Chemistry IGCSE Topics 8 and 9

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Crude Oil

a) Crude oil is a mixture of a very large number of compounds. 


b) Most of the compounds in crude oil are hydrocarbons, which are molecules made up of hydrogen and carbon atoms only.


c) The many hydrocarbons in crude oil may be separated into fractions, each of which contains molecules with a similar number of carbon atoms, by evaporating the oil and allowing it to condense at a number of different temperatures.  This process is called fractional distillation.

Candidates should know and understand the main processes in continuous fractional distillation in a fractionating column. Knowledge of the names of specific fractions or fuels is not required.  

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a) Most of the hydrocarbons in crude oil are saturated hydrocarbons (all the carbon–carbon bonds are single covalent bonds) called alkanes.  The general formula for the homologous series of alkanes is CnH2n+2.

 b) Alkane molecules can be represented in the following forms: C2H6  or   displayed structure

Candidates should know that in displayed structures a — represents a covalent bond. Candidates should be able to recognise alkanes from their formulae in any of the forms.

c) Some properties of hydrocarbons depend on the size of their molecules.  These properties influence how hydrocarbons are used as fuels. Knowledge of trends in properties of hydrocarbons is limited to:

·  boiling points

·  viscosity

·  flammability.

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Fuels 1

a) Most fuels, including coal, contain carbon and/or hydrogen and may also contain some sulfur.  The gases released into the atmosphere when a fuel burns may include carbon dioxide, water (vapour), carbon monoxide, sulfur dioxide and oxides of nitrogen.  Solid particles (particulates) may also be released.

Candidates should be able to relate products of combustion to the elements present in compounds in the fuel and to the extent of combustion. The details of how the oxides of nitrogen are formed are the fact that they are formed at high temperatures. Solid particles may contain soot (carbon) and unburnt fuels.

b) The combustion of hydrocarbon fuels releases energy.  During combustion, the carbon and hydrogen in the fuels are oxidised. 

c) Sulfur dioxide and oxides of nitrogen cause acid rain, an increase in carbon dioxide results in climate change, and solid particles cause global dimming.

Candidates should know at least one effect of, but are not required to know details of any other causes of, acid rain or climate change.

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Fuels 2

d) Sulfur can be removed from fuels before they are burned, eg in vehicles.  Sulfur dioxide can be removed from the waste gases after combustion, in power stations.

Knowledge of the methods of removing sulfur is not required.

e) Biofuels, including biodiesel and ethanol, are produced from plant material, and are possible alternatives to hydrocarbon fuels

Candidates should know and understand the benefits and disadvantages of biofuels in terms of:

·  use of renewable resources

·  their impacts on land use

·  their carbon footprint.

Candidates should know that ethanol for use as a biofuel is produced from a dilute solution of ethanol obtained by the fermentation of plant materials at a temperature between 20 °C and 35 °C. 

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Fuels 3

f) Hydrogen can be burned as a fuel in combustion engines or can be used in fuel cells that produce electricity to power vehicles.

Candidates should be able to compare the advantages and disadvantages of the combustion of hydrogen with the use of hydrogen fuel cells from information that is provided. Candidates should know and understand the benefits and disadvantages of hydrogen fuel in terms of:

·  storage and use

·  products of combustion. 

Knowledge of the details of the reactions in fuel cells is not required.   

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Obtaining useful substances from crude oil 1

a) Hydrocarbons can be broken down (cracked) to produce smaller, more useful molecules. This process involves heating the hydrocarbons to vaporise them.  The vapours are either passed over a hot catalyst or mixed with steam and heated to a very high temperature so that thermal decomposition reactions then occur.

b) The products of cracking include alkanes and unsaturated hydrocarbons called alkenes.  The general formula for the homologous series of alkenes is CnH2n.

Candidates should know that in unsaturated hydrocarbons some of the carbon–carbon bonds are double covalent bonds.

c) Unsaturated hydrocarbon molecules can be represented in the following forms: C3H6  or   displayed structure

Candidates should know that in displayed structures an   =    represents a double bond.

Candidates should be able to recognise alkenes from their names or formulae, but do not need to know the names of individual alkenes other than ethene and propene. 

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Obtaining useful substances from crude oil 2

d) Alkenes react with bromine water, turning it from orange to colourless.


e) Some of the products of cracking are useful as fuels.


f) Ethanol can be produced by reacting ethene with steam in the presence of a catalyst.

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Polymers 1

a) Alkenes can be used to make polymers such as poly(ethene) and poly(propene). In polymerisation reactions, many small molecules (monomers) join together to form very large molecules (polymers).  For example: ethene à poly(ethene)   - structural formulae – balanced equation with “n”

Candidates should be able to recognise the molecules involved in these reactions in the forms shown in the subject content.  They should be able to represent the formation of a polymer from a given alkene monomer.

b) The properties of polymers depend on what they are made from and the conditions under which they are made. For example, low-density (LD) and high-density (HD) poly(ethene) are produced using different catalysts and reaction conditions. 

c) Thermosoftening polymers consist of individual, tangled polymer chains.  Thermosetting polymers consist of polymer chains with cross-links between them so that they do not melt when they are heated.

Candidates should be able to explain thermosoftening polymers in terms of intermolecular forces. 

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Polymers 2

d) Polymers have many useful applications and new uses are being developed. Examples include: new packaging materials, waterproof coatings for fabrics, dental polymers, wound dressings, hydrogels, and smart materials (including shape memory polymers).

Candidates should consider the ways in which new materials are being developed and used, but will not need to recall the names of specific examples.

e) Many polymers are not biodegradable, ie they are not broken down by microbes. This can lead to problems with waste disposal.

Knowledge of specific named examples is not required, but candidates should be aware of the problems that are caused in landfill sites and in litter.

f) Plastic bags are being made from polymers and cornstarch so that they break down more easily.  Biodegradable plastics made from cornstarch have been developed.

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