C 3 Chemistryrevisionnotes
C 3 Chemistryrevisionnotes
C 3 Chemistryrevisionnotes
Guide to Chemistry
AQA Specification A
Unit Chemistry C3 CHY3H
Daniel Holloway
Contents
1 Acids & Bases
2 Energy Calculations
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5 Chemical Analysis
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Titrations
Adding an acidic solution to an alkaline solution will produce a neutralisation reaction. They
react together and neutralise each other, producing a salt in the process. When a
neutralisation reaction takes place, the quantities of each solution used must be correct,
because if a very strong acid and a very strong alkali were mixed, if there was more acid
solution, the whole alkali solution would be neutralised, but not all of the acid solution
would be so the mixture would become slightly acidic overall. We can measure precise
volumes of acids and alkalis needed to react with each other using titrations.
In the neutralisation reaction, the point at which the acid and the alkali have completely
reacted is called the end point. We can show the end point using a chemical indicator.
Indicators change colour over different pH ranges. We have to choose suitable indicators
when carrying out titrations with different combinations of acids and alkalis:
strong acid + strong alkali use any indicator
weak acid + strong alkali use phenolphthalein
strong acid + weak alkali use methyl orange
These are the steps to carry out a titration to calculate how much acid is needing to react
with an alkaline solution:
1 Measure an known volume of the alkali solution into a conical flask using a pipette
2 Add an indicator solution to the alkali in the flask
3 Now put the acidic solution into a burette. This long tube has measurements down
the side, and a tap on one end and can accurately measure the amount entering the
flask. So record the reading on the burette (i.e. starting volume)
4 Open the tap to release the acid solution. The solution from the burette is released
one drop at a time, alongside swirling of the flask to ensure the solutions are mixed
5 Keep repeating Step 4 until the indicator changes colour to let you know the acid and
the alkali have completely mixed
6 Record the amount of acid you entered by reading the measurement on the burette
Be sure to repeat the entire process two or three times at least to ensure accuracy.
The worked examples below are more complicated calculations involving titrations:
Because a reaction makes and breaks bonds, reactions are sort of both exo- and
endothermic. For this reason, it is the balance between exo- and endothermic reactions
which decides the overall reaction type; for example if more energy is released in the
making of new bonds than is taken in to break the bonds, it is overall exothermic because
the exothermic > endothermic.
The Greek letter delta (written as ) is often used in the sciences and maths to represent
change. In chemical energies, we use H to abbreviate energy change. So +H means
energy increases, -H means energy decreases.
The amount of energy needed to start a reaction
is called the activation energy of the reaction.
Adding a catalyst will significantly reduce this
amount of energy (see Rates of Reaction, C2).
This in turn increases the proportion of reacting
particles which will have enough energy to react.
This has many advantages, especially industrially,
as it means reactions are more efficient and the
catalysts are economical also.
Bond Energies
The energy required to break apart a bond between two particular atoms is known as bond
energy. Bond energies are measured in kJ/mol and we can use them to work out H in
energy calculations. Some of the most common bond energies are displayed below:
To calculate energy change we need to know: a) the amount of energy needed to break the
bonds between the atoms; and b) the amount of energy released in the formation of new
chemical bonds.
For example, the bond energy for an H-H bond is 436kJ/mol. This means that the bond
energy for forming a new H-H bond is -436kJ/mol.
These two energy level diagrams show H-H bonds being made and broken. The left diagram
shows an already bonded H-H bond being broken. This has a bond energy of +436kJ/mol, so
we write H = +436kJ/mol on the diagram next to the change in height arrow. The right side
is a diagram representing two separate hydrogen atoms bonding. Obviously, this is bond
making which releases energy so the energy change is -436kJ/mol, written the same way
as before, except with a minus sign.
To clarify, the left diagram is endothermic, the right is exothermic.
Making and breaking the same bond always involves the same amount of energy, just
different + and signs.
Solution:
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Solubility
We call the amount of solute which we can dissolve in a certain amount of solvent the
solubility of that substance. This is usually measured in grams (of solute) per 100g (of
solvent). The most common solvent used is water. Generally speaking, solubility of solid
solutes increases with temperature. A saturated solution is a solution in which as much
solute as possible has been dissolved. Heating the solution will allow more of the solvent to
be dissolved until it becomes saturated again. Of course, this means when the hot saturated
solution cools, some of the solute will have to come back so it crystallises back out of the
solution.
We can show the different amounts of solute which will dissolve into solution at different
temperatures using special graphs called solubility curves. These can be used to a) predict
how much solute will dissolve into a solvent at any given temperature, and b) predict how
much solute will form again when we cool down a hot solution.
The solubility curves for potassium nitrate, sodium
nitrate and sodium chloride are shown here. As you
can see, the solubility of each one increases with
temperature as the rule states but the rate of
increase differs between solutes. As you can see,
sodium chloride barely increases in solubility between
0C and 100C, whereas potassium nitrate increases
eightfold in the same period.
The thing that all of these solutes have in common is
that they are all solid solutes. The solubility of gases
works in exactly the opposite way as temperature
increases, the amount of solute which will dissolve into
solution decreases. However, pressure is another
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factor affecting the solubility of gases. So gas solubility only decreases with temperature as
long as the pressure is kept constant but if temperature is kept constant solubility of
gases increases as pressure increases.
The solubility curve here (note:
solubility curves may be straight lines)
shows the solubility of oxygen in
water at 10C. Temperature has to be
kept the same here, because if
temperature is not kept constant, it
has a knock-on effect of the changing
solubility. We measure the pressure in
atmospheres.
Water Hardness
Believe it or not there are different types of water coming through our taps in different
areas of the country. In certain places, when we wash with soap, the water forms a rich
lather easily, but in others it is more reluctant to this is because the water is hard. Hard
water makes it more difficult to wash, but also, more difficult to clean the bath or sink when
finished, because hard water contains dissolved substances which react with soap to form
scum. The scum floats on the water and will stick to the bath.
Generally speaking, hard water tends to contain calcium and
magnesium compounds. These get dissolved into the water
or rivers/streams when they run over rocks containing these
elements. For example, limestone which contains calcium
carbonate, gets dissolved in water droplets which even
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makes the water slightly acidic. The water is then taken to reservoirs, and forwarded to our
homes. It is the dissolved substances which react with hard water to form scum.
In terms of economic factors, hard water is more expensive because more soap is required
for the same wash. The soap reacts with the magnesium and calcium ions in the water,
forming salts called stearates (the chemical name for scum). Only after all of the calcium
and magnesium have reacted can the soap begin to form a lather and this is why so much
more soap is needed per wash.
The below simple equation shows the ion exchanges in the reactions:
Scum is not the only problem hard water can potentially cause. Pipes can suffer from scale
(also limescale). Scale is also common in heating elements and other parts of our hot water
system. Pipes which have a lot of scale eventually block up and stop functioning. The same
problem occurs in kettles. When it happens to them, they tend to become less energy
efficient by being more slow and heating to lower temperatures because scale is such a
poor conductor of heat.
The simple equation below shows limescale formation chemically:
Removing Hardness
The other type of water of course does not contain the dissolved substances that cause
scum and scale. This is soft water. We soften hard water by removing the calcium and
magnesium ions in the hard water. This benefits us in terms of washing our bodies, our
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clothes and heating our water, but of course, we are advised to continue drinking hard
water for health purposes. Industry is another reason to soften water, there are a number
of manufacturing concerns related to hard water. There are two main ways to soften
water
The first method is to use washing soda (chemical name sodium carbonate). All we do is add
washing soda to the hard water. When added, it precipitates out calcium and magnesium
ions as insoluble carbonates (see below). Once these ions which cause the hardness are no
longer in the solution, they cannot react with soap (to form scum, etc). This means the
water has become soft.
Ca2+ (aq) + CO32- (aq) CaCO3 (s)
Method number two is to use an ion-exchange column. This also removes the calcium and
magnesium ions from the hard water. They are columns containing sodium ions which are
exchanged for the calcium and magnesium ions in hard water when it is passed through.
This is good for homes, where the columns can be fitted in certain places to ensure all hard
water used for washing ourselves, our clothes and providing showering water is softened
(also, dishwashers contain their own water-softening system similar to this the majority of
the time). Once the sodium ions have been exchanged for the calcium and magnesium ones,
the column is washed with a salt solution to exchange the dumped calcium and magnesium
ions with more sodium ions, ready for the next time. For this reason, water-softeners must
be topped up with salt (sodium chloride) every so often to keep it functioning.
Water Treatment
There are so many uses for water for everyone all around the world. In a developed country
such as our own, we always need to treat the water to make sure its clean and safe. A
simple way of treating water is to use a filter jug around the home. This is fitted with a filter,
so that as you pour water in, it runs through the filter, purifying it. This normally contains
activated carbon, an ion-exchange column and some silver. Firstly, the carbon in the filter
reduces the levels of chlorine, pesticides and other organic impurities. Then the ionexchange column removes the dissolved substances as described earlier, including calcium,
magnesium, lead, aluminium and copper ions. Finally, the silver (fitted in most, but not all
filters) discourages bacterial growth in the filter, keeping it clean and functioning. However,
the filter usually needs changing every few weeks.
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Even after passing through a filter, water does not become pure. There are still dissolved
impurities in it. However, the water is still definitely cleaner after going through a filter
and it is always sensible to filter water you are putting into a kettle to be boiled, as it
reduces the limescale build up.
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than potassium (K) and so would be placed after potassium with the reactive metals but
argon is a noble gas! Therefore, argon was put before potassium, even if the argon atom
was heavier.
In the early 20th century, scientists began to look more closely at the atomic structure and
decided that the way to solve problems like the one described above was to arrange all the
elements in order of atomic number (or proton number). This put them in exactly the right
order, and they all are in their correct groups and periods. You can tell this nowadays
because of the way you know how certain elements behave based on electrons (depending
on what group they are in).
So the modern periodic table today is now a reliable model which we can use for many
functions, split into groups and periods.
Reactivity Groups
You should know that an elements placing in a group tells you something about its
reactivity, but the rules differ between the groups. The reason they all have similar
properties is because atoms in one group all share the same number of electrons in the
outer shell (energy level).
Within a group, the properties of elements are affected by the number of energy levels
beneath the outer one. As you go down a group, the size of the atoms gets bigger this is
because the number of energy levels from the nucleus has increased. Because the energy
levels are further away from the positive nucleus as you go down the group:
the larger atoms GAIN electrons LESS easily
the larger atoms LOSE electrons MORE easily
bigger, the single atom on the outer energy level (see the diagrams on the right) gets further
away from the nucleus and becomes less attracted to the positive nucleus. Therefore, it is
easier for that electron to be lost to another atom.
Alkali metals have quite a low density, in fact, lithium, potassium and sodium will float on
water. They are all also very soft, so can be cut with a knife. As for appearance, they are
shiny, metallic, however, when they react with oxygen, they turn a dull colour as they form
a layer of oxide.
They also melt and boil at fairly low temperatures compared to other metals. Melting and
boiling points decrease as you go down the group.
Alkali metals can only form ionic compounds. This means that they can only bond with nonmetals (see Atoms and Bonding, C2). They always react by losing their sole outer electron
and formed +1 ions.
These metals react with water, some more violently than others. Adding sodium, lithium or
potassium will cause the metal to float on the water and move around fizzing, because the
metal reacts with water to form hydrogen gas. Potassium reacts more violently than the
other two, because it is so vigorous the hydrogen actually catches fire and burns with a lilac
flame. The other three metals react insanely with water. Whenever an alkali metal reacts
with water, it also forms a metal hydroxide. These are soluble in water and give the solution
a high pH alkaline, hence the name alkali metals.
It is not just water that these alkali metals will react with. Other non-metals such as chlorine
will react with them. When they react with chlorine, chlorides are produced (white solids).
They dissolve into the solution and make a colourless solution. Again, the reactions get
more and more vigorous as they go down the group. You will already know the equation:
sodium + chlorine sodium chloride
The alkali metals react similarly with fluorine, bromine and iodine.
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complete their outer shell in ionic bonding. In this case, the two electrons have gone to the
chlorine one electron to each chlorine atom, because chlorine atoms only need one extra
electron each to complete their outer energy level.
The halogens melting and boiling points vary drastically. Melting and boiling points increase
as you go down the group.
We can use a more reactive halogen to displace a less reactive halogen from a solution of its
salt. For example, bromine displaces iodine from its solution because bromine is more
reactive, and chlorine will displace both iodine and bromine. Chlorine will displace bromine
if we bubble the gas through a solution of potassium bromide:
Cl2 + 2KBr 2KCl + Br2
Obviously, fluorine, the most reactive halogen, would displace all of the other halogens, but
since its reaction with water is so incredibly violent, we cannot do displacement reactions
involving fluorine.
Most of these elements have similar physical properties to each other, yet their properties
differ to other elements around the table. Their typical metallic structure explains the
majority of their properties. They have giant structures held together by metallic bonds,
with free roaming electrons (see Atoms and Bonding, C2). They are good conductors of heat
and electricity, like all metals, this is because delocalised electrons carry the current/heat
energy around the metal. They are also quite strong metals, but malleable. With the
exception of mercury (which is liquid at room temperature), they all have very high melting
and boiling points (usually ranging between 1200C and 2000C).
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In the transition elements, a lower energy level (or inner shell) is filled up between Group 2
and Group 3. This partly-filled lower energy level explains why transition metals form
brightly coloured compounds and results in their use as catalysts.
These metals are far less reactive than the alkali metals (Group 1). They do not react as
readily with oxygen or water, i.e. they corrode more slowly. Having these chemical
properties combined with the physical properties described above makes the transition
metals very suitable for structural materials. They are especially useful when mixed together
with each other or other elements to make alloys (see Rocks and Metals, C1). Iron mixed
with carbon in steels is a good example of an alloy. Other useful alloys of transition
elements include brass (a combination of copper and zinc) and cupro-nickel - a very hard
alloy of copper and nickel which is used to make coins in British currency.
Many of the transition metals form coloured compounds. For example:
potassium dichromate(VI) is orange, where the orange colour is there due to the
presence of chromium ions
copper(II) sulphate is blue, from copper ions
and potassium manganate(VII) is purple, from manganese ions
The colours produced play a part in our lives. For example, the colours of rocks, minerals
and gemstones are the direct result of transition elements ions. A red-brown colour in a
rock usually means there are iron ions present. Likewise, the blue in sapphire and the green
in emerald is due to the ions in the crystal-structure.
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Flame Tests
Identifying Group 1 and Group 2 metals is a piece of cake when we burn them, as they tend
to have unique flames which we can associate with the elements. We call these tests flame
tests. We perform a flame test by putting a small amount of a compound to be tested in a
platinum wire loop which has been dipped in hydrochloric acid, and then we hold the
substance over a blue Bunsen flame. The flame should show a particular colour which can
be used to identify the unknown substance.
As well as this, NaOH can be used to detect if ammonium ions (NH4+) are present in an
unknown substance. Ammonium ions react with NaOH to form ammonia and water:
NH4+ (aq) + OH- (aq) NH3 (aq) + H2O (l)
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Combustion Analysis
We can work out the empirical formula (see Chemical Calculations, C2) of an organic
compound by burning it and measuring the amounts of the formed products. For example,
an organic substance A contains hydrogen and carbon. A sample of A is burnt in an excess of
oxygen, producing 1.80g of water and 3.52g of carbon dioxide. To work out the empirical
formula for A we would:
1 Firstly, calculate the moles of carbon dioxide:
The relative atomic mass of carbon dioxide is 12 + (2 x 16) = 44g
Amount of carbon dioxide = 3.52 44 = 0.08 moles
2 Then calculate the moles of water:
The relative atomic mass of water is (2 x 1) + 16 = 18g
Amount of water = 1.80 18 = 0.1 moles
3 Each molecule of carbon dioxide formed requires one carbon atom from a molecule
of A. So for every mole of carbon dioxide formed, A must contain one mole of carbon
atoms
Amount of carbon atoms in sample of A = 0.08 moles
4 Similarly, every molecule of water formed requires two hydrogen atoms from A. So
for every mole of water formed, A must contain two moles of hydrogen atoms
Amount of hydrogen atoms in sample of A = 0.1 x 2 = 0.2 moles
5 So A contains carbon atoms and hydrogen atoms in the ratio:
0.08 : 0.20 = 2:5
6 The empirical formula of A is then C2H5
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Instrumental Analysis
There are a large number of reasons why instrumental analysis is a preferred method of
analysis. The ability to quickly, and still accurately, check products in industry to make sure
there are no contaminated substances or by-products in manufacturing is a particularly
beneficial advantage. The analysis of compounds is especially significant in terms of health
(e.g. kidney dialysis machines can build up dangerous levels of aluminium in the water, so
the water is often monitored to check the aluminium content is low).
The recent boom in terms of electronics and computing has aided the progression of
analysis, and new industrial methods have been developed. Instrumental methods are
preferred in general because they are highly accurate, quicker, and enable tiny quantities of
chemicals to be monitored.
However, the main disadvantages of these methods include it being very expensive, people
with special skills and a lot of training need to do it and it only gives results which can be
compared to those we already have.
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Chromatography
An example of a simple technique used to separate compounds within a mixture is
chromatography. This process determines the separate compounds based on how well they
dissolve in a particular solvent. We can tell their solubility based on how far they travel up a
piece of chromatography paper. Here are the different methods of chromatography
available:
gas-liquid chromatography to separate compounds which are easily vapourised
high performance liquid chromatography to separate compounds in solution
gel permeation chromatography to separate compounds according to the size of their
molecules
ion-exchange chromatography to separate compounds containing differently-charged
particles
After the substance has been tested, the results are compared to already known chemicals
to assess what they might be. If those are not available, an alternative is to use more
technical instruments to analyse the data.
Gel Electrophoresis
A technique called gel electrophoresis is used to analyse DNA. This technique is used for
commonly called DNA tests. They can be used to find out if people are related or to
identify if a suspect was at a crime scene by the police.
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Chemical Analysis
Chemical analysis can be used in society for a number of reasons. One of these reasons is to
stop doping (performance-enhancing drugs being used in sport). Chemical analysis can be
done to test if any of these have been taken by athletes. Athletes are advised to ask a
doctor about any medication, no matter how normal it looks, in case it contains these
agents and may be accused of cheating.
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