Chemistry C2 1.1-2.5
Atoms are made of protons, neutrons and electrons.
Protons have a positive + charge and electrons have a
negative – charge. Atoms are arranged in the periodic table
in order of their atomic number. Neutrons have no electric
charge so are neutral.
Atomic Structure
The element Lithium has a symbol of
Li.
3 electrons (atomic number) and an
atomic mass of 7
The electrons in an atom are arranged in shells.
Atoms with the same number of electrons in their
outer shell belong in the same group of the periodic
table. The number of electrons in the outer shell
of an atom determines the way that the atom
behaves in chemical reactions.
Covalent Bonding
Ionic Bonding
Ionic compounds are held together by
strong forces between the oppositely
charged ions. This is called ionic bonding.
Group 2 & Group 6 elements can form
ionic compounds. This happens between a
metal and non-metal. These form giant
structures of ions in a lattice.
This takes place between non-metals.
Covalent bonds are formed when atoms
share electrons. Many substances contain
covalent bonds consist of molecules, but
some have giant covalent structures. The
electrons are shared so there is an outer
shell of 8 electrons.
Chemical Bonding
Elements react to form compounds by gaining or
losing electrons or by sharing electrons.
The elements in Group 1 react with the elements
in Group 7 because group 1 needs to lose an
electron from its outer shell and group 7 need to
gain an electron to have a full outer shell.
This can be shown by using dot and cross
diagrams.
Remember: An outer shell which
does not have a full shell of 8
electrons is unstable. So all elements
are looking to become stable.
Metallic Bonding
The atoms (or ions) in metals are arranged in regular layers. The positive ions in
metals are held together by electrons from the outer shell of each metal atom.
These form giant structures. The diagram on the left shows this.
Ionic compounds – it takes a lot of energy to break the bonds which hold a
giant ionic lattice together. Very high melting points and are solid at room
temperatures. They conduct electricity when melt or dissolve them in
water because their ions can then move freely.
Chemistry C2 3.1-4.6
Equations & calculations
Mass Numbers
Chemical equations tell us the number of
moles of substances in the chemical
reaction. These can be used to calculate
the masses of reactants and products in
a chemical reaction from the masses of
one mole of each of the substances
involved in the reaction.
The relative mass of protons and
neutrons is 1.
The mass number of an atom tells you
the total number of protons and
neutrons in its nucleus.
Isotopes are atoms of the same element
with different numbers of neutrons.
Work out the relative formula mass of a
compound from the relative atomic
masses of the elements in it.
Yield: of a chemical reaction describes how much
product is made.
Making Ammonia – the Haber process
Ammonia is important for making other
chemicals, e.g. fertilisers. It is made from
nitrogen and hydrogen in the Haber process.
This is where the process is carried out under
conditions which are chosen to give a
reasonable yield of ammonia very quickly. Any
unused nitrogen and hydrogen are recycled in
the Haber process. See diagram of Haber
process.
Reversible reactions
In this reaction the products of the reaction
can react to make the original reactants.
The rate of the reaction in each direction is
controlled by Enzymes.
Collision theory- as particles react they begin
to collide, the faster they collide the more
energy is made. The rate of a chemical reaction
increases if the surface area of any solid
reactants is increased.
Effects of temperature, concentration &
increasing pressures – faster reactions, as
particles collide more often and have more
energy.
A Catalyst – speeds up the
rate of a chemical reaction. It
is not used up during a
chemical reaction.
We can measure the rate of
reaction by following the rate at
which reactants are used up.
Chemistry C2 5.1 -6.4
Thermal decomposition
These reactions are also
endothermic e.g. calcium carbonate
to form calcium oxide and carbon
dioxide.
Exothermic & Endothermic Reactions
Energy may be transferred to or from the
reacting substances in a chemical reaction.
A reaction that produces heat energy is called an
exothermic reaction.
A reaction where energy is taken to the reactants
is called an endothermic reaction.
Electrolysis the basics
This involves splitting up a substance using
electricity. Ionic substances can be electrolysed
when they are molten or in solution.
Positive ions move to the negative electrode
(cathode) and negative ions move to the positive
electrode (anode).
Changes at the electrodes
In electrolysis the ions move towards the
oppositely charged electrodes.
The negative ions are oxidised while
positive ions are reduced.
Electrolysing brine
This gives us 3 products – chlorine gas,
hydrogen gas and sodium hydroxide solution.
Reactions where reduction and oxidation
happen are called REDOX reactions.
When electrolysis happens in water, the
less reactive element is usually produced
at an electrode.
Purifying Copper
Copper extracted from its ore
contains impurities of gold & silver.
Copper is purified by electrolysis to
remove these impurities.
Energy & reversible reactions
In any reversible reaction, the amount of
energy released when the reaction goes in
one direction is exactly equal to the energy
absorbed when the reaction goes in the
opposite direction.
Chemistry C2 7.1 – 7.3
Acids & Alkalis
Acids are substances which produce H+ ions
when we add them to water. Bases are
substances that will neutralise acids. An
alkali produce OH- ions when we add them to
water.
The pH scale is used to show how acidic or
alkaline a solution is.
Making salts from solutions
An indicator is needed when we produce a
salt by reacting an alkali with an acid to make
a soluble salt.
Insoluble salts can be made by reacting 2
solutions to produce a precipitate.
Precipitation is an important way of removing
some substances from wastewater.
Making salts from metals or bases
An acid reacted with an alkali makes a
neutralisation reaction.
The reaction between an acid and a base
produces a salt and water.
Salts can also be made by reacting a metal with
an acid. This reaction also produces hydrogen
gas as well as a salt.