Unit 3 Full notes

Everything you need to know for the unit 3 exam in may 2013. It also includes organic chemistry.

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Chemistry Unit 3
AQA GCSE Chemistry Syllabus
C3.1.1 The early periodic table
C3.1.2 The modern periodic table
C3.1.3 Trends within the periodic table
The modern periodic table has been developed from work begun by Newlands and Mendeleev.
There are trends in chemical properties within the periodic table linked to how easily the element
gains or loses electrons.
C3.1 The periodic table
a) Newlands, and then Mendeleev, attempted to classify the elements by arranging them in order of
their atomic weights. The list can be arranged in a table so that elements with similar properties are
in columns, known as groups. The table is called a periodic table because similar properties occur at
regular intervals.
b) The early periodic tables were incomplete and some elements were placed in inappropriate
groups if the strict order of atomic weights was followed. Mendeleev overcame some of the
problems by leaving gaps for elements that he thought had not been discovered.
C3.1.2 The modern periodic table
a) When electrons, protons and neutrons were discovered early in the 20th century, the periodic
table was arranged in order of atomic (proton) numbers. When this was done, all elements were
placed in appropriate groups.
b) The modern periodic table can be seen as an arrangement of the elements in terms of their
electronic structures. Elements in the same group have the same number of electrons in their
highest occupied energy level (outer shell).
C3.1.3 Trends within the periodic table
a) The elements in Group 1 of the periodic table (known as the alkali metals):
are metals with low density (the first three elements in the group are less dense than water)
React with non-metals to form ionic compounds in which the metal ion carries a charge of
+1.
The compounds are white solids that dissolve in water to form colourless solutions
react with water, releasing hydrogen
Form hydroxides that dissolve in water to give alkaline solutions.
b) In Group 1, the further down the group an element is:
the more reactive the element
the lower its melting point and boiling point
c) Compared with the elements in Group 1, transition elements:
have higher melting points (except for mercury) and higher densities
are stronger and harder
are much less reactive and so do not react as vigorously with water or oxygen
d) Many transition elements have ions with different charges, form coloured compounds and are
useful as catalysts.

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Chemistry Unit 3
e) The elements in Group 7 of the periodic table (known as the halogens) react with metals to form
ionic compounds in which the halide ion carries a charge of ­1.…read more

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Chemistry Unit 3
C3.2.1 Hard and soft water
a) Soft water readily forms lather with soap. Hard water reacts with soap to form scum and so more
soap is needed to form lather. Soapless detergents do not form scum.
b) Hard water contains dissolved compounds, usually of calcium or magnesium. The compounds are
dissolved when water comes into contact with rocks.
c) There are two types of hard water. Permanent hard water remains hard when it is boiled.
Temporary hard water is softened by boiling.…read more

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Chemistry Unit 3
However if calcium (Ca2+) or magnesium (Mg2+) ions which are positive cations are present then
their presence produces permanent hard water.
Temporary hard water is easy to treat, boiling the water is sufficient.
Permanent water requires more sophisticated water softening methods.…read more

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Chemistry Unit 3
When heated to 1000c, only the water boils, it turns into a gas, which is then cooled down so it
condenses back into a liquid. Any ion impurities remain in the original heating vessel.
Because this method involves heating up water this is a very expensive method and an
uneconomical way of producing large quantities of water.
Water treatment
All water has to be treated before humans can consume it.…read more

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Chemistry Unit 3
f) In an exothermic reaction, the energy released from forming new bonds is greater than the energy
needed to break existing bonds.
g) In an endothermic reaction, the energy needed to break existing bonds is greater than the energy
released from forming new bonds.
h) Catalysts provide a different pathway for a chemical reaction that has lower activation energy.
i) Hydrogen can be burned as a fuel in combustion engines.…read more

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Chemistry Unit 3
In an endothermic reaction the total bond energy of the reactants is less than the total bond energy
of the products, this extra energy. So in photosynthesis there is less energy in the reactants water
and carbon dioxide, than there is in the products glucose and oxygen
Activation energy
This means the energy required to start a reaction. It is represented as EA, in energy level diagrams
and calculations.…read more

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Chemistry Unit 3
e) Halide ions in solution produce precipitates with silver nitrate solution in the presence of dilute
nitric acid. Silver chloride is white, silver bromide is cream and silver iodide is yellow.
f) Sulfate ions in solution produce a white precipitate with barium chloride solution in the presence
of dilute hydrochloric acid.
g) The volumes of acid and alkali solutions that react with each other can be measured by titration
using a suitable indicator.…read more

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Chemistry Unit 3
3) Work out the concentration of the unknown substance
Concentration = number of moles / volume
25cm of sodium hydroxide of concentration 0.1moles/dm3 is in the flask and it takes 30cm3 of
3
sulphuric acid to neutralise the sodium hydroxide. Work out the concentration of the acid in moles
per dm3.
· 0.1mol/dm3 x (25/1000)dm3 = 0.0025 moles of sodium hydroxide
· 2NaOH + H2SO4 Na2SO4 + 2H2O
· (2)NaOH:(1)H2SO4 2mol:1mol 0.0025mol:0.00125mol
· 0.00125 moles of sulfuric acid
· 0.…read more

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Chemistry Unit 3
Ammonia has the chemical formula NH3, The raw ingredients for its manufacture comes from:
Nitrogen - the air
Hydrogen - methane gas
The reaction between these two gases is neither simple nor easy to control, it is a reversible
reaction.…read more

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