1.8 Chemical Reactions Revision
Element
Symbol
Oxygen
O
Sodium
Na
Carbon
C
Zinc
Zn
Everything is made from atoms, including you. Atoms are tiny particles that are far too small to see, even with a microscope. If people were
the same size as atoms, the entire population of the world would fit into a box about a thousandth of a millimetre across!
There are over a hundred different types of atom, and these are called elements. Each element has a special name. For example carbon,
oxygen and hydrogen are all elements. Lead and gold are elements too. A piece of pure gold contains only gold atoms. A piece of pure lead
contains only lead atoms. Chemical reactions join or split atoms to rearrange them. But they cannot change one element into another
element, or anything simpler. A chemical reaction cannot turn lead into gold, because it can't change the atoms into different elements.
Elements are represented by one or two letters. The first letter is always a capital and the second is always lower case
Evidence for a chemical
reaction
Formulae of Compounds
If the molecule contains more than one atom of an element we use numbers to show this. The numbers are
Look for:



Change in temperature
A colour change
Bubbles of gas
written below the element symbol. For example, the formula for carbon dioxide is CO2. It tells you that each
molecule has one carbon atom and two oxygen atoms.
Take care when writing these formulae. The small number go at the bottom. For example CO 2 is correct but CO2
is wrong.
Combination Reactions
A combination reaction happens when two
elements react to form a compound.
Some formulae are more complicated. For example, the formula for sodium sulphate is Na2SO4. It tells you that
sodium sulphate contains two sodium atoms (Na2), one sulphur atom (S) and four oxygen atoms (O4).
Remember that metals react with oxygen in the
air to produce metal oxides, like magnesium
oxide.
Non-metals react with oxygen in the air to
produce non-metal oxides. Here are two
examples for the non-metals carbon and
sulphur.
Carbon burns in air to form carbon dioxide:
carbon + oxygen → carbon dioxide
All compounds have a definite composition. Let's look at water as an example. A water molecule always has two
hydrogen atoms and one oxygen atom - it cannot be a water molecule if it has different numbers of these atoms.
Its formula is always H2O.
Decomposition Reactions
Precipitation reactions
These reactions happen when metal carbonates break down when heated strongly. This is called
thermal decomposition. Here are the equations for the thermal decomposition of calcium carbonate:
calcium carbonate
These reactions happen between two metal
compounds.
Soluble salts dissolve in water. Insoluble salts do not
calcium oxide + carbon dioxide
dissolve in water. An insoluble salt is called a
CaCO3
CaO + CO2
precipitate.
Other metal carbonates decompose in the same way. Here are the equations for the thermal decomposition
of copper carbonate:
copper carbonate
CuCO3
Soluble and Insoluble Salts – IMPORTANT. Use
this information to work out whether an
insoluble salt will be produced in a reaction.
copper oxide + carbon dioxide
CuO + CO2
soluble
insoluble
all nitrates
none
most sulphates
lead sulphate, barium sulphate
most chlorides,
bromides and
iodides
silver chloride, silver bromide, silver
iodide, lead chloride, lead bromide,
lead iodide
sodium carbonate,
potassium carbonate
most other carbonates
sodium hydroxide,
potassium hydroxide
most other hydroxides
Notice that in both examples the products are a metal oxide and carbon dioxide. The carbon dioxide gas can
be detected using limewater. Limewater turns cloudy white when carbon dioxide is bubbled through it.
Metals high up in the reactivity series - such as calcium - have carbonates that need a lot of energy to
decompose them. Metals low down in the reactivity series - such as copper - have carbonates that are easily
decomposed. This is why copper carbonate is often used at school to show these reactions. It is easily
decomposed, and its colour change, from green copper carbonate to black copper oxide showing clearly the
reaction.
Displacement Reactions
These reactions happen between a metal and a metal compound.
If magnesium is placed in copper sulphate solution, a displacement reaction occurs.
Copper solid is displaced and the magnesium loses electrons to copper ions. Magnesium atoms become clear
Notice that nitrates and most chlorides are soluble.
magnesium ions and the blue copper ions lose their colour as they become solid copper.
This is why many of the chemicals you use in the
But placing copper metal in magnesium sulphate has no effect.
laboratory are nitrates or chlorides. If we want to
This is because copper is less reactive than
magnesium and can not displace magnesium from it compound of magnesium sulphate.
make an insoluble salt, we can react together two
soluble salts in a precipitation reaction.
Formulae (the use of brackets)
Working out the Formula of a Compound
Name
Formula
Sodium
Na+
Potassium
K+
Lithium
Li+
Magnesium
Mg2+
The reactivity table
Aluminium
Al3+
is useful to help us predict
displacement and decomposition
reactions.
Zinc
Zn2+
Copper(II)
Cu2+
Iron(II)
Fe2+
Iron(III)
Fe3+
CaCO3
Manganese
Mn2+
CaCl2
Barium
Ba2+
magnesium
chloride
MgCl2
Silver
Ag+
copper
sulphate
CuSO4
Calcium
Ca2+
oxide
O2-
chloride
Cl-
carbonate
CO32-
The formula of a compound can usually be deduced if the ions in it are
Sometimes brackets are used.
known. For example, the compound formed from Na+ and SO42- will
consist of two Na+ ions to every one SO42- ion so that the compound is
For example: Fe(OH)3 is the formula for Iron(III) hydroxide.
neutral overall. The formula is therefore Na2SO4.
Iron(III) hydroxide consists of one iron atom joined with three
It's helpful to remember some of the common ions so that the formula
oxygen and three hydrogen atoms. The formula is written like this
of most compounds can be deduced.
because the oxygen and hydrogen atom often act together.
Balancing equations
You should be able to write balanced equations if you are given
the name or formulae of the reactants and products.
Example
To work out the formula use the drop and swap method!
Formulae of some
substances
name
formula
oxygen
O2
Write the balanced equation for the reaction between calcium and
water to produce calcium hydroxide and hydrogen.
water
Step 1
magnesium
oxide
Write the formulae for each substance:
Ca + H2O
→
Ca(OH)2 + H2
Step 2
Check for one unbalanced element, for example, O. Adjust the
number of each atom or molecule needed, but never change a
formula. We need two Os on each side:
Ca + 2H2O
→
Ca(OH)2 + H2
Step 3
carbon dioxide
calcium
carbonate
calcium chloride
H2O
MgO
CO2
Check for another unbalanced element. In this example, the
equation is now balanced.