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New AQA UNIT 3 GCSE REVISION NOTES
The Periodic Table
History
At the start of the 1800s quite a lot of elements had been discovered, but there had been no
real attempt to organize them.
Newlands, an English chemist and Mendeleev, a Russian chemist both worked on the problem
by arranging the elements or order of their atomic weight (mass) and looking for patterns in
their properties.
Both were successful to some extent, but Mendeleev was much more successful because.
a) He recognized that there were some elements still to be discovered, so he left gaps
in his table
b) He was not afraid to change the order of some elements in the table where it
seemed to make sense.
c) He was able use the table to successfully predict the properties of some elements
which had not yet been discovered.
Later when more elements had been discovered and chemists realized that the elements
should be in order of atomic number rather than atomic mass, Mendeleev’s Table was shown
to be extremely successful.
The Modern Table
The periodic tables patterns are now known to be based on the structure of the atom.
Elements in the periodic table are arranged in order of atomic number (number of protons in
the nucleus)
The group number of an element shows the number of electrons in the outer shell.
The period number shows the number of shells of electrons
e.g. Sodium is element number 11 and has the structure 2.8.1 It therefore has 11 protons, it is
in Group1, period 3 of the periodic table.
GROUP 1 , The Alkali Metals.(Li, Na, K, Rb, Cs, Fr)
Similarities
1) All elements have 1 electron in the outer shell:
Li: 2.1 Na: 2.8.1 K: 2.8.8.1 etc.
2) For this reason they all form 1+ ions when they react:
Ions are Li+, Na+, K+ etc.
3) They have similar physical properties. All are very light, soft metals which are shiny when cut.
4) They have similar chemical properties. In particular they react vigorously with water giving
hydrogen and an alkaline solution of the hydroxide.
(e.g. 2Na + 2H2O = 2NaOH + H2)
For this reason, they are called alkali metals
Because of the violence of this reaction with water, alkali metals are stored under oil, and only
very small amounts should ever be reacted with water using safety screen, goggles etc.
Differences
1) The metals are all soft and light, but going down the group from Li to Cs, they
a) Have decreasing melting points (Li = 180oC, Na = 98oC, K = 64oC etc.)
b) Become softer
2) There is a definite increase in reactivity going down the group from Li to Cs. Li reacts quite
mildly, Na more vigorously, K quite violently etc.
This is because the atoms lower down have more inner shells of electrons. These 'shield' the
outer electrons from the attraction of the nucleus ,the outer electron is further away from the
positive nucleus reducing the electrostatic attraction this allows the outer electron to be lost
more easily. Since metals react by losing electrons, they become more reactive.
GROUP 7 , The Halogens. (F, Cl, Br, I, At)
Similarities
1) All are non-metals
2) They are diatomic (F2, Cl2, Br2 etc.)
3) All elements have 7 electrons in the outer shell:
F: 2.7 Cl: 2.8.7 Br: 2.8.18.7 etc.
4) For this reason they all form 1- ions when they react with metals
Ions are F-, Cl-, Br- etc.
(They get the extra electron by sharing if they combine with another non-metal)
Differences
APPEARANCE
MELTING POINT
REACTIVITY
Fluorine
Yellow gas
-220
Extremely reactive
Chlorine
Green gas
-101
Very Reactive
Bromine
Brown liquid
-7
Quite reactive
Iodine
Black solid
113
Not very reactive
Note:
1) The differences in appearance and state
2) The clear increase in melting point
3) The clear decrease in reactivity.
Reactivity.
Reactivity decreases down the group because it is harder for each atom to hold on to the new
added electron due to:
1. Decreased electrostatic attraction as the outer shell electron being added is getting
further away from the nucleus
2. Increase in the level of shielding as there are more inner electron shell0 decreasing the
electrostatic attraction
REACTIONS
1. They react readily with metals to form salts (chlorine does this most readily, then bromine,
then iodine)
e.g.
Mg + Cl2 = MgCl2
2Na + I2 = 2NaI
2. Like metals, a more reactive halogen will displace a less reactive one:
e.g. chlorine + potassium bromide = bromine + potassium chloride
Cl2 + 2KI = Br2 + 2KCl
or bromine + sodium iodide = iodine + sodium bromide
Br2 + 2NaI = I2 + 2KBr
THE TRANSITION METALS
This is the block which appears in the middle of the periodic table. It contains many of the
metals in everyday use, such as iron, nickel and copper.
Properties
These metals tend to be strong and dense, with a fairly high melting point.
Their reactions are similar to other metals, and their reactivity is medium to low.
Compounds
Like other metals, compounds are ionic, but many transition metals can have ions with different
charges.
e.g. Iron ions can be Fe2+ or Fe3+, so that it can form 2 sets of compounds called iron (II)
compounds and iron (III) compounds. e.g.
Iron (II) chloride contains Fe2+ ions and is pale green in colour
Iron (III) chloride contains Fe3+ ions and is brown-yellow in colour
Transition metal compounds have a variety of colours
Iron (II) compounds are pale green
Iron (III) compounds are yellow-brown
Copper compounds are blue
Nickel compounds are green
TITRATION CALCULATIONS (H)
1. The concentration of solutions is measured as the number of moles dissolved in one dm3 of
water (1dm3 = 1000cm3 = 1 litre). This concentration is given in mol dm-3 or is sometimes
written M.
e.g. if 2 moles of a substance are dissolved in 1 dm3 of water, the solution is said to have
a concentration of 2 mol/dm3.
2. The concentration of a solution can be found by using the relationship
Moles
Conc Vol
Concentration is measured in mol/dm3 and volume is measured in dm3
1000cm3 = 1 dm3, so always divide cm3 by 1000 before using this relationship.
e.g.
a) What is the molar concentration if 0.2 moles are dissolved in 250cm3 of water?
volume = 250/1000 = 0.25 dm3
concentration = 0.2/0.25 = 0.8 mol/dm3
Questions
Find the number of moles of
a) NaCl in 3 dm3 of a 0.1M solution.
b) CuSO4 in 200cm3 of a 0.2M solution.
c) HCl in 50cm3 of a 0.5M solution.
Complete the following table:
Moles
0.15
0.8
0.01
0.2
Concentration (mol dm-3)
0.1
0.5
2
0.5
Volume
3dm3
200cm3
1dm3
600cm3
CALCULATIONS FROM EQUATIONS
Once the number of moles has been calculated, the balanced equation can be used in the
normal way:
1) Use ' moles =conc x vol' to find number of moles of one substance.
2) Use 'big' numbers in equation to find moles of other substances.
3) Use conc = moles/volume to find the molar concentration of the other substance if
required.
Examples:
a) 250 cm3 of a solution of NaOH of concentration 0.1 moles dm-3 reacts with hydrochloric acid.
How much acid is used?
2NaOH + H2S04 = Na2SO4 + 2H2O
Moles = Concentration x Volume
Conc = 0.1M
Vol = 250/1000 = 0.25 dm3
So Moles of NaOH = 0.1 x 0.25 = 0.025
From the equation, the moles of H2SO4 is half this: 0.025/2 = 0.0125
b) 50 cm3 of a solution of sodium carbonate of concentration 0.2 moles dm-3 reacts with
hydrochloric acid. How much hydrochloric acid is used?
Na2CO3 + 2HCl= NaCl + H2O + CO2
Moles = Concentration x Volume
Conc = 0.2mol dm-3
Vol = 50/1000 = 0.05 dm3
So Moles of NaOH = 0.2 x 0.05 = 0.01
From the equation, the moles of HCl is twice this: 0.01 x 2 = 0.02 moles
c) 100 cm3 of a solution of sodium hydroxide of concentration 0.5 moles dm-3 reacts with
25cm3 of hydrochloric acid. NaOH + HCl= NaCl + H2O
i) How many moles of hydrochloric acid is used?
ii) What is the concentration of the hydrochloric acid?
i) Moles = Concentration x Volume
Conc = 0.5mol dm-3
Vol = 100/1000 = 0.1 dm3
So Moles of NaOH = 0.5 x 0.1 = 0.05
From the equation, the moles of HCl is the same: 0.05 moles
ii) For the HCl, Concentration = Moles/ Volume
Moles = 0.05 Volume = 25/1000 = 0.025 dm3
Concentration = 0.05/0.025 = 2 mol dm-3
WATER CHEMISTRY
WATER
Water is the most common chemical on the planet. One of its most important properties is as a
solvent. It is an extremely good solvent for a wide variety of chemicals, and for this reason it is
difficult to obtain pure.
Sea water is formed when rainwater runs through rocks & soils, dissolving out minerals. These
are taken by rivers to the seas where the minerals are concentrated through evaporation. The
most common is sodium chloride (salt) but there are many others as well.
HARD & SOFT WATER
Hard water has the following properties:
1) It is difficult to make a good lather.
2) Scum is formed with soap.
3) Lime scale forms in kettles, pipes etc.
Hard water contains Ca2+ or Mg2+ ions in solution. These ions react with soap to form scum.
This is because soap is sodium stearate, which reacts with calcium ions to form insoluble
calcium stearate which is scum.
Calcium is usually dissolved in the form of calcium sulphate or calcium hydrogen carbonate.
If calcium hydrogen carbonate is present, it can be removed by boiling. This decomposes the
calcium hydrogen carbonate to calcium carbonate, which forms scale. This is called temporary
hardness.
If other calcium /magnesium compounds are present, hardness cannot be removed by boiling.
This is called permanent hardness (although, it can in fact be removed by other watersoftening methods).
The hardness usually comes about by rainwater falling on limestone or chalk. Rainwater is
made acidic by dissolved CO2, and this dissolves the calcium carbonate to form soluble calcium
hydrogen carbonate.
CaCO3 + H2O + CO2 = Ca(HCO3)2
The presence of dissolved Ca2+ ions from the calcium hydrogen carbonate makes the water
hard.
If water containing calcium hydrogen carbonate is heated, insoluble calcium carbonate is
formed again as lime scale in your kettle, pipes etc.
Ca(HCO3)2 = CaCO3 + CO2 + H2O
Some people choose to soften their water because
1) Less soap is used.
2) No scum is formed.
3) No lime scale is formed.
WATER QUALITY
Temporary and permanent hardness
We have seen how scale can form inside a kettle. This has the effect of softening the hard water
as the calcium/magnesium ions are removed. Water that can have it hardness removed by
boiling is called temporary hard water.
Temporary hard water will contain Hydrogencarbonate ions(HCO3-)
When temporary hard water is heated the hydrogencarbonate ions decompose to form
carbonate ions( CO32-). These carbonate ions react with the calcium and magnesium ions in the
temporary
water and
form
precipitate
s and are
so
removed
from the
watersoftening
the water.
Boiling will not remove hardness from permanent hard water. The Following methods are
ways we can remove the hardness from permanent water
1. Treating with washing soda (sodium carbonate) this works by precipitating out the
calcium and magnesium ions, which means if the ions are no longer present they can
not react with soap to from a scum, or form limescale in appliances such as dish
washers. This means the water is soft.
Ca2+(aq)
(hardness)
+
CO32-(aq) →
(from Na2CO3)
CaCO3 (s)
2. Using an ion-exchange column.
Water can also be softened by removing the calcium and magnesium ions using an ionexchange column. These columns contain lots of sodium ions which are exchanged for calcium
and magnesium ions in hard water as it passes through the column. This is how domestic water
softening units work. They use them to remove the hardness of all the water they use to
prevent limescale forming in pipes and boilers. Once all the sodium ions have been removed
from the resin in the column they can be replaced by washing it with a salt solution, this
exchanges the calcium & magnesium ions for sodium ions.
Water treatment
Providing people with clean drinking water which is uncontaminated by diseases, sewage or
chemicals is a major issue all over the world.
Water that comes from bore holes is usually fairly clean because it has been filtered by the
rocks around the around the boreholes. Normally we just have to disinfect/sterilise this water
with chlorine.
When we take water from rivers and reservoirs we need to give it more treatment than this.
This treatment involves a number of physical and chemical processes. To start with water is
chosen that contains as few dissolved chemicals as possible. The water then passes through the
five stages shown in the water treatment diagram( at bottom of page).
Even with modern treatment of water some people like to treat it further in their home by
using a filter jug to improve the taste and quality. These contain activated carbon, an ionexchange column and silver.

The carbon reduces the levels of chlorine, pesticides and other organic impurities in the
water.
 The ion-exchange column removes calcium, magnesium, lead, copper and aluminium
ions.
 The silver discourages the growth of bacteria in the cartridge.
In most jugs the cartridge needs to be changed every few weeks.
Even water that has been treated and passed through a filter jug is not pure. It will still contain
many dissolved substances. But despite this it is safe to drink. We can get totally pure water by
distilling(boiled then condensed) it, which is what we often do if we need pure water in a
chemistry lab. It would be far too expensive, as it would require lots of energy, to distil water
for domestic use and it would have no taste.
Fluoride is also added to the water in some areas to prevent tooth decay
Pure water can be produced by distillation but this requires lots of energy so is very expensive
ENERGY, FUELS AND POLLUTION
EXOTHERMIC reactions are reactions which release heat or other forms of energy to the
surroundings. Most reactions are exothermic, the most obvious ones are combustion reactions.
Other examples are respiration & fermentation
e.g. Magnesium + oxygen = Magnesium oxide
When an exothermic reaction occurs, the products of the reaction have less energy than the
reactants, since some energy has been released. This can be shown on an 'Energy level
diagram'
Magnesium + Oxygen
Energy
Heat given out to surroundings (-1245kJ)
Magnesium oxide
e.g. 2Mg + O2 = 2MgO
ΔH = -1254 kJ/mole
exothermic reactions.
ENDOTHERMIC reactions are reactions which take in heat from the surroundings the most usual
examples are thermal decompositions, i.e. reactions in which a substances is split up by heat.
Another example is photosynthesis.
e.g. Calcium carbonate = Calcium Oxide + carbon dioxide
When an endothermic reaction occurs, the products of the reaction have more energy than the
reactants, since some energy has been taken in. This can be shown on an 'Energy Profile'
Energy
Calcium oxide + Carbon dioxide
Heat taken in from surroundings (+345kJ)
Calcium carbonate
The energy change is represented by Δ H, which is measured in kJ per mole, and is positive for
endothermic reactions. e.g.
CaCO3 = CaO + CO2 Δ H = +345 kJ/mole
Practical Measurement of Energy of Reaction
When a reaction is carried out, it is usually carried out in such a way that the heat evolved is
transferred to some water (in some cases a dilute solution is used in place of water, but the
effect is very similar). The formula used is
q= mcΔT
Where q= Energy change is measure in Joules
m = mass in grams of water or solution whose temperature is raised.
(remember 1cm3 of water weighs 1g)
c = heat capacity of water (usually given as 4.2 J/K/g)
ΔT = change in temperature
If the value of ΔH is required in kJ, the enthalpy change is divided by 1000
If the value is required is in kJ /mol, then divide by the number of moles of reactant.
i.e.
ΔH = mcΔT
where n is the number of moles used.
1000 n
Remember that if the temperature increased, the reaction is exothermic (ΔH -ve) and vice
versa. May need to work out the energy content of a certain mass of fuel- so always work out
the energy content of 1g and then then you can work out the energy in any amount, the same
method can be used to work out the energy per mole, just work out the Mr of the fuel, and that
will give you the mass in a mole, and multiple by the energy in 1g
Energy level diagrams/bond breaking and bond making
We can find out more about what’s happening in a reaction by looking at the energy level
diagram. These diagrams show us the relative amounts of energy of reactants and products. The
energy is measured in Kilojoules per
moles (KJ/mol)
Exothermic reactions
This diagram shows the energy level
diagram of an exothermic reaction. The
products are at a lower energy level than
the products. So going from reactants to
products energy has be lost/releasedexothermic.
The difference in energy between
reactants and products is released into the
surroundings, resulting in the
surroundings getter hotter- a sign of an exothermic reaction
Endothermic reactions
This diagram shows the energy level diagram of an endothermic reaction. Here the products are
at a higher energy level than the reactants. As the reactants form products energy is absorbed
from the surroundings. This means the
surrounding get colder and the products are
at a higher energy level.
Activation energy and catalysts
From the rate of reaction topic we know that the
activation energy is the minimum about of energy
required to start a reaction. We can show the activation
energy on an energy level diagram
Catalysts can increase the rate reaction. The way they
do this is they provide an alternative pathway to the
products, which has a lower activation energy. This
means a greater proportion of reactant particles will
have the activation energy. This can be shown on an
energy level diagram.
Bond breaking and bond making
In a chemical reaction, we can think the chemical bonds
between the reactant atoms being broken and new bond
forming to produce new products.
 Energy must be supplied to break bonds. So bond
breaking is an endothermic process. Energy is
taken in from the surroundings
 When new bonds form, energy is released. So
making bonds is an exothermic reaction process
The above diagram shows the reaction of Hydrogen and oxygen to produce water. It shows that 2
H-H bonds and 1 O=O must be broken and 4 new H-O must be made.
Bon
d
Energies
1. When a chemical reaction occurs, the bonds between the atoms in the reactants must be
broken, before the new bonds in the products can be formed.
e.g. H2 + Cl2 = 2HCl
Showing the bonds: H-H + Cl-Cl = H-Cl + H-Cl
The bonds between the 2 hydrogen atoms in H2 and between the 2 chlorine atoms in Cl2 must
be broken before new bonds between H & Cl can be made
The process of bond breaking is endothermic (takes in energy)
The process of bond making is exothermic (gives out energy)
To calculate the energy change for the above reaction, we need to know the bond energies:
Bond energy of H-H bond = 436 kJ/mole
Bond energy of Cl-Cl bond = 242 kJ/mole
Bond energy of H-Cl bond = 431 kJ/mole
Energy needed to break 1 H-H bond and 1 Cl-Cl bond = 436 + 242 = 678 kJ
Energy given out when 2 H-Cl bonds are formed = 2 x 431 = 862 kJ
The difference is 862-678 = 184 kJ given out.
So we can write
H2 + Cl2 = 2HCl
ΔH = -184 kJ/mole
(remember that a minus ΔH means an exothermic reaction)
Activation Energy
Many exothermic reactions need a small input of energy before the main reaction will start
(e.g. the match that lights the bonfire). This is called ACTIVATION ENERGY, and is needed to
start breaking the bonds in the reactants so that new ones can be formed.
Endothermic Reaction
Exothermic Reaction
Fuels &
Pollution
Pollution of
the
atmosphere is
mainly caused
by the
burning of
ACTIVATION
ENERGY
REACTANTS
ENERGY OF
REACTION
PRODUCTS
ACTIVATION
ENERGY
PRODUCTS
ENERGY OF
REACTION
REACTANTS
fossil fuels (coal, oil and gas). Power stations use huge amounts of these fuels and so are a
major source of pollution. Cars and lorries are another major source of atmospheric pollution,
however the use of fuels is essential for our economy, and viable alternatives to fossil fuels will
need to be developed.
CHEMICAL ANALYSIS
TESTS FOR METAL IONS
FLAME TEST
Clean a piece of nichrome wire with sandpaper, then water, and dip it into your test substance.
Now hold the wire at the edge of a blue Bunsen flame.
Bright orange flame - SODIUM (Na+)
Lilac - POTASSIUM (K+)
Crimson - LITHIUM (Li+)
red
- CALCIUM (Ca2+)
Apple green – BARIUM (Ba2+)
TESTING WITH SODIUM HYDROXIDE
Since most metal hydroxides are insoluble, adding sodium hydroxide solution to a solution of a
metal salt will give a precipitate.
e.g. Copper sulphate + Sodium hydroxide = Copper Hydroxide + sodium sulphate.
CuSO4(aq) + 2NaOH(aq)
= Cu(OH)2(s) + Na2SO4(aq)
The copper hydroxide is seen as a blue precipitate.
Method
The substance is dissolved in water. Sodium hydroxide solution is added until in excess.
White Precipitate which does not redissolve in excess NaOH
CALCIUM (Ca2+) or MAGNESIUM (Mg2+) must be present, as Ca(OH)2 and Mg(OH)2 are both
white and insoluble.
White Precipitate which dissolves in when excess NaOH is added
ALUMINIUM (Al3+) present. Al(OH)3 is insoluble in water, but soluble in sodium hydroxide
solution.
Coloured Precipitate
COPPER (Cu2+) ions give a blue Precipitate of Cu(OH)2
IRON (II) (Fe2+) ions give a pale green Precipitate of Fe(OH)2
IRON (III) (Fe3+) ions give a brown Precipitate of Fe(OH)3
TESTS FOR NON-METAL negative IONS
1) Test for Carbonates
Add dilute acid.
If it fizzes, test for CO2.with lime water (goes cloudy). This indicates a CARBONATE (CO32-)
2) Test for Halides (Chloride, Bromide & Iodide)
Add a little nitric acid, followed by a few drops of silver nitrate solution.
CHLORIDE (Cl-) gives a white Precipitate of AgCl,
BROMIDE (Br-) gives a cream coloured Precipitate of AgBr
IODIDE(I-) gives a yellow Precipitate of AgI
3)Test for Sulphates
Add a little dilute HCl, followed by barium chloride solution.
SULPHATE (SO42-) gives a white Precipitate of BaSO4.
Haber process
Making Ammonia- the Haber process
We need plants for food and to maintain the amount of oxygen in the atmosphere. Plants need
nitrogen to grow, even though the atmosphere contains 80% nitrogen most plants can’t use it
directly.
Instead plants absorb nitrates from the soil through their roots. When these plants are harvested
the farmer needs to replace the lost nitrates in the soil. He does this by adding fertilizers to the
soil.
The Haber process
We can use this process to make ammonia, which can be used as a fertilizer. We use this process
as a way of turning nitrogen in the air into ammonia.
The raw materials needed to make ammonia are:
1. Nitrogen from the air
2. Hydrogen, we can get from natural gas (methane, CH4)
The hydrogen and nitrogen are purified and passed over an iron catalyst at a temperature of
450oC and at a pressure of 200 atmospheres. The product of this reversible reaction is ammonia.
The reaction is reversible so the ammonia will break down and form nitrogen and hydrogen
again. To stop this the ammonia is removed by cooling the gases so that the ammonia liquefies
and it can be separated from the unreacted nitrogen and hydrogen which remain as gases and are
recycled back into the reacting mixture.
N2(g)
+
3H2(g)
⇌
2NH3(g)
nitrogen
hydrogen
ammonia
we carry out the Haber process at
certain conditions. We have certain
conditions to consider:
1. The yield of ammonia
2. The cost
3. The rate of the reaction
4. The safety of the reaction
Altering conditions
Pressure and equilibrium
In closed system with a reversible reaction. If we alter the conditions, equilibrium will try to
cancel/reverse any change we make. It will do this by changing the rate of the forward or reverse
reactions to oppose our change.
This is true if we change concentration, as we’ve already seen. It is also true if we change the
pressure
In many reversible reactions, there are more molecules of gas on one side of the reaction the
other, so by changing the pressure we carry out the reaction at we can increase the amount of
Products
If the forward reaction produces more
molecules of gas………..
……an increase in pressure decreases the
amount of products
……a decrease in pressure increases the
amount of products
If the forward reaction produces fewer
molecules of gas…….
….an increase in pressure increases the
amount of products
…..a decrease in pressure decreases the
amount of products
In the example of:
2NO2(g)
Brown gas
⇌
N2O4(g)
pale yellow gas
We can see that there are 2 molecules of gas on the left –hand side and only 1 molecule of gas on
the right-hand side.
If we increase the pressure of the reaction, equilibrium will shift to reduce that pressure increase
(cancel/oppose it). It will move in favour of the reaction which produces the least gas molecules.
In this case the right side, so it will favour the forward reaction. So more N2O4 is made and the
reaction turns lighter.
Temperature (energy) and equilibrium
In a closed system reversible reaction at equilibrium the relative amounts of the reactants and
products depends on the temperature.
As before, if alter the reactions conditions, Equilibrium will shift to cancel/reverse that change.
So if we increase the temperature of the reaction -equilibrium will shift to lower
temperature, this means it will favor the reaction direction which is endothermic- as this
reduces temperature
If we were to decrease temperature, the equilibrium will shift to increase temperature, so it
will favor the reaction direction which is exothermic- as this increases temperature.
If the forward reaction is exothermic…….
…An increase in temperature will decrease
the amount of products formed
….A decrease in temperature will increase
the amount of products formed
If the forward reaction is endothermic….
….an increase in temperature will increase
the amount of products formed
….a decrease in temperature will decrease
the amount of products formed
In reaction example of;
2NO2(g)
⇌
N2O4(g)
The forward reaction is exothermic and thus, the reverse reaction must be endothermic.
So, if we were to lower the temperature of the reaction, we would shift equilibrium to right as it
would favor the exothermic reaction as this would increase the temperature. So more N2O4
would be made
The economics of
The balanced
the Haber process
reaction for the Haber process is:
N2(g)
+
3H2(g)
⇌
2NH3(g)
The effect of
pressure
As the balanced
equation shows there are 4 molecules of gas on
the left-hand side
only 2 molecules of gas on the right-hand side.
This means an
increase in pressure will shift equilibrium to the
right and favor the production of more ammonia.
So we could use a very high pressure to produce a high yield of ammonia. However, very high
pressures needs lots of energy/costs and can be dangerous resulting in explosions. Very
expensive equipment would be needed. So a as compromise, the reaction uses a lower pressurethis gives a lower yield/amount of ammonia but it does reduce the costs and makes the reaction
safer.
The effect of temperature
Exothermic
N2(g)
+
3H2(g)
⇌
Endothermic
2NH3(g)
The effect of temperature on the Haber process is a bit more complicated. As the forward
reaction is exothermic. This means that a low temperature will favor the production of a high
yield of ammonia.
However, at a low temperature the rate of reaction will be too slow, and we need to make
ammonia quickly.
So we make another compromise. We use a reasonably high temperature, 450oC, this allows a
reasonably high rate of reaction but it does lower the yield of ammonia.
A lower temperature also reduces the effectiveness of the iron catalyst.
The effect of a catalyst
We use an iron catalyst to speed up the reaction rate. This lowers the activation energy, and
allows the reaction to take place at the reasonable temperature of 450oC. The catalyst does not
affect the actual yield of ammonia but allows it to be made faster.
Chemical analysis
Chemists have instruments to analyse unknown substances. These can be qualitative tests, like
flame tests, that tell what is in a sample. The can
also so be quantitative tests, like titrations
which can tell you how much ions are present.
When instrumental or ‘wet” chemistry is used
the accuracy of the data collected depends on
the skill of the tester.
Analysis in forensic science
Forensic science uses qualitative and quantitative analysis. They solve crimes by analyzing;
 drugs
 paints
 remnants of explosives
 fire debris
 gunshot residues
 fibres
 soil samples
 toxic samples
 biological samples
 DNA
A technique called Gel electrophoresis is used to analyse DNA. Analysing DNA with this
technique produces a plate which carries a number of bands according to the composition of the
DNA. The bands are unique to each person (except identical twins).
Analysis in pollution control
Environmental scientists need to monitor environmental pollution. For example, if a river gets
polluted they will test the water and trace the origin of the pollution.
Analysis in medicine
Another use of genetic fingerprinting is the treatment of
leukemia (a blood disease). Bone marrow is transplanted
from a healthy donor to the patient. After the operation
samples of blood from the donor and patient or analysed for
their DNA. The doctors are looking for a match between the
two bands on the electrophoresis plates. If they find this
then the transplant has been successful.
Doctors can also study the concentrations of metal ions in
the body of a few parts per million(PPM) in patients. They can look for the concentrations of
cobalt and chromium ions in the blood of patients with hip replacements. A concentration of
metal ions above 7PPM indicates that the hip replacement will fail
Structure of alcohols, carboxylic acids and esters
Organic compounds all contain Carbon. We have already met some as alkanes and alkenes.
There are over families of organic compounds, these include:
1. Alcohols
2. Carboxylic acids
3. Esters
Alcohols
Imagine removing a H from an alkane molecule and replacing it with a O-H group. This would
give us an alcohol. The O-H group is an example of a functional group. The functional groups
identify what family the molecule comes from.
A family of molecules with the same functional group is called a Homologous group.
You need to know the first 3 members of the homologous series of alcohols
CH3OH
C2H5OH
C3H7OH
The general formula of an alcohol is CnH2n+1 OH
If you were asked for the structure of ethanol, you would probably say C2H5OH, which is
correct. This is the molecular formula
However, there is also the structural formula. This shows what is bonded to each carbon atom,
so the structural formula of ethanol becomes CH3CH2OH.
Alcohols are often used as solvents and fuels
Carboxylic acids
Ethanoic acid is a carboxylic acid and it is the main acid in vinegar. All carboxylic acids contain
the functional group –COOH, the first 3 members of the homologous series of carboxylic acids
are;
CH3CH2COOH ( C2H5COOH)
Carboxylic acids will react with carbonates to form carbon dioxide and will form solutions of a
pH less than 7 when dissolved in water
Carboxylic acids can be called weak acids as they will not be completely ionised in water, that
means it will not form as much H+ ions as say Hydrochloric acid would, as it form a lot more H+
ions when dissolved in water
Esters
Esters are closely related to carboxylic acids. If we replace the H atom in the –COOH group with
a hydrocarbon group we get an ester. Here is the ester called ethly ethanoate
CH3COOCH2CH3 (structural formula)
CH3COOC2H5 (molecular formula)_
An ester’s structural formula always has the functional group –COO- the structural formula of
ethyl ethanoate is CH3COOCH2CH3 or CH3COOC2H5
Esters are quite sweet smelling- used in flavourings and perfumes
Properties and uses of alcohols
Ethanol is the main alcohol in alcoholic drinks. Alcohols are also important as fuels.
Alcohols will dissolve in solution, water, to make neutral solutions. Alcohol can also dissolve
other organic compounds so are very useful as solvents.
Combustion of alcohols
The use of ethanol, and methanol, as fuels shows that alcohols are flammable. Ethanol is used in
spirit burners. It burns with a clean blue flame
Ethanol
C2H5OH
+
oxygen
+
3O2
carbon dioxide
+
water
2CO2
+
3H2O
Reactions with sodium
When sodium is added to alcohol, it reacts in a similar way when sodium is added to water. The
sodium fizzes and hydrogen gas is given of.
Oxidation
Combustion is one way to oxidise an alcohol. However, when we use chemical oxidising agents,
like potassium dichromate(vi), we get different products. An alcohol is oxidised to a carboxylic
acid when boiled with an acidified potassium dichromate(vi) solution.
So ethanol can be oxidised to ethanoic acid. The same reaction takes place when ethanol is left
exposed to air. Microbes in the air produce ethanoic acid from ethanol. This is why, when bottle
of beer or wine is left open they begin to smell like vinegar
Carboxylic acids and esters
The most well-known carboxylic acid is ethanoic acid - vinegar
Carboxylic acids form acidic solutions, less than pH 7, when
dissolved in water. Carboxylic acids also have typical reactions
with carbonates- they produce carbon dioxide gas:
Ethanoic acid + sodium carbonate
+ water + carbon dioxide
sodium ethanoate
Carboxylic acids are weak acids
Carboxylic acids are weak acids compared to Hydrochloric acid which is a strong acid. Acids
must dissolve in water to show their acidic properties. This is because in water the acids ionisewhich means the acids split into a negative ion and the H+(aq) ion. It is the H+ (aq) ions that
make a solution acidic. The greater the concentration of H+ ions the more acidic the solution
is.( lower pH)
Hydrochloric acid will completely ionise in water, producing a high concentration of H+ ions.
The reaction is:
H+(aq)
HCl (aq)
+ Cl- (aq)
However, with a carboxylic acid it will not completely ionise in water, the reaction is
⇌
CH3COOH(aq)
CH3COO-(aq)
+
H+(aq)
An Equilibrium is reached where the molecules and ions are both present
Only some of molecules split into ions, and so less H+ ions are formed, so carboxylic acids form
weak acids and have a higher pH than strong acids like hydrochloric acid
Making esters
Carboxylic acids will react with alcohols to make esters. Water is also formed in the reversible
reaction. An acid, usually sulphuric acid, is used as a catalyst.
Ethanoic acid
+ ethanol
⇌
ethyl ethanoate + water
In general, the reaction is
Carboxylic acid + alcohol ⇌ ester +
Here is another example:
water
Strong acid catalyst
Ethanoic acid
+
methanol
⇌
methyl ethanoate
+
water
The esters formed have distinctive smells, many smell sweet and fruity, so they are used in
perfumes and food flavourings.
Ethanol in drinks
An alcoholic drink can be good in moderation. However, too many people are drinking more that
the recommended amount, binge drinking, alcoholics. This puts their health at risk. Alcohol is
linked to high blood pressure and heart disease. Excess alcohol can cause liver damage, in
extreme cases, a liver transplant is the only way to avoid death.
Alcohol is an acceptable drug. However, like other drugs, people can become dependent on it.
Alcoholics are addicted to ethanol.
Ethanol is used to methylated spirits as a solvent. This has toxic methanol added to it, it also
contains emetics (substances that make you vomit). But some addicts will drink this.
Drinking ‘meths’ causes liver failure, blindness and an early death. Other chemicals are added to
the meths to make it difficult to distil of the ethanol. These other chemicals have a similar boiling
point to ethanol so people can’t separate the ethanol to drink.
Alcoholic drinks are more expensive than methylated spirits because they have a tax added on.
The government use the income for many good causes. However, we should weigh up the costs
of dealing with:
 The health problems
 Days lost at work
 Policing antisocial behaviour
Ethanol and esters as biofuels
Ethanol can be used as a biofuel. It is made by fermenting sugars from crops. We also looked at
biodiesel. This is made from plant oils which are esters.in processing these esters, the oils are
broken down in to long chain carboxylic acids. They are reacted with methanol or ethanol (in the
presence of a strong acid catalyst) to make an ester used as a biofuel.
However, the land used for biofuel crops could be used for food crops- leading to food shortages
The new farm land is often made by cutting to rain forests. This destroys habitats and wildlife,
and contributes to an increasing amount of carbon dioxide in the atmosphere. An alternative to
crude oil is needed but at what cost?
Fuel Issues
Industrial societies around the world rely heavily on fossil fuels for their energy, but our supplies
of fossil fuels are running out and they cause a lot of CO2 pollution. So the search for new
alternative fuels is becoming more urgent.
Hydrogen-powered vehicles
Scientists are developing hydrogen as a fuel. It burns well and produces no pollution
Hydrogen + oxygen
2H2
+ O2
water
2H2O
This could help reduce global warming as no carbon dioxide is produced. However, there are
problems of safety and storage. And it is very expensive and there is no real infrastructure in
place in society to provide hydrogen- like we have petrol stations.
Supplying the hydrogen to burn in car engines is also a problem. If we use electrolysis, then
generating the electricity from non-renewable fossil fuels does not help the environment.
Hydrogen Fuel Cell
A more efficient use of the energy oxidising hydrogen is in a fuel cell. These cells are supplied
with hydrogen and oxygen which produces water. Most of the energy released is transferred to
electrical energy and this can run the car.
The challenge is to match the performance, convenience and price of petrol or diesel.
Titrations
An Acid added to an Alkali (a soluble base) react together and neutralize each other, they form a
salt (plus water) in the process.
Suppose we add a strong acid and strong alkali. The solution will produce a neutral solution only
if we add the exact same amounts of acid and alkali.
If we start off with more acid than alkali the final solution will be acidic.
If we start off with more alkali than acid the final solution will be alkaline.
We can measure the exact volume of an acid and alkali needed to neutralize each other with a
technique called Titration. The point at which the acid and alkali reacted completely is called
the End point- we show this by using an indicator. All readings from the burette and pipette are
taken from the bottom of the meniscus
C3
revision
booklet