AQA unit 2 chemistry GCSE notes

GCSE AQA revision notes for unit 2 chemistry hope it helps.... oh yh it is for higher!!!!

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Atoms, Molecules and Ions.
AN ATOM is the smallest particle of an element. They cannot be split into smaller particles in
chemical reactions. Iron is made of iron atoms (Fe). Sulphur is made of sulphur atoms (S)
A MOLECULE is a small group of atoms joined together.
The atoms may be the same (e.g. O2) or different (e.g. H2O). The chemical formula shows
the number and type of atoms present. Non-metal compounds are made of molecules:
Carbon dioxide contains CO2 molecules
Methane (natural gas) contains CH4 molecules
AN ION is an atom or group of atoms with an electrical charge (+ or -). Metal compounds
such as sodium chloride or copper sulphate contain ions.
Sodium chloride is made of Na+ and Cl- ions
Copper Sulphate is made of Cu2+ and SO42- ions
Note that metals form positive ions while non-metals form negative ions.
A solid is represented by (s). e.g. H2O(s) is ice.
A liquid is represented by (l) e.g. Fe(l) is molten iron.
A gas is represented by (g) e.g. H2O(g) is steam.
A solution in water is represented by (aq). Salt dissolved in water is NaCl(aq).
You should remember that the common gases are diatomic (have 2 atoms in each
molecule). These are Oxygen O2; Hydrogen H2; Nitrogen N2; and Chlorine Cl2.
Elementary Particles
Atoms are made up of smaller particles called protons, neutrons and electrons.
The protons and neutrons cluster together in a small nucleus at the centre of the atom
while the electrons orbit the nucleus.
The main properties of the particles are:
Particle Mass Charge
ELECTRON Very Small -1
Every element has an atomic number, which is the number of protons in the nucleus.
Atoms are neutral, so that the number of electrons is the same as the number of
The mass of an atom is almost entirely made up of protons and neutrons, which have the
same mass as each other (the mass of electrons can be ignored).
These relations can be summarised as:
Atomic Number Protons Electrons Neutrons Mass Number
Na 11 11 11 12 23
C 6 6 6 6 12
U 92 92 92 146 238
Electronic Structure

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The electrons orbit the nucleus in 'shells'. These can hold the following numbers of
the innermost shell can contain up to 2 electrons
the next shell can contain up to 8 electrons
the next shell can contain up to 8 electrons (although this can be expanded up to 18)
Fluorine has atomic number 9. Its electronic structure is 2.7
Sodium has atomic number 11. Its electronic structure is 2.8.1
Calcium has atomic number 20. Its electronic structure is 2.8.8.…read more

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Because the bonds between all the atoms are very
1) They have very high melting points.
2) They are very hard (graphite is an exception)
3) They do not conduct electricity (graphite is an exception)
4) They are insoluble in all solvents.
The important examples are diamond (C), graphite (C) and silicon dioxide (SiO2). The
last-mentioned is the base for the structure of many rocks, such as flint or granite.…read more

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Mr of ammonium nitrate = 14 + 4 x 1+ 14 + 3 x 16 = 80
Percentage of nitrogen = 2x14x 100 = 35%
Empirical Formulae
This is the formula which shows the lowest whole number ratio of the atoms
e.g.…read more

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The actual mass obtained in an experiment was 3.5g. Find the % yield.
Answer = 3.5 x 100 = 76.1%
The Mole
The mole is an important quantity used in all calculations you will meet later in chemistry.
What you need to know now is
1) A mole of any substance contains approximately 6x1023 particles (this number is
called the Avogadro constant)
2) The mass of 1 mole of any substance is its formula mass (Mr) in grams.…read more

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Similarly, at the start of the reaction, concentrations of C & D will be low, but as the
reaction proceeds their concentrations will increase and so the reverse reaction becomes
Eventually, as the forward reaction gets slower and the reverse reaction gets faster, the
rates will eventually become equal. This is known as equilibrium.
When this state is achieved, the amounts of both reactant and product remain constant.…read more

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Under these conditions, about 28% of the gases are converted to ammonia. This can be
separated from the unreacted nitrogen and hydrogen by cooling. Ammonia turns to a liquid
at -33C, while the other gases do not turn to liquids until much lower temperatures. The
unreacted gases can then be recycled to make more ammonia.
Properties of Ammonia (NH3)
It is a colourless gas with a pungent smell.
It is alkaline (turns pH paper blue).
It is very soluble in water.…read more

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The concentration of a solution is the amount of solute present in a solution. The
concentration is usually measured in moles per dm3 (1dm3 = 1 litre)
This means that equal volumes of solution of the same concentration contain equal number
of particles.
e.g. 25cm3 of a solution of HCl contains the same number of particles as 25cm3 of a
solution of NaOH of if they both have a concentration of concentration 0.…read more

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That the curves start steeply, but then level off. This is because the concentration of the
acid is high at the start of the reaction, but gets less as the acid is used up and eventually
c) That the amount of gas made by the end of the reaction is the same as long as the
amounts of reactants are the same. The rate of a reaction does not affect the final amount
of product made.…read more

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Metals and graphite are the only solids which conduct electricity, but no chemical change is
involved. Liquid (melted) metals also conduct, but again there is no chemical change.
These are liquids which conduct electricity, and are decomposed by it. They are ionic
substances which are dissolved in water or have been melted. This includes all acids and
metal compounds.
Examples: Copper sulphate solution, iron chloride solution, molten sodium chloride, dilute
sulphuric acid.
Non-electrolytes are covalent substances, e.g. pure water, sugar solution, alcohol, petrol.…read more


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