NCEA AS 90947
S1.8 Chemical Reactions
NCEA L1 Science 2012
1
Types of reactions
In this standard a selection of the main types of chemical reactions are
studied:
1. combination reaction.
2.precipitation (exchange) reaction.
3.displacement reaction.
4.thermal decomposition reaction.
The objectives include:
Recognise changes that indicate one of the above reactions have taken place.
Recall the identifying features of each type of reaction.
Write Chemical equations for each type of reaction.
Produce new products by selecting the appropriate reaction type. (practical)
Predict outcomes of reactions.
Application of knowledge of reaction types to a technology.
2
Chemical reactions - naming reactants & products - and recognising
chemical change
A chemical reaction is a process
that produces a chemical change
to one or more substances.
A chemical reaction will produce a
new substance. Other
observations may include a
temperature change, a colour
change or production of gas.
Chemicals that are used in a
chemical reaction are known as
reactants. Those that are formed
are known as products.
Reactants
3
→
Products
Chemical equations
Compounds and elements can react together to form new substances in a chemical
reaction. We use a chemical equation to show the substances we start with called
reactants, and the substances that are formed called products.
4
Balancing Chemical equations
Balanced equations must have the same number of atoms on each side of the equation
i.e. reactants and products.
2Na
+
2H2O
=
2NaOH
O
Na
Na
H
H
O
H
H
Na
O
5
+ H2
H
O
Na
H
H H
Combination reaction
Combination reactions
occur when two or more
reactants combine for form
one product.
An example is a metal and
oxygen forming a metal
oxide or a metal and a nonmetal reacting to form a
ionic compound.
These are limited to simple reactions of elements with other elements (such as
magnesium or sulphur with oxygen, iron with sulphur etc).
6
Combination - Metals form oxides by reaction with oxygen
Metals react with oxygen in the air to produce metal oxides, like magnesium
oxide.
Electrons are lost from the metal to form a cation (positive ion) and gained by
the oxygen to form an anion – oxide (negative ion). The cation and anion then
join together to form a neutral metal oxide.
7
Combination - Metals form oxides by reaction with oxygen
2Mg
+
Magnesium
O2
Oxygen
+
→
12p
+ve
-ve
12p
8
2MgO
Magnesium Oxide
Ionic compounds are the product of chemical reactions between metal
and non-metal ions
1. Write down the ions (with charges) that react to form the compound.
Cation comes before Anion.
Al3+
O2-
2. Cross and drop the charge numbers.
3. Place brackets around a compound ion.
Al2O3
4. If the numbers are both the same remove.
5. If any of the numbers are a 1 they are removed
6. Remove any brackets if not followed by a number
H+
SO4-2
H2(SO4)1
H2SO4
9
Precipitation (exchange) reactions
Precipitation reactions occur when
two solutions react together to form
a solid that settles out of the
solution. The solid formed is called
the precipitate.
An example is a lead (II) nitrate
solution mixed with a potassium
iodide solution to form a lead iodide
precipitate.

These are limited to precipitation reactions such as the formation of:
 chlorides of silver and lead
 sulfates of barium and lead
 hydroxides of copper, iron(II), iron(III), calcium, and magnesium
 carbonates of copper, iron(II), zinc, calcium, and magnesium ions.
Solubility Rules
Some ions will join together to form precipitates and are insoluble. Other ions will not
form precipitates and are soluble.
Ion
Rule
Exceptions
nitrate
soluble
chloride, iodide
soluble
silver and lead
sulfate
soluble
lead, calcium, barium
carbonate
insoluble
sodium, potassium ammonium
hydroxides
insoluble
sodium, potassium
sodium
potassium
ammonium
all soluble
11
Solubility Grid
When adding one ionic solution to another we use a solubility grid to decide if a
precipitate has formed or not.
e.g.
CaCl2(aq) + Na2CO3(aq) g ?
Ca2+
2Cl-
Na+
CO32-
?
?
-
The Na+ and the Cl- ions are dissolved in solution in the beginning and remain in
solution at the end. They are not involved in the precipitation reaction so they are
known as spectator ions. They do not need to be written in the equations for the
reactions.
12
Ionic Equations
Ionic compounds in solution break down into their ions.
NaCl(s)
dissolving
Na+(aq)
+
Cl-(aq)
Write ionic solution equations for the following
a. potassium hydroxide
b. sodium nitrate
c. magnesium chloride
d. copper sulfate
e. sodium carbonate
f. aluminum nitrate
13
Ionic Equations
Write only the ions that are used in forming the precipitate. The other ions
are known as spectators and can be left out of the equation.
Include the states of the reactants - (aq) aqueous and products - (S) solid.
14
Displacement reaction
Displacement reactions
occur when a metal and salt
(metal + non-metal ionic
compound) solution are
mixed and the metal
replaces the metal in the
salt.
An example would be
reacting magnesium metal
and copper sulfate to
produce magnesium sulfate
plus copper metal.
These are limited to the displacement of metal ions in solution by other metals.
15
Displacement reaction
Metal displacement equations
A more reactive metal will displace a less active metal from a solution. For
example, if you place magnesium in a solution of copper sulfate, the
magnesium will dissolve and the copper will be deposited on the bottom of
the container. This can be represented by a word equation:
magnesium + copper sulfate →
Mg
+ CuSO4
→
magnesium sulfate + copper
MgSO4
+ Cu
If you place calcium in a solution of zinc nitrate, the calcium will dissolve and
zinc will be deposited on the bottom of the container. The word equation
for this reaction is as follows:
calcium + zinc nitrate
2Ca + Zn(NO3)2
→ calcium nitrate + zinc
→
2CaNO3
+ Zn
16
Thermal decomposition reaction
Thermal decomposition
reactions occur when one
substance is broken apart
with the use of heat
energy into smaller
substances.
An example is copper
carbonate heated which
breaks down into carbon
dioxide and copper oxide.
These are limited to thermal decomposition of carbonates and hydrogen carbonates.
17
Thermal decomposition reaction
Equations for thermal decomposition of calcium carbonate:
calcium carbonate
CaCO3
calcium oxide + carbon dioxide
CaO
+
CO2
Equations for the thermal decomposition of copper carbonate:
copper carbonate
copper oxide + carbon dioxide
CuCO3
CuO +
CO2
18
Precipitation
Combination
Is there a
precipitate?
yes
no
Is there a
precipitate?
yes
no
Displacement
NCEA L1 Science 2012
Thermal Decomposition
19
Combination
Reaction summary
Metal
Metal
Oxide
Oxygen
Displacement
Metal (2)
Salt
Precipitation
Cation (1)
Anion (1)
Solution
Cation (2)
Anion (1)
Cation (2)
Anion (2)
Solution
Cation (1)
Anion (2)
Is there a
precipitate?
yes
Is there a
precipitate?
yes
Thermal Decomposition
Metal
(2)
Carbon
Dioxide
Metal
Carbonate
Metal
(1)
NCEA L1 Science 2012
Metal (1)
Salt
Metal
Oxide
20
no
no