Notes

5.3.2.4 Limiting reactants (HT only)
Content Key opportunities for
skills development
In a chemical reaction involving two reactants, it is common to use
an excess of one of the reactants to ensure that all of the other
reactant is used. The reactant that is completely used up is called
the limiting reactant because it limits the amount of products.
Students should be able to explain the effect of a limiting quantity of
a reactant on the amount of products it is possible to obtain in terms
of amounts in moles or masses in grams.
WS 4.1
5.3.2.5 Concentration of solutions
Content Key opportunities for
skills development
Many chemical reactions take place in solutions. The concentration
of a solution can be measured in mass per given volume of
solution, eg grams per dm3 (g/dm3
).
Students should be able to:
• calculate the mass of solute in a given volume of solution of
known concentration in terms of mass per given volume of
solution
• (HT only) explain how the mass of a solute and the volume of
a solution is related to the concentration of the solution.
MS 1c
Use ratios, fractions and
percentages.
MS 3b
Change the subject of an
equation.
5.4 Chemical changes
Understanding of chemical changes began when people began experimenting with chemical
reactions in a systematic way and organizing their results logically. Knowing about these different
chemical changes meant that scientists could begin to predict exactly what new substances would
be formed and use this knowledge to develop a wide range of different materials and processes. It
also helped biochemists to understand the complex reactions that take place in living organisms.
The extraction of important resources from the earth makes use of the way that some elements
and compounds react with each other and how easily they can be ‘pulled apart’.
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5.4.1 Reactivity of metals
5.4.1.1 Metal oxides
Content Key opportunities for
skills development
Metals react with oxygen to produce metal oxides. The reactions
are oxidation reactions because the metals gain oxygen.
Students should be able to explain reduction and oxidation in terms
of loss or gain of oxygen.
5.4.1.2 The reactivity series
Content Key opportunities for
skills development
When metals react with other substances the metal atoms form
positive ions. The reactivity of a metal is related to its tendency to
form positive ions. Metals can be arranged in order of their reactivity
in a reactivity series. The metals potassium, sodium, lithium,
calcium, magnesium, zinc, iron and copper can be put in order of
their reactivity from their reactions with water and dilute acids.
The non-metals hydrogen and carbon are often included in the
reactivity series.
A more reactive metal can displace a less reactive metal from a
compound.
Students should be able to:
• recall and describe the reactions, if any, of potassium,
sodium, lithium, calcium, magnesium, zinc, iron and copper
with water or dilute acids and where appropriate, to place
these metals in order of reactivity
• explain how the reactivity of metals with water or dilute acids
is related to the tendency of the metal to form its positive ion
• deduce an order of reactivity of metals based on experimental
results.
The reactions of metals with water and acids are limited to room
temperature and do not include reactions with steam.
AT 6
Mixing of reagents to
explore chemical changes
and/or products.
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5.4.1.3 Extraction of metals and reduction
Content Key opportunities for
skills development
Unreactive metals such as gold are found in the Earth as the metal
itself but most metals are found as compounds that require
chemical reactions to extract the metal.
Metals less reactive than carbon can be extracted from their oxides
by reduction with carbon.
Reduction involves the loss of oxygen.
Knowledge and understanding are limited to the reduction of oxides
using carbon.
Knowledge of the details of processes used in the extraction of
metals is not required.
Students should be able to:
• interpret or evaluate specific metal extraction processes when
given appropriate information
• identify the substances which are oxidised or reduced in
terms of gain or loss of oxygen.
5.4.1.4 Oxidation and reduction in terms of electrons (HT only)
Content Key opportunities for
skills development
Oxidation is the loss of electrons and reduction is the gain of
electrons.
Student should be able to:
• write ionic equations for displacement reactions
• identify in a given reaction, symbol equation or half equation
which species are oxidised and which are reduced.
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5.4.2 Reactions of acids
5.4.2.1 Reactions of acids with metals
Content Key opportunities for
skills development
Acids react with some metals to produce salts and hydrogen.
(HT only) Students should be able to:
• explain in terms of gain or loss of electrons, that these are
redox reactions
• identify which species are oxidised and which are reduced in
given chemical equations.
Knowledge of reactions limited to those of magnesium, zinc
and iron with hydrochloric and sulfuric acids.
5.4.2.2 Neutralisation of acids and salt production
Content Key opportunities for
skills development
Acids are neutralised by alkalis (eg soluble metal hydroxides) and
bases (eg insoluble metal hydroxides and metal oxides) to produce
salts and water, and by metal carbonates to produce salts, water
and carbon dioxide.
The particular salt produced in any reaction between an acid and a
base or alkali depends on:
• the acid used (hydrochloric acid produces chlorides, nitric acid
produces nitrates, sulfuric acid produces sulfates)
• the positive ions in the base, alkali or carbonate.
Students should be able to:
• predict products from given reactants
• use the formulae of common ions to deduce the formulae of
salts.
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5.4.2.3 Soluble salts
Content Key opportunities for
skills development
Soluble salts can be made from acids by reacting them with solid
insoluble substances, such as metals, metal oxides, hydroxides or
carbonates. The solid is added to the acid until no more reacts and
the excess solid is filtered off to produce a solution of the salt.
Salt solutions can be crystallised to produce solid salts.
Students should be able to describe how to make pure, dry samples
of named soluble salts from information provided.
Required practical activity 8: preparation of a pure, dry sample of a soluble salt from an insoluble
oxide or carbonate, using a Bunsen burner to heat dilute acid and a water bath or electric heater to
evaporate the solution.
AT skills covered by this practical activity: chemistry AT 2, 3, 4 and 6.
This practical activity also provides opportunities to develop WS and MS. Details of all skills are
given in Key opportunities for skills development (page 180).
5.4.2.4 The pH scale and neutralisation
Content Key opportunities for
skills development
Acids produce hydrogen ions (H+
) in aqueous solutions.
Aqueous solutions of alkalis contain hydroxide ions (OH–
).
The pH scale, from 0 to 14, is a measure of the acidity or alkalinity
of a solution, and can be measured using universal indicator or a
pH probe.
A solution with pH 7 is neutral. Aqueous solutions of acids have pH
values of less than 7 and aqueous solutions of alkalis have pH
values greater than 7.
In neutralisation reactions between an acid and an alkali, hydrogen
ions react with hydroxide ions to produce water.
This reaction can be represented by the equation:
Students should be able to:
• describe the use of universal indicator or a wide range
indicator to measure the approximate pH of a solution
• use the pH scale to identify acidic or alkaline solutions.
AT 3
This is an opportunity to
investigate pH changes
when a strong acid
neutralises a strong alkali.
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5.4.2.5 Strong and weak acids (HT only)
Content Key opportunities for
skills development
A strong acid is completely ionised in aqueous solution. Examples
of strong acids are hydrochloric, nitric and sulfuric acids.
A weak acid is only partially ionised in aqueous solution. Examples
of weak acids are ethanoic, citric and carbonic acids.
For a given concentration of aqueous solutions, the stronger an
acid, the lower the pH.
As the pH decreases by one unit, the hydrogen ion concentration of
the solution increases by a factor of 10.
Students should be able to:
• use and explain the terms dilute and concentrated (in terms of
amount of substance), and weak and strong (in terms of the
degree of ionisation) in relation to acids
An opportunity to measure
the pH of different acids at
different concentrations.
• describe neutrality and relative acidity in terms of the effect on
hydrogen ion concentration and the numerical value of pH
(whole numbers only).
MS 2h
Make order of magnitude
calculations.
5.4.3 Electrolysis
5.4.3.1 The process of electrolysis
Content Key opportunities for
skills development
When an ionic compound is melted or dissolved in water, the ions
are free to move about within the liquid or solution. These liquids
and solutions are able to conduct electricity and are called
electrolytes.
Passing an electric current through electrolytes causes the ions to
move to the electrodes. Positively charged ions move to the
negative electrode (the cathode), and negatively charged ions move
to the positive electrode (the anode). Ions are discharged at the
electrodes producing elements. This process is called electrolysis.
(HT only) Throughout Section 4.4.3 Higher Tier students should be
able to write half equations for the reactions occurring at the
electrodes during electrolysis, and may be required to complete and
balance supplied half equations.
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5.4.3.2 Electrolysis of molten ionic compounds
Content Key opportunities for
skills development
When a simple ionic compound (eg lead bromide) is electrolysed in
the molten state using inert electrodes, the metal (lead) is produced
at the cathode and the non-metal (bromine) is produced at the
anode.
Students should be able to predict the products of the electrolysis of
binary ionic compounds in the molten state.
A safer alternative for
practical work is anhydrous
zinc chloride.
5.4.3.3 Using electrolysis to extract metals
Content Key opportunities for
skills development
Metals can be extracted from molten compounds using electrolysis.
Electrolysis is used if the metal is too reactive to be extracted by
reduction with carbon or if the metal reacts with carbon. Large
amounts of energy are used in the extraction process to melt the
compounds and to produce the electrical current.
Aluminium is manufactured by the electrolysis of a molten mixture
of aluminium oxide and cryolite using carbon as the positive
electrode (anode).
Students should be able to:
• explain why a mixture is used as the electrolyte
• explain why the positive electrode must be continually
replaced.
5.4.3.4 Electrolysis of aqueous solutions
Content Key opportunities for
skills development
The ions discharged when an aqueous solution is electrolysed
using inert electrodes depend on the relative reactivity of the
elements involved.
At the negative electrode (cathode), hydrogen is produced if the
metal is more reactive than hydrogen.
At the positive electrode (anode), oxygen is produced unless the
solution contains halide ions when the halogen is produced.
This happens because in the aqueous solution water molecules
break down producing hydrogen ions and hydroxide ions that are
discharged.
Students should be able to predict the products of the electrolysis of
aqueous solutions containing a single ionic compound.
WS 1.2
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Required practical activity 9: investigate what happens when aqueous solutions are electrolysed
using inert electrodes. This should be an investigation involving developing a hypothesis.
AT skills covered by this practical activity: chemistry AT 3 and 7.
This practical activity also provides opportunities to develop WS and MS. Details of all skills are
given in Key opportunities for skills development (page 180).
5.4.3.5 Representation of reactions at electrodes as half equations (HT only)
Content Key opportunities for
skills development
During electrolysis, at the cathode (negative electrode), positively
charged ions gain electrons and so the reactions are reductions.
At the anode (positive electrode), negatively charged ions lose
electrons and so the reactions are oxidations.
Reactions at electrodes can be represented by half equations, for
example:
2H+
+ 2e-
→ H2
and
4OH-
→ O2 + 2H2O + 4eor
4OH-
– 4e-
→ O2 + 2H2O
5.5 Energy changes
Energy changes are an important part of chemical reactions. The interaction of particles often
involves transfers of energy due to the breaking and formation of bonds. Reactions in which energy
is released to the surroundings are exothermic reactions, while those that take in thermal energy
are endothermic. These interactions between particles can produce heating or cooling effects that
are used in a range of everyday applications. Some interactions between ions in an electrolyte
result in the production of electricity. Cells and batteries use these chemical reactions to provide
electricity. Electricity can also be used to decompose ionic substances and is a useful means of
producing elements that are too expensive to extract any other way.
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5.5.1 Exothermic and endothermic reactions
5.5.1.1 Energy transfer during exothermic and endothermic reactions
Content Key opportunities for
skills development
Energy is conserved in chemical reactions. The amount of energy in
the universe at the end of a chemical reaction is the same as before
the reaction takes place. If a reaction transfers energy to the
surroundings the product molecules must have less energy than the
reactants, by the amount transferred.
An exothermic reaction is one that transfers energy to the
surroundings so the temperature of the surroundings increases.
Exothermic reactions include combustion, many oxidation reactions
and neutralisation.
Everyday uses of exothermic reactions include self-heating cans
and hand warmers.
An endothermic reaction is one that takes in energy from the
surroundings so the temperature of the surroundings decreases.
Endothermic reactions include thermal decompositions and the
reaction of citric acid and sodium hydrogencarbonate. Some sports
injury packs are based on endothermic reactions.
Students should be able to:
• distinguish between exothermic and endothermic reactions on
the basis of the temperature change of the surroundings
• evaluate uses and applications of exothermic and endothermic
reactions given appropriate information.
Limited to measurement of temperature change. Calculation of
energy changes or ΔH is not required.
AT 5
An opportunity to measure
temperature changes when
substances react or
dissolve in water.
Required practical activity 10: investigate the variables that affect temperature changes in
reacting solutions such as, eg acid plus metals, acid plus carbonates, neutralisations, displacement
of metals.
AT skills covered by this practical activity: chemistry AT 1, 3, 5 and 6.
This practical activity also provides opportunities to develop WS and MS. Details of all skills are
given in Key opportunities for skills development (page 181).
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5.5.1.2 Reaction profiles
Content Key opportunities for
skills development
Chemical reactions can occur only when reacting particles collide
with each other and with sufficient energy. The minimum amount of
energy that particles must have to react is called the activation
energy.
Reaction profiles can be used to show the relative energies of
reactants and products, the activation energy and the overall
energy change of a reaction.
Students should be able to:
• draw simple reaction profiles (energy level diagrams) for
exothermic and endothermic reactions showing the relative
energies of reactants and products, the activation energy and
the overall energy change, with a curved line to show the
energy as the reaction proceeds
• use reaction profiles to identify reactions as exothermic or
endothermic
• explain that the activation energy is the energy needed for a
reaction to occur.
5.5.1.3 The energy change of reactions (HT only)
Content Key opportunities for
skills development
During a chemical reaction:
• energy must be supplied to break bonds in the reactants
• energy is released when bonds in the products are formed.
The energy needed to break bonds and the energy released when
bonds are formed can be calculated from bond energies.
The difference between the sum of the energy needed to break
bonds in the reactants and the sum of the energy released when
bonds in the products are formed is the overall energy change of
the reaction.
In an exothermic reaction, the energy released from forming new
bonds is greater than the energy needed to break existing bonds.
In an endothermic reaction, the energy needed to break existing
bonds is greater than the energy released from forming new bonds.
Students should be able to calculate the energy transferred in
chemical reactions using bond energies supplied.
MS 1a
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5.6 The rate and extent of chemical change
Chemical reactions can occur at vastly different rates. Whilst the reactivity of chemicals is a
significant factor in how fast chemical reactions proceed, there are many variables that can be
manipulated in order to speed them up or slow them down. Chemical reactions may also be
reversible and therefore the effect of different variables needs to be established in order to identify
how to maximise the yield of desired product. Understanding energy changes that accompany
chemical reactions is important for this process. In industry, chemists and chemical engineers
determine the effect of different variables on reaction rate and yield of product. Whilst there may be
compromises to be made, they carry out optimisation processes to ensure that enough product is
produced within a sufficient time, and in an energy-efficient way.
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5.6.1 Rate of reaction
5.6.1.1 Calculating rates of reactions
Content Key opportunities for
skills development
The rate of a chemical reaction can be found by measuring the
quantity of a reactant used or the quantity of product formed over
time:
mean rate o f reaction =
quantity o f reactant used
time taken
mean rate o f reaction =
quantity o f product f ormed
time taken
The quantity of reactant or product can be measured by the mass in
grams or by a volume in cm3
.
The units of rate of reaction may be given as g/s or cm3
/s.
For the Higher Tier, students are also required to use quantity of
reactants in terms of moles and units for rate of reaction in mol/s.
Students should be able to:
• calculate the mean rate of a reaction from given information
about the quantity of a reactant used or the quantity of a
product formed and the time taken
• draw, and interpret, graphs showing the quantity of product
formed or quantity of reactant used up against time
• draw tangents to the curves on these graphs and use the
slope of the tangent as a measure of the rate of reaction
• (HT only) calculate the gradient of a tangent to the curve on
these graphs as a measure of rate of reaction at a specific
time.
MS 1a
Recognise and use
expressions in decimal
form.
MS 1c
Use ratios, fractions and
percentages.
MS 1d
Make estimates of the
results of simple
calculations.
MS 4a
Translate information
between graphical and
numeric form.
MS 4b
Drawing and interpreting
appropriate graphs from
data to determine rate of
reaction.
MS 4c
Plot two variables from
experimental or other data.
MS 4d
Determine the slope and
intercept of a linear graph.
MS 4e
Draw and use the slope of a
tangent to a curve as a
measure of rate of change.
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5.6.1.2 Factors which affect the rates of chemical reactions
Content Key opportunities for
skills development
Factors which affect the rates of chemical reactions include: the
concentrations of reactants in solution, the pressure of reacting
gases, the surface area of solid reactants, the temperature and the
presence of catalysts.
Students should be able to recall how changing these factors
affects the rate of chemical reactions.
This topic offers
opportunities for practical
work and investigations in
addition to required
practical 11.
Required practical activity 11: investigate how changes in concentration affect the rates of
reactions by a method involving measuring the volume of a gas produced and a method involving a
change in colour or turbidity.
This should be an investigation involving developing a hypothesis.
AT skills covered by this practical activity: chemistry AT 1, 3, 5 and 6.
This practical activity also provides opportunities to develop WS and MS. Details of all skills are
given in Key opportunities for skills development (page 182)
5.6.1.3 Collision theory and activation energy
Content Key opportunities for
skills development
Collision theory explains how various factors affect rates of
reactions. According to this theory, chemical reactions can occur
only when reacting particles collide with each other and with
sufficient energy. The minimum amount of energy that particles
must have to react is called the activation energy.
Increasing the concentration of reactants in solution, the pressure of
reacting gases, and the surface area of solid reactants increases
the frequency of collisions and so increases the rate of reaction.
Increasing the temperature increases the frequency of collisions
and makes the collisions more energetic, and so increases the rate
of reaction.
Students should be able to :
• predict and explain using collision theory the effects of
changing conditions of concentration, pressure and
temperature on the rate of a reaction
WS 1.2
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Content Key opportunities for
skills development
• predict and explain the effects of changes in the size of pieces
of a reacting solid in terms of surface area to volume ratio
• use simple ideas about proportionality when using collision
theory to explain the effect of a factor on the rate of a
reaction.
MS 5c
MS 1c
5.6.1.4 Catalysts
Content Key opportunities for
skills development
Catalysts change the rate of chemical reactions but are not used up
during the reaction. Different reactions need different catalysts.
Enzymes act as catalysts in biological systems.
Catalysts increase the rate of reaction by providing a different
pathway for the reaction that has a lower activation energy.
A reaction profile for a catalysed reaction can be drawn in the
following form:
Students should be able to identify catalysts in reactions from their
effect on the rate of reaction and because they are not included in
the chemical equation for the reaction.
Students should be able to explain catalytic action in terms of
activation energy.
Students do not need to know the names of catalysts other than
those specified in the subject content.
AT 5
An opportunity to
investigate the catalytic
effect of adding different
metal salts to a reaction
such as the decomposition
of hydrogen peroxide.
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5.6.2 Reversible reactions and dynamic equilibrium
5.6.2.1 Reversible reactions
Content Key opportunities for
skills development
In some chemical reactions, the products of the reaction can react
to produce the original reactants. Such reactions are called
reversible reactions and are represented:
A + B C + D
The direction of reversible reactions can be changed by changing
the conditions.
For example:
5.6.2.2 Energy changes and reversible reactions
Content Key opportunities for
skills development
If a reversible reaction is exothermic in one direction, it is
endothermic in the opposite direction. The same amount of energy
is transferred in each case. For example:
5.6.2.3 Equilibrium
Content Key opportunities for
skills development
When a reversible reaction occurs in apparatus which prevents the
escape of reactants and products, equilibrium is reached when the
forward and reverse reactions occur at exactly the same rate.
WS 1.2
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5.6.2.4 The effect of changing conditions on equilibrium (HT only)
Content Key opportunities for
skills development
The relative amounts of all the reactants and products at equilibrium
depend on the conditions of the reaction.
If a system is at equilibrium and a change is made to any of the
conditions, then the system responds to counteract the change.
The effects of changing conditions on a system at equilibrium can
be predicted using Le Chatelier’s Principle.
Students should be able to make qualitative predictions about the
effect of changes on systems at equilibrium when given appropriate
information.
5.6.2.5 The effect of changing concentration (HT only)
Content Key opportunities for
skills development
If the concentration of one of the reactants or products is changed,
the system is no longer at equilibrium and the concentrations of all
the substances will change until equilibrium is reached again.
If the concentration of a reactant is increased, more products will be
formed until equilibrium is reached again.
If the concentration of a product is decreased, more reactants will
react until equilibrium is reached again.
Students should be able to interpret appropriate given data to
predict the effect of a change in concentration of a reactant or
product on given reactions at equilibrium.
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5.6.2.6 The effect of temperature changes on equilibrium (HT only)
Content Key opportunities for
skills development
If the temperature of a system at equilibrium is increased:
• the relative amount of products at equilibrium increases for an
endothermic reaction
• the relative amount of products at equilibrium decreases for
an exothermic reaction.
If the temperature of a system at equilibrium is decreased:
• the relative amount of products at equilibrium decreases for
an endothermic reaction
• the relative amount of products at equilibrium increases for an
exothermic reaction.
Students should be able to interpret appropriate given data to
predict the effect of a change in temperature on given reactions at
equilibrium.
5.6.2.7 The effect of pressure changes on equilibrium (HT only)
Content Key opportunities for
skills development
For gaseous reactions at equilibrium:
• an increase in pressure causes the equilibrium position to
shift towards the side with the smaller number of molecules as
shown by the symbol equation for that reaction
• a decrease in pressure causes the equilibrium position to shift
towards the side with the larger number of molecules as
shown by the symbol equation for that reaction.
Students should be able to interpret appropriate given data to
predict the effect of pressure changes on given reactions at
equilibrium.
5.7 Organic chemistry
The chemistry of carbon compounds is so important that it forms a separate branch of chemistry. A
great variety of carbon compounds is possible because carbon atoms can form chains and rings
linked by C-C bonds. This branch of chemistry gets its name from the fact that the main sources of
organic compounds are living, or once-living materials from plants and animals. These sources
include fossil fuels which are a major source of feedstock for the petrochemical industry. Chemists
are able to take organic molecules and modify them in many ways to make new and useful
materials such as polymers, pharmaceuticals, perfumes and flavourings, dyes and detergents.
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5.7.1 Carbon compounds as fuels and feedstock
5.7.1.1 Crude oil, hydrocarbons and alkanes
Content Key opportunities for
skills development
Crude oil is a finite resource found in rocks. Crude oil is the remains
of an ancient biomass consisting mainly of plankton that was buried
in mud.
Crude oil is a mixture of a very large number of compounds. Most of
the compounds in crude oil are hydrocarbons, which are molecules
made up of hydrogen and carbon atoms only.
Most of the hydrocarbons in crude oil are hydrocarbons called
alkanes. The general formula for the homologous series of alkanes
is CnH2n+2
The first four members of the alkanes are methane, ethane,
propane and butane.
Alkane molecules can be represented in the following forms:
C2H6 or
Students should be able to recognise substances as alkanes given
their formulae in these forms.
Students do not need to know the names of specific alkanes other
than methane, ethane, propane and butane.
WS 1.2
Make models of alkane
molecules using the
molecular modelling kits.
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5.7.1.2 Fractional distillation and petrochemicals
Content Key opportunities for
skills development
The many hydrocarbons in crude oil may be separated into
fractions, each of which contains molecules with a similar number of
carbon atoms, by fractional distillation.
The fractions can be processed to produce fuels and feedstock for
the petrochemical industry.
Many of the fuels on which we depend for our modern lifestyle,
such as petrol, diesel oil, kerosene, heavy fuel oil and liquefied
petroleum gases, are produced from crude oil.
Many useful materials on which modern life depends are produced
by the petrochemical industry, such as solvents, lubricants,
polymers, detergents.
The vast array of natural and synthetic carbon compounds occur
due to the ability of carbon atoms to form families of similar
compounds.
Students should be able to explain how fractional distillation works
in terms of evaporation and condensation.
Knowledge of the names of other specific fractions or fuels is not
required.
WS 1.2
5.7.1.3 Properties of hydrocarbons
Content Key opportunities for
skills development
Some properties of hydrocarbons depend on the size of their
molecules, including boiling point, viscosity and flammability. These
properties influence how hydrocarbons are used as fuels.
Students should be able to recall how boiling point, viscosity and
flammability change with increasing molecular size.
The combustion of hydrocarbon fuels releases energy. During
combustion, the carbon and hydrogen in the fuels are oxidised. The
complete combustion of a hydrocarbon produces carbon dioxide
and water.
Students should be able to write balanced equations for the
complete combustion of hydrocarbons with a given formula.
Knowledge of trends in properties of hydrocarbons is limited to:
• boiling points
• viscosity
• flammability.
WS 1.2, 4.1
Investigate the properties of
different hydrocarbons.
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5.7.1.4 Cracking and alkenes
Content Key opportunities for
skills development
Hydrocarbons can be broken down (cracked) to produce smaller,
more useful molecules.
Cracking can be done by various methods including catalytic
cracking and steam cracking.
Students should be able to describe in general terms the conditions
used for catalytic cracking and steam cracking.
The products of cracking include alkanes and another type of
hydrocarbon called alkenes.
Alkenes are more reactive than alkanes and react with bromine
water, which is used as a test for alkenes.
Students should be able to recall the colour change when bromine
water reacts with an alkene.
There is a high demand for fuels with small molecules and so some
of the products of cracking are useful as fuels.
Alkenes are used to produce polymers and as starting materials for
the production of many other chemicals.
Students should be able to balance chemical equations as
examples of cracking given the formulae of the reactants and
products.
Students should be able to give examples to illustrate the
usefulness of cracking. They should also be able to explain how
modern life depends on the uses of hydrocarbons.
(Students do not need to know the formulae or names of individual
alkenes.)
WS 1.2
5.8 Chemical analysis
Analysts have developed a range of qualitative tests to detect specific chemicals. The tests are
based on reactions that produce a gas with distinctive properties, or a colour change or an
insoluble solid that appears as a precipitate.
Instrumental methods provide fast, sensitive and accurate means of analysing chemicals, and are
particularly useful when the amount of chemical being analysed is small. Forensic scientists and
drug control scientists rely on such instrumental methods in their work.
AQA GCSE Combined Science: Trilogy 8464. GCSE exams June 2018 onwards. Version 1.1 04 October 2019
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5.8.1 Purity, formulations and chromatography
5.8.1.1 Pure substances
Content Key opportunities for
skills development
In chemistry, a pure substance is a single element or compound,
not mixed with any other substance.
Pure elements and compounds melt and boil at specific
temperatures. Melting point and boiling point data can be used to
distinguish pure substances from mixtures.
In everyday language, a pure substance can mean a substance
that has had nothing added to it, so it is unadulterated and in its
natural state, eg pure milk.
Students should be able to use melting point and boiling point data
to distinguish pure from impure substances.
WS 2.2, 4.1
5.8.1.2 Formulations