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GCSE SCIENCE REVISION
CHEMISTRY (P2)
OK it’s time to stop faffin about and get stuck in to some revision. If you want to do well in your science GCSE you will need to know all of the stuff on this revision booklet.
SECTION C2.1
You should know and / or be able to use / do the following
To represent the electronic structure of the first twenty elements of the periodic table in the following forms:
for sodium (and 2,8,1)
To represent the electronic structure of the ions in sodium, chloride, magnesium oxide and calcium chloride in the following forms:
for sodium ion (Na+) and [2,8]+
To represent the covalent bonds in molecules such as water, ammonia, hydrogen, hydrogen chloride, chlorine, methane and oxygen and in giant structures such as diamond and silicon dioxide in the following forms:
To represent the bonding in metals in the following form:
To write balanced chemical equations for reactions.
Atoms have a small central nucleus made up of protons and neutrons around which there are electrons.
The relative electrical charges are as shown:
Name of particle ChargeProton +1Neutron 0Electron .1
In an atom, the number of electrons is equal to the number of protons in the nucleus. Atoms have no overall electrical charge. All atoms of a particular element have the same number of protons. Atoms of different elements have different numbers of protons.
The number of protons in an atom is called its atomic number (proton number).
Atoms are arranged in the modern periodic table in order of their atomic number (proton number).
Electrons occupy particular energy levels. Each electron in an atom is at a particular energy level (in a particular shell). The electrons in an atom occupy the lowest available energy levels (innermost available shells).
Elements in the same group in the periodic table have the same number of electrons in the highest energy levels (outer electrons).
Compounds are substances in which atoms of two, or more,el ements are not just mixed together but chemically combined.
Chemical bonding involves either transferring or sharing electrons in the highest occupied energy levels (shells) of atoms.
When atoms form chemical bonds by transferring electrons, they form ions. Atoms that lose electrons become positively charged ions. Atoms that gain electrons become negatively charged ions. Ions have the electronic structure of a noble gas (Group 0).
The elements in Group 1 of the periodic table, the alkali metals, have similar chemical properties. They all react with non-metal elements to form ionic compounds in which the metal ion has a single positive charge.
The elements in Group 7 of the periodic table, the halogens, have similar chemical properties. They react with the alkali metals to form ionic compounds in which the halide ions have a single negative charge.
An ionic compound is a giant structure of ions. Ionic compounds are held together by strong forces of attraction between oppositely charged ions. These forces act in all directions in thelattice and this is called ionic bonding.
When atoms share pairs of electrons, they form covalent bonds. These bonds between atoms are strong. Some covalently bonded substances consist of simple molecules such as H2, Cl2, O2, HCl, H2O and CH4.
Others have giant covalent structures (macromolecules), such as diamond and silicon dioxide.
Metals consist of giant structures of atoms arranged in a regular pattern. The electrons in the highest occupied energy levels (outer shell) of metal atoms are delocalised and so free to move through the whole structure. This corresponds to a structure of positive ions with electrons between the ions holding them together bystrong electrostatic attractions.
Now try these exam style questions
Structures and bonding1 The diagram represents an atom of an element.
(a) Write the electronic structure of this atom as numbers and commas.
...........................................................................................................................................
(1)
(b) How many protons are in the nucleus of this atom?
...........................................................................................................................................
(1)
(c) Name the other particles that are in the nucleus.
...........................................................................................................................................
(1)
(d) In which group of the periodic table is this element?
...........................................................................................................................................
(1)
(e) Draw a similar diagram to show the ion formed by this atom in ionic
compounds. Show the charge on the ion.
2 Complete the missing information (a) to (f) in the table.
Atomic number
Symbol Electronic structure of atom
Formula of ion
Electronic structure of ion
9 F (a) (b) [2,8]–
11 (c) 2,8,1 Na+ (d)
(e) S 2,8,6 S2– (f)
(6)
3 A hydrogen atom can be represented by the diagram:
(a) Draw a similar diagram to show the electrons in the outer shell of a chlorine
atom. (1)
(b) Draw a dot and cross diagram to show the bonding in a molecule of
hydrogen chloride. (2)
(c) Explain why hydrogen and chlorine form a single covalent bond.
...........................................................................................................................................
...........................................................................................................................................
(2)
(d) Explain why silicon can form giant structures.
...........................................................................................................................................
...........................................................................................................................................
...........................................................................................................................................
(3)
4 (a) Draw a dot and cross diagram to show the arrangement of electrons in a
magnesium ion. Show the charge on the ion. (3)
(b) Draw a dot and cross diagram to show the arrangement of electrons in an
oxide ion. (3)
(c) What is the formula of magnesium oxide?
...........................................................................................................................................
(1)
5 Berzelius (1779–1848) carried out experiments to discover the atomic mass of
many elements.
He wrote about the fact that bodies combine in definite proportions and that led
him to suggest the existence of a cause.
(a) Suggest an observation that Berzelius might have made.
...........................................................................................................................................
(1)
(b) Is what Berzelius wrote a prediction or a hypothesis? Explain your answer.
...........................................................................................................................................
(1)
(c) Berzelius gave oxygen the number 100 to represent its relative atomic mass.
He then set out to compare the mass of other elements with oxygen.
However, he could not measure these directly because they could not be
turned into gases – the temperature needed was too high and he did not
have the equipment to do this.
(i) Explain, in general terms, the problem he had.
.....................................................................................................................................
(1)
(ii) Use this example to explain the relationship between technology and
science.
.....................................................................................................................................
(1)
6 The diagram represents atoms of potassium in the solid metal.
(a) What is the electronic structure of a potassium atom?
...........................................................................................................................................
(1)
(b) Explain as fully as you can how the atoms are held together in solid
potassium metal.
...........................................................................................................................................
...........................................................................................................................................
...........................................................................................................................................
(3)
SECTION C2.2
You should know and / or be able to use / do the following
Substances that have simple molecular, giant ionic and giant covalent structureshave very different properties. Ionic, covalent and metallic bonds are strong. Theforces between molecules are weaker, eg in carbon dioxide and iodine. Nanomaterials have new properties because of their very small size.
To relate the properties of substances to their uses
To suggest the type of structure of a substance given its properties
To evaluate developments and applications of new materials, eg nanomaterials, ad smart materials.
Substances that consist of simple molecules are gases, liquids or solids that have relatively low melting points and boiling points.
Substances that consist of simple molecules have only weak forces between the molecules (intermolecular forces). It is these intermolecular forces that are overcome, not the covalent bonds, when the substance melts or boils.
Substances that consist of simple molecules do not conduct electricity because the molecules do not have an overall electric charge.
Ionic compounds have regular structures (giant ionic lattices) in which there are strong electrostatic forces in all directions between oppositely charged ions. These compounds have high melting points and high boiling points.
When melted or dissolved in water, ionic compounds conduct electricity because the ions are free to move and carry the current.
Atoms that share electrons can also form giant structures or macromolecules. Diamond and graphite (forms of carbon) and silicon dioxide (silica) are examples of giant covalent structures (lattices) of atoms. All the atoms in these structures are linked to other atoms by strong covalent bonds and so they have very high
melting points.
In diamond, each carbon atom forms four covalent bonds with other carbon atoms in a giant covalent structure, so diamond is very hard.
In graphite, each carbon atom bonds to three others, forming layers. The layers are free to slide over each other and so graphite is soft and slippery.
In graphite, one electron from each carbon atom is delocalised. These delocalised electrons allow graphite to conduct heat and electricity.
Metals conduct heat and electricity because of the delocalised electrons in their structures.
The layers of atoms in metals are able to slide over each other and so metals can be bent and shaped.Nanoscience refers to structures that are 1-100 nm in size, of the order of a few hundred atoms. Nanoparticles show different properties to the same materials in bulk and have a high surface area to volume ratio, which may lead to the development of new computers, new catalysts, new coatings, highly selective sensors and stronger and lighter construction materials.
(7)
Structures and properties1 The table contains information about some substances.
Complete the missing information (a) to (g).
Melting point (C)
Boiling point (C)
Electrical conductivity when solid
Electrical conductivity when molten
Solubility in water
Type of bonding
Type of structure
1660 3287 (a) good insoluble metallic giant
–101 –35 poor (b) soluble covalent (c)
712 1418 poor good soluble (d) giant
–25 144 (e) poor insoluble (f) small molecules
1410 2355 poor poor insoluble covalent (g)
2 Quartz is a very hard mineral that is used as an abrasive. It is insoluble in water.
It is a form of silica, SiO2. It can form large, attractive crystals that are
transparent and can be used for jewellery. It melts at 1610C. It does not conduct
electricity when solid or when molten. It is used in the form of sand in the
building and glass-making industries.
(a) Give three pieces of evidence from the passage that tell you that quartz has a giant structure..................................................................................................................................................................................................................................................................................................................................................................................................................................
(3)(b) What type of bonding is in quartz? Explain your answer.
...........................................................................................................................................
...........................................................................................................................................(2)
3 Copper can be used to make electrical wires, water pipes, and cooking pans.
(a) Suggest three reasons why copper is used to make cooking pans..................................................................................................................................................................................................................................................................................................................................................................................................................................
(3)(b) Which two properties of copper depend on the ability of delocalised
electrons to flow through the metal?......................................................................................................................................................................................................................................................................................
(2)
(c) Explain what happens to the atoms in the metal when a piece of copper is pulled
into a wire.
...........................................................................................................................................
...........................................................................................................................................
(2)
[Higher]
4 Nanotechnology promises to revolutionise our world. Nanoparticles and new
devices are being rapidly developed but production is still on a very small scale.
The properties of nanoparticles that make them useful can cause problems if
they are made in large quantities. These include explosions because of
spontaneous combustion on contact with air.
(a) What are nanoparticles?
...........................................................................................................................................
...........................................................................................................................................
(2)
(b) Suggest two reasons why nanotechnology is being developed rapidly.
...........................................................................................................................................
...........................................................................................................................................
(2)
(c) Why are nanoparticles more likely to catch fire when exposed to air
compared with normal materials?
...........................................................................................................................................
...........................................................................................................................................
(2)
5 Piezoceramics are smart materials that can be made to vibrate by passing an
electric current through them. They can be made small enough to work inside
mobile phones.
(a) Suggest a possible economic advantage of piezoceramics.
...........................................................................................................................................
(1)
(b) Suggest an environmental advantage of piezoceramics.
...........................................................................................................................................
(1)
Some smart materials can only be seen at higher temperatures. They can be
used in the manufacture of clothing.
(c) Suggest how this feature could be useful.
...........................................................................................................................................
(1)
6 A molecule of pentane can be represented as shown:
(a) What do the letters C and H represent?
...........................................................................................................................................
(1)
(b) What do the lines between each C and H represent?
...........................................................................................................................................
...........................................................................................................................................
(2)
(c) Explain why liquid pentane does not conduct electricity.
...........................................................................................................................................
...........................................................................................................................................
(2)
(d) Pentane boils at 36°C. Explain what happens to the molecules of pentane
when liquid pentane boils and becomes a gas.
...........................................................................................................................................
...........................................................................................................................................
(2)
SECTION C2.3
You should know and / or be able to use / do the following
The relative masses of atoms can be used to calculate how much to react and howmuch we can produce, because no atoms are gained or lost in chemical reactions. Inindustrial processes, atom economy is important for sustainable development.
To calculate chemical quantities involving formula mass (Mr) and percentages of elements in compounds
To calculate chemical quantities involving empirical formulae, reacting masses and percentage yield.
To calculate the atom economy for industrial processes and be able to evaluate sustainable development issues related to this economy.
Atoms can be represented as shown:
Mass number 23
NaAtomic Number 11
The relative masses of protons, neutrons and electrons are:
Name of particle MassProton 1Neutron 1Electron Very small (0)
The total number of protons and neutrons in an atom is called its mass number.
Atoms of the same element can have different numbers of neutrons; these atoms are called isotopes of that element.
The relative atomic mass of an element (Ar) compares the mass of atoms of the element with the 12C isotope. It is an average value for the isotopes of the element.
The relative formula mass (Mr) of a compound is the sum of the relative atomic masses of the atoms in the numbers shown in the formula.
The relative formula mass of a substance, in grams, is known as one mole of that substance.
The percentage of an element in a compound can be calculated from the relative mass of the element in the formula and the relative formula mass of the compound.
The masses of reactants and products can be calculated from balanced symbol equations.
Even though no atoms are gained or lost in a chemical reaction, it is not always possible to obtain the calculated amount of a product because:− the reaction may not go to completion because it is reversible− some of the product may be lost when it is separated from the reaction mixture− some of the reactants may react in ways different to the expected reaction.
The amount of a product obtained is known as the yield. When compared with the maximum theoretical amount as a percentage, it is called the percentage yield.
The atom economy (atom utilisation) is a measure of the amount of starting materials that end up as useful products. It is important for sustainable development and for economical reasons to use reactions with high atom economy.
In some chemical reactions, the products of the reaction can react to produce the original reactants. Such reactions are called reversible reactions.
When a reversible reaction occurs in a closed system, equilibrium is reached when the reactions occur at exactly the same rate in each direction.
The relative amounts of all the reacting substances at equilibrium depend on the conditions of the reaction.
Although reversible reactions may not go to completion, they can still be used efficiently in continuous industrial processes, such as the Haber process that is used to manufacture ammonia.
The raw materials for the Haber process are nitrogen and hydrogen. Nitrogen is obtained from the air and hydrogen may be obtained from natural gas or other sources.
The purified gases are passed over a catalyst of iron at a high temperature (about 450 °C) and a high pressure (about 200 atmospheres). Some of the hydrogen and nitrogen reacts to form ammonia.
The reaction is reversible so ammonia breaks down again into nitrogen and hydrogen.
On cooling, the ammonia liquefies and is removed. The remaining hydrogen and nitrogen is re-cycled.
The reaction conditions are chosen to produce a reasonable yield of ammonia quickly.
Now try these exam style questions
How much?1 Hydrogen has three isotopes
(a) What are isotopes?
...........................................................................................................................................
...........................................................................................................................................
(2)
(b) How many protons, neutrons and electrons are there in an atom of 3H?
..........................................................................................................................................
...........................................................................................................................................
...........................................................................................................................................
(3)
(c) Heavy water contains atoms of the isotope hydrogen 2instead of hydrogen 1
What is the mass of one mole of heavy water?
...........................................................................................................................................
...........................................................................................................................................
(2)
2 Tablets taken by people with iron deficiency anaemia contain 0.200 g of
anhydrous iron(II) sulfate, FeSO4.
(a) Calculate the relative formula mass of iron(II) sulfate, FeSO4.
...........................................................................................................................................
...........................................................................................................................................
(2)
(b) Calculate the percentage of iron in iron(II) sulfate.
...........................................................................................................................................
...........................................................................................................................................
(2)
(c) Calculate the mass of iron in each tablet.
...........................................................................................................................................
...........................................................................................................................................
(2)
3 The equation for the main reaction to make ammonia is:
N2 + 3H2 ⇋ 2NH3
(a) What does the symbol ⇋ tell you about this reaction?
...........................................................................................................................................
(1)
(b) The flow diagram shows the main stages in making ammonia.
(i) Name the two raw materials A and B.
.....................................................................................................................................
.....................................................................................................................................
(2)
(ii) What is the purpose of the iron in the reactor?
.....................................................................................................................................
(1)
(iii) Why do the nitrogen and hydrogen not react completely.
.....................................................................................................................................
(1)
(iv) How is wastage of unreacted nitrogen and hydrogen prevented?
.....................................................................................................................................
(1)
4 The equation for the reaction of calcium carbonate with hydrochloric acid is:
CaCO3 + 2HCl CaCl2 + CO2 + H2O
(a) How many moles of hydrochloric acid react with one mole of calcium
carbonate?
...........................................................................................................................................
(1)
(b) How many moles of calcium chloride are produced from one mole of calcium
carbonate?
...........................................................................................................................................
(1)
(c) What is the mass of calcium chloride that can be made from one mole of
calcium carbonate?
...........................................................................................................................................
...........................................................................................................................................
(2)
(d) What is the mass of one mole of calcium carbonate?
...........................................................................................................................................
...........................................................................................................................................
(2)
(e) A student reacted 10 g of calcium carbonate with hydrochloric acid and
collected 7.4 g of calcium chloride. What was the percentage yield?
...........................................................................................................................................
...........................................................................................................................................
(2)
[Higher]
5 Chromium can be obtained from chromium oxide, Cr2O3, by reduction with
aluminium or carbon. For the first reaction, chromium is mixed with aluminium
and ignited in a crucible. The reaction using carbon is done at high temperatures
in a low-pressure furnace. The equations for the reactions are:
Cr2O3 + 2Al 2Cr + Al2O3
2Cr2O3 + 3C 4Cr + 3CO2
(a) Calculate the maximum mass of chromium that can be obtained from one
mole of chromium oxide.
...........................................................................................................................................
...........................................................................................................................................
(2)
(b) Calculate the percentage atom economy for both reactions to show which
reaction has the better atom economy.
...........................................................................................................................................
...........................................................................................................................................
...........................................................................................................................................
...........................................................................................................................................
(4)
(c) Suggest one advantage and one disadvantage of using carbon to
manufacture chromium.
...........................................................................................................................................
...........................................................................................................................................
(2)
er]
6 Ibuprofen is used as a pain killer throughout the world. You might know it as
Nurofen or Ibuleve. The traditional way to manufacture ibuprofen involved a lot of
chemical reactions and produced a lot of waste. The atom economy was just
32%.
Recently it became possible for any pharmaceutical (drug) company to make
ibuprofen. As there was a lot of money to be made, the race was on to find the
most economic way to make it. This meant cutting down waste. The new method
involves catalysts, some of which can be completely recovered and do not go
out as waste. The atom economy is increased to 77%, partly because only the
active form of ibuprofen is made. This also means that lower doses are needed
and they take a shorter time to kill any pain.
Evaluate the two methods of manufacture in terms of the social, economic and
environmental issues involved.
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(6)
SECTION C2.4
You should know and / or be able to use / do the following
Being able to speed up or slow down chemical reactions is important in everyday lifeand in industry. Changes in temperature, concentration of solutions, surface area ofsolids and the presence of catalysts all affect the rates of reactions.
To interpret graphs showing the amount of product formed (or reactant used up) with time, in terms of the rate of the reaction
To explain and evaluate the development, advantages and disadvantages of using catalysts in industrial processes.
The rate of a chemical reaction can be found by measuring the amount of a reactant used or the amount of product formed over time:
The rate of a chemical reaction increases:− if the temperature increases− if the concentration of dissolved reactants or the pressure of gases increases− if solid reactants are in smaller pieces (greater surface area)− if a catalyst is used.
Chemical reactions can only occur when reacting particles collide with each other and with sufficient energy. The minimum amount of energy particles must have to react is called the activation energy.
Increasing the temperature increases the speed of the reacting particles so that they collide more frequently and more energetically. This increases the rate of reaction.
Increasing the concentration of reactants in solutions and increasing the pressure of reacting gases also increases the frequency of collisions and so increases the rate of reaction.
Concentrations of solutions are given in moles per cubic decimetre (mol/dm3). Equal volumes of solutions of the same molar concentration contain the same number of moles of solute, ie the same number of particles.
Equal volumes of gases at the same temperature and pressure contain the same number of molecules. (Candidates will not be expected to find concentrations of solutions or volumes of gases in this Unit.)
Catalysts change the rate of chemical reactions but are not used up during the reaction. Different reactions need different catalysts.
Catalysts are important in increasing the rates of chemical reactions used in industrial processes to reduce costsNow try these exam style questions
1 Marble chips (calcium carbonate) react with hydrochloric acid as shown in the
equation.
CaCO3(s) + 2HCl(aq) CaCl2(aq) + H2O(l) + CO2(g)
Some students investigated the effect of the size of marble chips on the rate of
this reaction. They did the reactions in a conical flask, which they
put onto a balance connected to a computer to
record their results. They used three different sizes
of marble chips and kept all of the other conditions
the same. The graphs show the total mass of the
flask and reaction mixture plotted against time for the
three experiments.
(a) Which curve, A, B or C, shows the results for the fastest reaction?
...........................................................................................................................................
(1)
(b) Which curve, A, B or C, shows the results for the largest marble chips?
...........................................................................................................................................
(1)
(c) Explain, using collision theory, why changing the size of marble chips
changes the rate of reaction.
...........................................................................................................................................
...........................................................................................................................................
(2)
(d) (i) Use curve A to describe how the rate of reaction changes from the start
to the finish of the reaction.
.....................................................................................................................................
.....................................................................................................................................
.....................................................................................................................................
(3)
(ii) Explain why the rate of reaction changes in this way.
.....................................................................................................................................
.....................................................................................................................................
(2)
2 A student investigated the reaction between magnesium ribbon and hydrochloric
acid.
Mg(s) + 2HCl(aq) MgCl2(aq) + H2(g)
The student reacted 20 cm3 of two different concentrations of hydrochloric acid
with 0.050 g of magnesium. All other conditions were kept the same. The
student’s results are shown in the table.
Concentration
of acid (moles
per dm3)
Time
(minutes)
0 1 2 3 4 5 6 7 8 9 10
1.0 Volume of
gas (cm3)
0 15 24 31 37 41 44 46 47 48 48
2.0 Volume of
gas (cm3)
0 30 39 45 47 48 48 48 48 48 48
(a) Name the independent variable.
...........................................................................................................................................
(1)
(b) Suggest a control variable.
...........................................................................................................................................
(1)
(c) Suggest how the student might have controlled this variable.
...........................................................................................................................................
(1)
(d) Plot these results on the same axes, with time on the horizontal axis and
volume of gas on the vertical axis. Draw a smooth line for each
concentration. Label each line with the concentration of acid.
(4)
(e) (i) What is the effect of doubling the concentration on the rate of reaction?
.....................................................................................................................................
(1)
(ii) Explain how the graphs show this effect.
.....................................................................................................................................
.....................................................................................................................................
(2)
(iii) Explain this effect in terms of particles and collision theory.
.....................................................................................................................................
.....................................................................................................................................
(2)
(f) Explain why the total volume of hydrogen is the same for both reactions.
...........................................................................................................................................
...........................................................................................................................................
(2)
(g) Draw a labelled diagram of the apparatus you would use to do this
experiment.
3 Hydrogen peroxide solution is colourless and decomposes very slowly at 20°C.
2H2O2(aq) 2H2O(l) + O2(g)
Manganese(IV) oxide, a black powder, is a catalyst for the reaction.
(a) Explain what the word catalyst means.
...........................................................................................................................................
...........................................................................................................................................
(2)
(b) What would you see if manganese(IV) oxide was added to hydrogen
peroxide solution?
...........................................................................................................................................
(1)
(c) Describe briefly one way that you could show that manganese(IV) oxide was
acting as a catalyst.
...........................................................................................................................................
...........................................................................................................................................
(2)
(d) Explain, using particle and collision theory, how a solid catalyst works.
...........................................................................................................................................
...........................................................................................................................................
(2)
(e) Hydrogen peroxide solution stored at 10°C decomposes at half the rate
compared to when it is stored at 20°C. Explain, in terms of particles, why the
rate of the reaction changes in this way.
...........................................................................................................................................
...........................................................................................................................................
...........................................................................................................................................
(3)
SECTION C2.5
You should know and / or be able to use / do the following
Chemical reactions involve energy transfers. Many chemical reactions involve therelease of energy. For other chemical reactions to occur, energy must be supplied. Inindustrial processes, energy requirements and emissions need to be considered both for economic reasons and for sustainable development.
To describe the effects of changing the conditions of temperature and pressure on a given reaction or process
To evaluate the conditions used in industrial processes in terms of energy requirements.
When chemical reactions occur, energy is transferred to or from the surroundings.
An exothermic reaction is one that transfers energy, often as heat, to the surroundings. Examples of exothermic reactions include combustion, many oxidation reactions and neutralisation.
An endothermic reaction is one that takes in energy, often as heat, from the surroundings. Endothermic reactions include thermal decompositions.
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.
When a reversible reaction occurs in a closed system, equilibrium is reached when the reactions occur at exactly the same rate in each direction.
The relative amounts of all the reacting substances at equilibrium depend on the conditions of the reaction.
If the temperature is raised, the yield from the endothermic reaction increases and the yield from the exothermic reaction decreases.
If the temperature is lowered, the yield from the endothermic reaction decreases and the yield from the exothermic reaction increases.
In gaseous reactions, an increase in pressure will favour the reaction that produces the least number of molecules as shown by the symbol equation for that reaction.
These factors, together with reaction rates, are important when determining the optimum conditions in industrial processes, including the Haber process.
It is important for sustainable development as well as economic reasons to minimise energy requirements and energy wasted in industrial processes. Non-vigorous conditions mean less energy is used and less is released into the environment.
Now try these exam style questions
Energy and reactions1 Match each of (a) to (g) with one of the following:
endothermic reaction exothermic reaction no reaction
(a) Burning petrol in a car engine.
(b) Respiration in living cells.
(c) Boiling water.
(d) Converting limestone into calcium oxide.
(e) Switching on an electric light bulb.
(f) Reducing lead oxide with carbon to produce lead.
(g) Carbon dioxide combining with water in cells of green plants. (7)
2 When heated continuously, pink cobalt chloride crystals can be changed into
blue crystals .
CoCl2.6H2O ⇋ CoCl2.2H2O + 4H2Opink blue
(a) What does the symbol ⇋ tell you about this reaction?
...........................................................................................................................................
(1)
(b) How can you tell that the reaction to produce blue crystals is endothermic?
...........................................................................................................................................
(1)
(c) (i) How could you change the blue crystals to pink crystals?
.....................................................................................................................................
(1)
(ii) What temperature change would you observe when this is done?
.....................................................................................................................................
(1)
(d) Suggest how the colour changes of these crystals could be used.
...........................................................................................................................................
(1)
3 The equation for the main reaction in the Haber process to make ammonia is:
N2 + 3H2 ⇋ 2NH3
The table shows the percentage yield of the Haber process at different
temperatures and pressures.
Temp. (°C)Pressure (atm)
0 100 200 300 400 500
400 99 91 78 55 32 20
200 96 87 66 40 21 12
100 94 79 50 25 13 6
50 92 71 36 16 5 2
(a) Why does the yield of ammonia decrease with increased temperature?
...........................................................................................................................................
...........................................................................................................................................
(2)
(b) Why does the yield of ammonia increase with increased pressure?
...........................................................................................................................................
...........................................................................................................................................
(2)
(c) Why are conditions of 200 atm pressure and 450°C used in the industrial
process?
...........................................................................................................................................
...........................................................................................................................................
...........................................................................................................................................
(3)
(d) Suggest a better way than a table to present this data.
...........................................................................................................................................
(1)
[Higher]
4 The reaction to produce poly(ethene) is exothermic.
n C2H4 ( CH2—CH2 ) n
ethene poly(ethene)
The conditions used in two processes to make poly(ethene) are shown in the
table.
Process Temperature (°C) Pressure (atm) Catalyst
A 150–300 1000–3000 no
B 40–80 1–50 yes
(a) What enables process B to be operated under less vigorous conditions?
...........................................................................................................................................
(1)
(b) Suggest one way to keep the energy used to a minimum in both processes.
...........................................................................................................................................
(1)
(c) Suggest two environmental advantages of using process B to make
poly(ethene).
...........................................................................................................................................
— —
...........................................................................................................................................
(2)
5 A student had learned that the reaction between hydrochloric acid and sodium
hydroxide solution was exothermic. She, therefore, predicted that when she
added more acid to the alkali more heat would be produced. She used a burette
to deliver exact amounts of hydrochloric acid to 20 cm3 of alkali in a flask. She
used a thermometer to measure the temperature. Her results are in this table:
Volume of acid added (cm3) Temperature recorded (°C)
0 17
10 21
20 24
30 21
40 21
50 20
(a) How should she have insulated the flask?
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(b) Explain why she should have taken the temperature of the acid before
adding it to the sodium hydroxide solution.
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(c) Did she actually measure the heat produced by the reaction? Explain your
answer.
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(d) How might she have used an indicator to increase the accuracy of her
method?
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SECTION C2.6
You should know and / or be able to use / do the following
Ionic compounds have many uses and can provide other substances. Electrolysis isused to produce alkalis and elements such as chlorine and hydrogen.
Oxidation reduction reactions do not just involve oxygen. Soluble salts can be made from acids and insoluble salts can be made from solutions of ions.
To predict the products of electrolysing solutions of ions to suggest methods to make a named salt
To explain and evaluate processes that use the principles described in this unit
To complete and balance supplied half equations for the reactionsoccurring at the electrodes during electrolysis.
The state symbols in equations are (s), (l), (g) and (aq).
When an ionic substance is melted or dissolved in water, the ions are free to move about within the liquid or solution.
Passing an electric current through ionic substances that are molten or in solution breaks them down into elements. This process is called electrolysis.
During electrolysis, positively charged ions move to the negative electrode, and negatively charged ions move to the positive electrode.
At the negative electrode, positively charged ions gain electrons(reduction) and at the positive electrode, negatively charged ions lose electrons (oxidation).
If there is a mixture of ions, the products formed depend on the reactivity of the elements involved.
Reactions at electrodes can be represented by half equations, for example:
2Cl– → Cl2 + 2e–
The electrolysis of sodium chloride solution produces hydrogen and chlorine. Sodium hydroxide solution is also produced. These are important reagents for the chemical industry.
Copper can be purified by electrolysis using a positive electrode made of the impure copper and a negative electrode of pure copper in a solution containing copper ions.
Insoluble salts can be made by mixing appropriate solutions of ions so that a precipitate is formed. Precipitation can be used to remove unwanted ions from solutions, for example in treating water for drinking or in treating effluent.
Soluble salts can be made from acids by reacting them with:− metals - not all metals are suitable, some are too reactive and others are not reactive enough− insoluble bases – the base is added to the acid until no more will react and the excess solid is filtered off− alkalis - an indicator can be used to show when the acid and alkali have completely reacted to produce a salt solution.
Salt solutions can be crystallised to produce solid salt.
Metal oxides and hydroxides are bases. Soluble hydroxides are called alkalis.
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 acidproduces nitrates, sulfuric acid produces sulfates)− the metal in the base or alkali.
Ammonia dissolves in water to produce an alkaline solution. It is used to produce ammonium salts. Ammonium salts are important as fertilisers.
Hydrogen ions H+(aq) make solutions acidic and hydroxide ions OH– (aq) make solutions alkaline. The pH scale is a measure of the acidity or alkalinity of a solution.
In neutralisation reactions, hydrogen ions react with hydroxide ions to produce water. This reaction can be represented by theequation:
H+(aq) + OH– (aq) → H2O(l).
Now try these exam Now style questions
Acids, alkalis and salts1 Magnesium hydroxide, Mg(OH)2, is used in many antacids for relieving acid
indigestion.
(a) Magnesium hydroxide is slightly soluble in water.
(i) Give the formulae of the ions produced when it dissolves.
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(ii) Give a value for the pH of the solution it forms.
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(b) Write a word equation for the reaction of magnesium hydroxide with
hydrochloric acid.
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(c) Write a balanced symbol equation for the reaction.
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(d) Suggest why sodium hydroxide would not be suitable for use as a cure for
indigestion.
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2 Copper(II) sulfate crystals can be made from an insoluble base and sulfuric acid.
(a) Name the insoluble base that can be used to make copper(II) sulfate.
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(b) Describe how to make a solution of copper(II) sulfate from 25 cm3 of dilute
sulfuric acid so that all of the acid is used.
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(c) Describe how you could make crystals of copper(II) sulfate from the solution.
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3 Salts are formed when acids react with alkalis.
(a) Complete the word equation:
acid + alkali ………. + ………
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(b) What type of reaction takes place when an acid reacts with an alkali?
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(c) (i) Name the acid and alkali used to make potassium nitrate.
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(ii) What would you use to show when the acid had completely reacted with
the alkali?
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(iii) Write a balanced symbol equation for the reaction that takes place.
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4 The effluent from nickel plating works is treated with sodium carbonate to
precipitate nickel ions from the solution. The precipitate is separated from the
solution by settlement in a tank. Filtration is not usually used as the main method
of removing the precipitate, but can be used to remove small amounts of solids
from the effluent after settlement.
(a) Write a word equation for the reaction between nickel sulfate solution and
sodium carbonate solution.
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(b) Name the precipitate that is formed.
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(c) How is most of the precipitate removed from the effluent?
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(d) Suggest one reason why filtration is not used to remove most of the
precipitate.
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(e) Why is it necessary to remove metal ions like nickel from effluents?
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5 There are four main methods of making salts:
A Acid + metal
B Acid + insoluble base
C Acid + alkali
D Solution of salt A + solution of salt B
(a) A student wanted to make some sodium sulfate.
(i) Which method would be the best one to use?
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(ii) Explain why you chose this method.
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(iii) Name the reagents you would use.
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(iv) Write a word equation for the reaction.
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(b) Another student wanted to make some magnesium carbonate.
(i) Which method would you use for this salt?
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(ii) Explain why you chose this method.
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(iii) Name the reagents you would use.
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(iv) Write a word equation for the reaction.
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