Module 2
Module 2
Inquiry question: What happens in chemical reactions?
Students:
 conduct practical investigations to observe and measure the quantitative
relationships of chemical reactions, including but not limited to:
o masses of solids and/or liquids in chemical reactions
o volumes of gases in chemical reactions (ACSCH046)
 relate stoichiometry to the law of conservation of mass in chemical reactions by
investigating:
o balancing chemical equations (ACSCH039)
o solving problems regarding mass changes in chemical reactions (ACSCH046)
 Practical investigations are an essential part of
chemistry and involve changes in the quantity of
a chemical present.
 You are expected to be able to propose methods
to measure changes in the quantity of a substance
that might occur during a chemical reaction.
 You could be expected to measure the change in
the mass of a substance as a result of a chemical
reaction.
 For example, if a strip of magnesium ribbon was
brunt in oxygen to form magnesium oxide, you
could measure the change in mass of the solid
substance, and work out what mass was added to
the material.
 You could also be expected to measure
volumes of gas produced in the reaction
using things like a syringe or a balloon.
 For example, hydrogen is produced when
metals react with acid, and can be
captured using a balloon.
 When asked to propose how an
experiment to measure mass or volume
could be done, remember to think in
practical terms: choose a method that is
straightforward and achievable in a basic
laboratory.
It is a chemical change in which one or more substances
are destroyed and one or more new substances are
created.
BEFORE
AFTER
H2 gas
H2O
liquid
and
O2 gas
Reactants  Products
Reactants: Substances that are destroyed by the
chemical change (bonds break).
Products: Substances created by the chemical change
(new bonds form).
The arrow () is read as “yields”.
 (s) = solid
 (l) = liquid
 (g) = gas
 (aq) = aqueous solution (the substance is dissolved in
H2O)
 “+” separates two or more reactants or products
 “” yield sign separates reactants from products
1) Evolution of light or heat. 2) Temperature change (increase
or decrease) to the
surroundings.
3) Formation of a gas (bubbling or an 4) Color change (due to the
odor) other than boiling.
formation of a new substance).
5) Formation of a precipitate (a new solid forms)
from the reaction of two aqueous solutions.
WORD EQUATIONS
 Statements that indicate the reactants and products in a
chemical reaction.
 Ex.
Iron (s) + chlorine (g)  iron (III) chloride (s)
 This is read as:
“Solid iron and chlorine gas react (combine) to
produce solid iron (III) chloride”
 A skeleton equation uses chemical formulas rather than words to
identify the reactants and products of a chemical reaction.
 The word equation
Iron (s) + chlorine (g)  iron (III) chloride (s)
 The skeleton equation
Fe(s) + Cl2(g)  FeCl3 (s)
A skeleton equation is not yet “balanced” by coefficients!

6 Na (s) + Fe2O3 (s)  3 Na2O (s) + 2 Fe (s)
 The numbers preceding the chemical formulae are coefficients.
They are used to balance the reaction.
 The numbers within the chemical formulae are subscripts.
 You can read the above balanced reaction as:

“6 atoms of solid sodium plus 1 formula unit of solid iron (III) oxide
yields 3 formula units of solid sodium oxide and 2 atoms of solid
iron” or…

“6 moles of solid sodium plus 1 mole of solid iron (III) oxide yields
3 moles of solid sodium oxide plus 2 moles of solid iron”

Chemical reactions can never be read in terms of grams, only in
terms of particles or groups of particles (moles).
 The Law of Conservation of Mass states that mass cannot
be created or destroyed by chemical reactions or
physical transformations. This means that the total mass
of the products in a chemical reaction are exactly the
same as the total mass of the reactions – matter can not
be produced from nothing!
 A consequence of the Law of Conservation of Mass is that all
chemical equations must be balanced, that is, you must show the
same number of each type of atom on each side of the reaction
arrow. In order for a chemical equation to be balances and
correct:
1. All reactants and products must be included
2. The chemical formula for each chemical species must be correct
3. The physical state at the room temperature and pressure must be
included
4. The number of each type of atom must be the same on each side of the
reaction arrow.
CONSERVATION OF MASS
During a chemical reaction, atoms are neither created nor
destroyed (Conservation of Mass).
Hydrogen and oxygen gas react to form water:
H2 (g) + O2 (g)  H2O (l)
H2 (g) + O2 (g)  H2O (l)
What is wrong with this equation above? Doesn’t it appear that one
oxygen atom “went missing”?
According to conservation of mass, the proper way to write this
reaction is:
2H2 (g) + 1O2 (g)  2H2O (l)
The red coefficients represent the # of molecules (or the # of moles)
of each reactant or product.
 For example, the balances chemical equation for the reaction between magnesium
metal and oxygen gas to produce magnesium oxide is as follows:
 As an additional note, you will need to know these polyatomic ions moving forward:
2Ca3(PO4)2 + 6 SiO2 + 10C 
6 CaSiO3 + P4 +10CO
Atom
Ca
P
O
Si
C
# Atoms on
Left Side
# Atoms on
Right Side
__H2 + __ O2  __H2O
Balancing is about finding the
right coefficients!
1) You can change the coefficients, but NEVER the
subscripts!
__H2 + __ O2  __H2O
Off Limits!
2) The coefficients must reduced to represent the
lowest possible numbers.
4H2 + 2 O2  4H2O
3) It is OK to use fraction coefficients, but you must get rid of
them in the end (multiply through by denominator).
H2 + ½ O2  H2O
4) Do a final balance check for each element!
2H2 + O2  2H2O
1) K + Br  KBr
2) HgO  Hg + O2
3) Na + H2O  NaOH + H2
4) CaO + H2O  Ca(OH)2
5) Al + HCl  AlCl3 + H2
 Page 206 from Pearson has the practical write up and students refer to worksheet
2.1 from skills and assessment workbook for questions.