethanol is used as fuel for cars. Ethanol is
manufactured in two ways.
a) Explain why burning a mixture of petrol and ethanol causes less air pollution than
burning petrol.
b) Ethanol can be manufactured from sugars, such as glucose. The manufacturing process
can be carried out in the set-up shown below.
i) Write a chemical equation for the reaction occurred in the flask.
ii) Suggest what X may be. State its function in the production of ethanol.
iii) Why is it essential that there is no oxygen in the flask?
iv) After a few days the reaction stops. Suggest a reason why the reaction stops after a
c)
d)
23
a)
few days.
v) The final mixture in the flask contains about 15% of ethanol by volume. What
technique can be used to concentrate the aqueous ethanol?
Ethanol can also be manufactured from a petroleum product.
i) Name the process for the manufacture.
ii) Write an equation for the reaction involved.
A country is densely populated. It is relatively rich and has its own petroleum reserves.
Suggest, with reasons, which method of ethanol production is likely to be used in this
country.
(11 marks)
A student used the following set-up to prepare an alkanoic acid Y from a compound X of
molecular formula C3H8O.
i)
ii)
b)
Name apparatus A.
Why was a water bath, instead of a naked flame, used for heating.
iii) Explain why some pumice stones were added to the reaction mixture before heating.
iv) Name the alkanoic acid Y produced in the reaction.
v) Write a chemical equation for the reaction involved.
vi) What was the function of acidified potassium dichromate solution in the experiment?
vii) Draw an experimental set-up for obtaining the alkanoic acid produced from the
reaction mixture.
An ester Z was prepared by heating ethanol and alkanoic acid Y under reflux in the
presence of concentrated sulphuric acid.
i)
Name ester Z.
ii) What was the function of concentrated sulphuric acid in the preparation?
iii) Suggest ONE reason why it was necessary to heat the mixture under reflux.
(12 marks)
24
Consider the following flow diagram.
a)
X is an alkene.
i) Give the systematic name of X.
ii) Suggest a process from which X is obtained industrially.
iii) Name Process 1. Write an equation for the reaction involved.
iv) X reacts with bromine dissolved in an organic solvent readily. State the expected
observation and write a chemical equation for the reaction involved.
b)
i) Suggest a reagent that can be used to convert ethanol into ethanoic acid.
ii) What is the type of reaction involved in Process 2? Explain your answer briefly.
When ethanol is heated with ethanoic acid under reflux in the presence of a catalyst Y,
ester W is formed.
i) Name Y.
ii) State TWO characteristics of a catalyst.
iii) Give the structural formula of W.
iv) Name the type of the reaction involved.
v) Draw a labelled diagram of the experimental set-up for the reaction.
c)
(16 marks)
25
A student tried to prepare methyl propanoate in the following experiment:
A mixture of reagents A and B was heated with a small amount of concentrated sulphuric acid
in a test tube for about 10 minutes. A water bath was used for heating. The resultant mixture
was then added to a beaker of sodium carbonate solution.
a)
Name reagents A and B.
b)
c)
d)
e)
What was the function of concentrated sulphuric acid in the experiment?
Write a chemical equation for the reaction involved.
Give the structural formula of methyl propanoate.
Why was a water bath, instead of a naked flame, used for heating the test tube and its
contents?
i) State TWO observable changes when the contents of the test tube were added to the
sodium carbonate solution.
ii) Suggest a way to obtain methyl propanoate from the resulting solution.
Which one of the following hazard warning labels should be displayed on a bottle of
methyl propanoate?
f)
g)
h)
26
Draw the structure of another ester which has the same molecular formula as methyl
propanoate, and give its systematic name.
(12 marks)
A student tried to prepare an ester by mixing 3.68 g of ethanol and 3.00 g of ethanoic acid with
the following set-up.
a)
b)
The student found that no ester was formed after the reaction mixture was heated under
reflux for an hour. Suggest how he could make the experiment feasible.
After the student made the modification, he could produce an ester successfully.
i) Write an equation for the production of ester in the above experiment.
ii) Give the systematic name of the ester obtained in this experiment.
iii) Name the apparatus labelled X. State its function.
c)
d)
e)
iv) What was the function of the anti-pumping granules?
The student found that only 1.80 g of ester were obtained.
i) What mass of ester would be obtained if the conversion was 100%?
ii) What was the percentage yield of the reaction?
iii) Suggest why the conversion was not 100%.
Suggest a hazard warning label that should be displayed on a bottle of the ester.
Ester is a common solvent for molecular substances but not ionic compounds.
i) Explain why esters cannot dissolve ionic compounds.
ii) State another common use of esters in our daily lives.
(Relative atomic masses: H = 1.0; C = 12.0; O = 16.0)
(16 marks)
27
Ethyl ethanoate (boiling point 77℃) can be prepared in the following way:
Stage 1
Stage 2
Stage 3
Ethanoic acid and ethanol are heated under reflux for about 40 minutes in the
presence of a little concentrated sulphuric acid.
The reaction mixture is then distilled. Everything that boils below 85℃ is collected.
The distillate is shaken with 30% sodium carbonate solution.
Stage 4
Stage 5
The organic product is treated with anhydrous calcium chloride.
The organic product is redistilled. The liquid that boils between 76℃ and 80℃ is
collected.
a) Suggest ONE reason why the reaction mixture is heated under reflux in Stage 1.
b) What is the function of the concentrated sulphuric acid in Stage 1?
c) Why is the distillate shaken with 30% sodium carbonate solution in Stage 3?
d) Why is the organic product treated with anhydrous calcium chloride in Stage 4?
e) Ethyl ethanoate is commonly used as a solvent. Explain why ethyl ethanoate can dissolve
iodine but not sodium iodide.
f)
28
Draw the structure of another ester which has the same molecular formula as ethyl
ethanoate, and give its systematic name.
(9 marks)
The following table shows some chemical properties of ethanol.
Experiment
1 Ethanol is heated with acidified
Result
The colour of the solution changes from orange to
potassium dichromate solution
green.
Compound A is formed which turns blue litmus paper
red.
2 Burning ethanol in a sufficient
supply of air.
A blue flame is observed.
3 Ethanol and compound B are
mixed and the reaction mixture
is …..
Compound C with five carbon atoms in each molecule
(with pleasant smell) is formed.
a)
b)
c)
29
Consider Experiment 1.
i) What is A? Write the structural forumla of A.
ii) Write an equation for the reaction involved.
iii) Explain, in terms of oxidation number, whether potassium dichromate is reduced or
oxidized.
iv) What will be observed when A is added to sodium carbonate solution? Write a
chemical equation for the reaction involved.
i) Is the combustion of ethanol an exothermic or an endothermic reaction?
ii) Write an equation for the complete combustion of ethanol.
iii) Suggest a chemical test for each of the products formed in the complete combustion
of ethanol.
iv) Explain why, under the same conditions, ethanol usually burns with a blue flame
while petrol does not.
i) Part of the procedure of Experiment 3 is missed. Complete the missing part.
ii) Name compound B.
iii) What type of compound is C? Give the systematic name of C.
(17 marks)
W, X, Y and Z were four unlabelled bottles, each containing one of the following chemicals:
butanoic acid, pent-1-ene, ethyl ethanoate and butan-1-ol.
A student tried to identify them with the following chemical tests.
Test
Result
1 Add bromine in 1,1,1-trichloroethane
to the chemicals.
Bromine was decolourized by the chemical in
X only.
2 Add sodium hydrogencarbonate
solution to the chemicals.
Effervescence occurred with the chemical in Z
only.
3 Add acidified potassium dichromate
solution to the chemicals.
The chemical in W turned acidified potassium
dichromate solution from orange to green.
There was no observable change for other
chemicals.
a)
Identify W, X and Y.
b)
c)
d)
Name the type of reaction in Test 1. Write an equation for the reaction involved.
Write a chemical equation for the reaction in Test 2.
Consider Test 3.
i) Explain, with the help of an ionic half-equation, why the acidified potassium
dichromate solution changed from orange to green.
ii) Explain whether acidified potassium permanganate solution can be used instead of
acidified potassium dichromate solution to distinguish W from other chemicals.
Apart from Test 2, suggest another chemical test for Z. Write an equation for the reaction
involved.
Explain, in general, why W, X, Y and Z have different chemical properties.
e)
f)
(13 marks)
30
Esters are compounds which provide the flavour of many fruits and the perfumes of many
flowers.
The ester A CH3(CH2)2COOCH3 contributes to the aroma of apples. A mixture of this ester
and dilute sodium hydroxide solution was allowed to react by heating under reflux.
a)
Give the systematic name of A.
b)
Name the process occurred when ester A was heated with dilute sodium hydroxide
solution under reflux.
i) Write a chemical equation for the process.
ii) Give the systematic name of the products.
c)
d)
e)
f)
Draw a labelled diagram for the set-up you would use for refluxing ester A with dilute
sodium hydroxide solution in the laboratory.
What is the function of the dilute sodium hydroxide solution in the process?
After reflux, the resulting mixture was transferred to the round-bottomed flask Y and
heated as shown below:
g)
h)
31
i)
What is the function of the piece of apparatus labelled X?
ii)
Identify the distillate collected in flask Z.
Suggest how ester A can be prepared in the laboratory.
Apart from their use as perfumes and food flavourings, state ONE major use of esters.
(13 marks)
A student prepared crystals of benzoic acid C6H5COOH by hydrolysing methyl benzoate
C6H5COOCH3 using the following method.
Step 1
Add excess sodium hydroxide solution to 2.70 g of methyl benzoate in a flask.
Step 2
Heat the mixture under reflux for 30 minutes.
Step 3
Distil the mixture and collect the first 2 cm3 of distillate.
Step 4
Pour the residue in the flask into a beaker and add dilute sulphuric acid until the
solution is acidic.
Step 5
Filter the crystals obtained.
The process can be summarized as:
a)
b)
What are the products formed in Step 2?
Name the distillate collected in Step 3.
32
c)
Why was the residue from the flask acidified in Step 4?
d)
The student obtained 1.50 g of benzoic acid.
i) Calculate the percentage yield of the benzoic acid.
ii) Suggest why the percentage yield was substantially below 100%.
(Molar mass of methyl benzoate = 136.0 g mol-1; molar mass of benzoic acid = 122.0 g
mol-1)
(9 marks)
“Cracking” is an important industrial process.
Explain why cracking is an important industrial process. Briefly describe how this process can
be carried out in a school laboratory.
(You are required to give a paragraph-length answer. In this question, 6 marks will be awarded
for chemical knowledge and 3 marks for effective communication.)
(9 marks)
33
Discuss global warming. Your essay should include the causes of and the problems associated
with global warming. Suggest TWO possible ways to reduce global warming.
(You are required to give a paragraph-length answer. In this question, 6 marks will be awarded
for chemical knowledge and 3 marks for effective communication.)
(9 marks)
34
Discuss the pros and cons of using ethanol as a fuel for motor cars.
(You are required to give a paragraph-length answer. In this question, 6 marks will be awarded
for chemical knowledge and 3 marks for effective communication.)
(9 marks)