Student Book
CAMBRIDGE ® IGCSE CHEMISTRY Chris Sunley and Sam Goodman
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Organic chemistry is distinct from other branches of chemistry, such as inorganic and physical chemistry. It may be described as the chemistry of living processes (often referred to as biochemistry) but extends beyond that. Organic chemistry focuses almost entirely on the chemistry of covalently bonded carbon molecules. As well as life processes, it includes the chemistry of other types of compounds including plastics, petrochemicals, drugs and paint. Early chemists never imagined that complex chemicals of living processes could ever be manufactured in a laboratory, but they were wrong. Today medical drugs can be made and then their structures modified to achieve improvements in their effectiveness. An understanding of organic chemistry begins with knowledge of the structure of a carbon atom and how it can combine with other carbon atoms by forming covalent bonds. In this section you will be introduced to a few of the ‘families’ or series of organic compounds. This knowledge will provide a sound basis for further work in chemistry or biology.
STARTING POINTS 1. Where is carbon in the Periodic Table of elements? What can you work out about carbon from its position? 2. What is the atomic structure of carbon? How are its electrons arranged? 3. How does carbon form covalent bonds? Show the bonding in methane (CH4), the simplest of organic molecules? 4. You will be learning about a series of organic compounds which are hydrocarbons. What do you think a hydrocarbon is? 5. You will be learning about methane. Where can methane be found and what it is used for? 6. You will also be learning about ethanol, which belongs to a particular series of organic compounds. Do you know where you could find ethanol in everyday products?
SECTION CONTENTS a) Fuels b) Alkanes c) Alkenes d) Alcohols e) Acids f) Macromolecules g) E xam-style questions
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4
Organic chemistry
∆∆Oil rigs are used to extract hydrocarbons from the Earth.
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Fuels INTRODUCTION
The most common fuels used today are either fossil fuels or are made from fossil fuels. There are problems associated with using fossil fuels – burning them produces a number of polluting gases and releases carbon dioxide, a greenhouse gas. Nevertheless, fossil fuels are a very important source of energy.
∆∆Fig. 4.1 Crude oil contains a mixture of hydrocarbons.
KNOWLEDGE CHECK ✓✓Know that the burning of fossil fuels produces carbon dioxide, a greenhouse gas. ✓✓Know that burning some fossil fuels can also produce pollutant gases such as sulfur dioxide and nitrogen oxides. ✓✓Know that there are alternative energy sources to fossil fuels.
WHAT ARE FOSSIL FUELS? Petroleum (crude oil), natural gas (mainly methane) and coal are fossil fuels. Crude oil was formed millions of years ago from the remains of animals and plants that were pressed together under layers of rock. It is usually found deep underground, trapped between layers of rock that it can’t seep through (impermeable rock). Natural gas is often trapped in pockets above crude oil.
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Organic chemistry
LEARNING OBJECTIVES ✓✓Know the fuels coal, natural gas and petroleum (crude oil). ✓✓Know that methane is the main constituent of natural gas. ✓✓Be able to describe petroleum as a mixture of hydrocarbons and its separation into useful fractions by fractional distillation. ✓✓Be able to describe the properties of molecules within a fraction. ✓✓Know the uses of the fractions obtained from petroleum.
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The supply of fossil fuels is limited – having taken millions of years to form, these fuels will eventually run out. They are called finite or non-renewable fuels. This makes them an extremely valuable resource that must be used efficiently. Fossil fuels contain many useful chemicals (known as fractions) and these must be separated so that they are not wasted. FRACTIONAL DISTILLATION The chemicals in petroleum are separated into useful fractions by a process known as fractional distillation.
∆∆Fig. 4.2 Fractional distillation takes place in oil refineries, like this one in the Netherlands.
number of carbon atoms in each fraction
65 °C
gases bubble up liquids trickle down
170 °C
250 °C preheated crude oil
340 °C 340 °C 500 °C
refinery gas (butane, propane for bottled gas)
1–4
gasoline (petrol)
5–8
naphtha (to make chemicals)
9–12
kerosene (paraffin, fuel)
10–14
gas oil (diesel oil)
14–20
lubricants, waxes and polishes
over 20
GAS
POLISH
bubble cap
kept hot (over 500 °C)
vapour
fuel oil for ships and home heating, bitumen for making roads
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Fuels
∆∆Fig. 4.3 A fractionating column converts crude oil into many useful fractions.
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The crude oil is heated in a furnace and passed into the bottom of a fractionating column. It gives off a mixture of vapours that rise up the column, and the different fractions condense out at different heights. The fractions that come off near the top are light-coloured, runny liquids. Those removed near the bottom of the column are dark and sticky. Thick liquids that are not runny, such as these at the bottom of the fractionating column, are described as viscous. How does fractional distillation work? The components of petroleum separate because they have different boiling points. A simple particle model explains why their boiling points differ. Petroleum is a mixture of hydrocarbon molecules, which contain only carbon and hydrogen. The molecules are chemically bonded in similar ways with strong covalent bonds but contain different numbers of carbon atoms. H
heptane
H
octane
H
H
H
H
H
H
H
C
C
C
C
C
C
C
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
C
C
C
C
C
C
C
C
H
H
H
H
H
H
H
H
H
H
∆∆Fig. 4.4 Octane has one more carbon atom and two more hydrogen atoms than heptane. Their formulae differ by CH2.
REMEMBER
The longer the molecule, the stronger the attractive force between the molecules. The weak attractive forces between the molecules must be broken for the hydrocarbon to boil. The longer a hydrocarbon molecule is, the stronger the intermolecular forces between the molecules. The stronger these forces of attraction, the higher the boiling point because more energy is needed to overcome the forces.
C 4 H10 strength of intermolecular forces boiling point
C 5 H12
C 6 H14
C 7 H16
C 8 H18
increases increases
∆∆Fig. 4.5 How the properties of hydrocarbons change as molecules get longer.
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Organic chemistry
size of molecule
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The smaller-molecule hydrocarbons are more volatile – they form a vapour easily. For example, we can smell petrol (with molecules containing between 5 and 10 carbon atoms) much more easily than we can smell engine oil (with molecules containing between 14 and 20 carbon atoms) because petrol is more volatile. Another difference between the fractions is how easily they burn and how smoky their flames are. size
small
large
ease of burning
ignite easily
difficult to light
smokiness of flame
blue flame almost no smoke
blue
yellow/grey yellow smoky flame
∆∆Fig. 4.6 How different hydrocarbons burn.
QUESTIONS 1. Petroleum is a ‘non-renewable’ fuel. What does this mean? 2. When drilling for oil, there is often excess gas to be burned off. What is this gas? Where does it come from?
3. One of the oil fractions obtained from the fractional distillation of crude oil is light-coloured and runny.
Is this fraction more likely to have a small chain of carbon atoms or a long chain?
4. Another of the oil fractions obtained from the fractional distillation of petroleum burns with a very sooty yellow flame.
Is this fraction more likely to have a small chain of carbon atoms or a long chain?
5. Some fractions obtained from petroleum are very ‘volatile’.
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Fuels
What does this mean?
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CRACKING THE OIL FRACTIONS The composition of petroleum varies in different parts of the world. Table 4.1 shows the composition of a sample of petroleum from the Middle East after fractional distillation. Fraction (in order of increasing boiling point) Liquefied petroleum gases (LPG) Gasoline Naphtha Kerosene Diesel Heavy oils and bitumen
Typical percentage produced by fractional distillation 3 13 9 12 14 49
∆∆Table 4.1 Oil fractions.
Small molecules are much more useful than the larger molecules. Larger molecules can be broken down into smaller ones by catalytic cracking. This requires a high temperature of between 600 to 700 °C and a catalyst of silica or alumina.
C 10 H 22 (g) decane
C 4 H10(g) butane
+
2C3H6(g) propene
∆∆Fig. 4.7 The decane molecule (C10H22) is converted into the smaller molecules butane (C4H10) and propene (C3H6) in cracking.
The butane and propene formed in this example have different types of structures. REMEMBER
Alkenes are much more reactive (and hence useful) than hydrocarbons like decane (an alkane).
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Organic chemistry
Propene belongs to a family of hydrocarbons called alkenes.
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Developing Investigative Skills A group of students set up an experiment to see if they could ‘crack’ some liquid paraffin. They soaked some mineral wool in liquid paraffin and assembled the apparatus as shown in Fig. 4.8. They then heated the pottery pieces very strongly, occasionally letting the flame heat the mineral wool. Bubbles of gas started to collect in the test tube. After a few minutes they had collected three test tubes full of gas and so they stopped heating. Almost immediately, water from the trough started to travel back up the delivery tube towards the boiling tube. test tube to collect gases mineral wool soaked in liquid paraffin
boiling tube pottery fragments
∆∆Fig. 4.8 Incorrectly set up apparatus for experiment.
Evaluating methods ➊➊The gas or gases produced in this reaction can be collected by
displacement of water. What property of gas(es) does this demonstrate? ➋➋Why did the water start to travel back up the delivery tube when heating was stopped?
Using and organising techniques, apparatus and materials ➌➌What are the hazards involved in this experiment? What safety precautions would minimise them? ➍➍The first test tube of gas collected did not burn, but the second one did. Explain this difference. ➎➎The third test tube of gas decolourised bromine water. What does this suggest about the gas present?
Interpreting observations and data ➏➏One of the students suggested that one of the two products was ethene
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Fuels
(C2H4). Assuming that liquid paraffin has the formula C14H30, write an equation for the cracking of the liquid paraffin used in this experiment.
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QUESTIONS 1. The cracking of hydrocarbons often produces ethene. To which homologous series does ethene belong?
2. Why is cracking needed in addition to the fractional distillation of crude oil?
3. What conditions are needed for the cracking of oil fractions?
● SCIENCE IN CONTEXT
THE FOSSIL FUEL DILEMMA
There is widespread agreement that supplies of the non-renewable fossil fuels – oil, gas and coal – will eventually run out. However, it is not easy to estimate exactly when they will run out. Many different factors need to be considered, including how much of each deposit is left in the Earth, how fast we are using each fossil fuel at the moment, whether or not countries that have supplies will sell to those that don’t, and how this is likely to change in the future. If we start switching to alternative fuel sources that are renewable, the reserves that we currently have will last longer. Current estimates suggest that crude oil (petroleum) will run out between 2025 and 2070. The estimate for natural gas is similar, with 2060 a possible date.
∆∆Fig. 4.9 A coal-fired power station.
So should we increase our efforts to develop renewable forms of energy such as wind and solar energy; should we put greater emphasis on nuclear power; or should we plan to make much greater use of coal? Perhaps we should do all three? Solving this dilemma is likely to depend as much on political decisions as scientific ones. What would you recommend?
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The situation with coal is very different. Most coal deposits have not yet been tapped, and the decline of the coal mining industry in countries such as the UK means that many coal seams are lying undisturbed. If we carry on using coal at the same rate as we do today, there could be enough to last well over a thousand years. However, as other fossil fuels run out, particularly oil, the use of coal may increase, reducing that timespan considerably.
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End of topic checklist Key terms alkene, catalytic cracking, fraction, fractional distillation, fossil fuel, hydrocarbon, non-renewable, viscous, volatile
During your study of this topic you should have learned:
❍❍About the fuels coal, natural gas and petroleum (crude oil). ❍❍That methane is the main constituent of natural gas. ❍❍How to describe petroleum as a mixture of hydrocarbons and its separation into useful fractions by fractional distillation.
❍❍How to describe the properties of molecules within a fraction. ❍❍About the uses of the following fractions obtained from petroleum: refinery gas for bottled gas for heating and cooking
●●
gasoline fraction for fuel (petrol) in cars
●●
naphtha fraction for making chemicals
●●
kerosene/paraffin fraction for jet fuel
●●
diesel/gas oil for fuel in diesel engines
●●
fuel oil fraction for fuel for ships and home heating systems
●●
lubricating fraction for lubricants, waxes and polishes
●●
bitumen for making roads.
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Fuels
●●
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End of topic questions Note: The marks awarded for these questions indicate the level of detail required in the answers. In the examination, the number of marks awarded to questions like these may be different.
1. a) How was crude oil (petroleum) formed?
(2 marks)
b) Why is crude oil called a ‘non-renewable’ fuel?
(1 mark)
2. The diagram shows a column used to separate the components present in petroleum. gases
petrol
naphtha level X kerosene
diesel oil petroleum vapours
heater
lubricating oil
liquids
a) Name the process used to separate petroleum into fractions.
fuel oil and bitumen
(1 mark)
b) What happens to the boiling point of the mixture as it goes up the column? (1 mark) c) The mixture of vapours arrives at level X. What now happens to the various (2 marks) parts of the mixture? 3. The cracking of decane molecules is shown by the equation C10H22 → Y + C2H4. a) Decane is a hydrocarbon. What is a hydrocarbon?
(2 marks)
c) Write down the molecular formula for hydrocarbon Y.
(1 mark)
d) What ‘family’ does hydrocarbon Y belong to?
(1 mark)
e) Why is the cracking of petroleum fractions so important?
(2 marks)
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Organic chemistry
b) What reaction conditions are needed for cracking?
(1 mark)
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4. Petrol is a hydrocarbon with a formula of C8H18. a) What are the products formed when petrol burns in a plentiful supply of air?
(2 marks)
b) Write a balanced equation, including state symbols, for the reaction when (2 marks) petrol burns in a plentiful supply of air.
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Fuels
c) When petrol is burned in a car engine, carbon monoxide may be formed. (2 marks) Explain why carbon monoxide is dangerous.
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