Section 2:
Chemical
Formulas and
Equations
Year 10 Science
2
The Language of Chemistry
 Early
chemists used symbols for elements and
compounds derived from alchemy, a ‘mystical
chemistry’.
3
The Language of Chemistry
 Currently
used symbols for the elements are those
found on the Periodic Table.
 Elements are represented by symbols, compounds
by formulas and reactions by equations.
4
Formulas
 Molecular(covalent)
formulas are those
made up of non-metals. They are listed on
the data sheet of the booklet.
 There are some flashcards on the website
to help you learn them
 Ionic formulas are those for metals joined
to non-metals. You need to be able to
write these given the ions on the data
sheet.
 There are activities in the booklet and the
website to help you revise them.
5
The Language of Chemistry
 In
a chemical reaction, at least one new
substance is formed.
 Chemical equations are a shorthand way
to represent long, complex reactions.
 Chemical equations take the form:
Reactants  Products
6
Conservation of mass
Brainpop: Conservation of mass
7
Conservation of mass
 The
Law of conservation of mass states that matter
can neither be destroyed or created.
8
Rearranging atoms
 During
chemical reaction the bonds in the
reactants are broken. The atoms are
rearranged and new bonds form.
Reactants → Products
A+B→C+D
A→B+C
A + B → C, etc..
9
Some Common Reaction
types
 Precipitation
 Decomposition
 Combustion
 Corrosion
 Acid-base
 Metal
displacement..
We will be studying
•Precipitation
•Some reactions of acids
10
Word Equations
 Chemical
reactions can be represented by
word equations.
 Photosynthesis
can be represented as:
Carbon dioxide + water  glucose + oxygen
 Reactants:
carbon dioxide, water
 Products: glucose, oxygen
11
Experiment 2:
Describing Chemical
Reactions
Remember to record:
 What
the reactants look like before the
experiment
 What the products look like at the end of the
experiment.
12
A. Magnesium in
hydrochloric acid
How do we know that a
chemical reaction has
occurred?
Describe this reaction
using a word equation.
13
B. Copper
sulfate + sodium
hydroxide
How do we know that a
chemical reaction has
occurred?
Describe this reaction
using a word equation.
14
C. Copper in
silver nitrate
solution
How do we know that a
chemical reaction has
occurred?
Describe this reaction
using a word equation.
15
Word Equations
 Complete
the report for experiment 2.
 Text:
Read pages 88-89 and complete
questions 1-7 page 89.
 Exercise
4 page 11 booklet.
16
Text page 89
17
Chemical Equations
 Brainpop:
Chemical Equations
18
Chemical Equations
A chemical equation gives the chemical formulas of
the reactants on the left of the arrow and the
products on the right.
Reactants

O2 (g)
C(s)
Products
CO2 (g)
19
Symbols Used in Equations
Symbols used in
chemical equations
show:
 -the states of the
reactants.
 -the
states of the
products.
 -the
reaction
conditions.
Can you write a word equation for this reaction?
What about the Law of Conservation of Mass?
Subscripts vs Coefficients
 Subscripts
tell
you how many
atoms of a
particular
element are in a
substance. The
coefficient tells
you about the
number of
molecules or
ion pairs of the
substance.
22
Learning Check

How many oxygen atoms are present in:
NaOH
1
3NaOH
3
H2O
1
2H2O
2
23
Learning Check
How many oxygen atoms are present in:
Cu(OH)2
3Cu(OH)2
2Na3PO4
2
6
8
24
Chemical Equations are Balanced
In a chemical reaction
atoms are neither
gained nor lost.
 In
a balanced chemical
equation the number of
reactant atoms is equal
to the number of
product atoms.
25
A Balanced Chemical Equation
In a balanced chemical equation:
 -there must be the same number of each type of atom
on the reactant side and on the product side of a
balanced equation.
 -numbers called coefficients are used in front of one or
more formulas.
Al +
2Al
S
+ 3S
 Al2S3
 Al2S3
Not Balanced
Balanced
26
Learning Check
State the number of atoms of each
element on the reactant side and the
product side for each of the following
balanced equations.
A. P4(s) + 6Br2(l)  4 PBr3(g)
4P
4P
12 Br
12 Br
B. 2Al(s) + Fe2O3(s)  2Fe(s) + Al2O3(s)
2 Al
2 Al
2 Fe
2 Fe
3O
3O
27
Learning Check
Decide if each equation is balanced or not.
A. Na(s) + N2(g)  Na3N(s)
No.
2 N on reactant side, 1 N on product side.
1 Na on reactant side, 3 Na on product
side.
B. C2H4(g) + H2O(l)  C2H5OH(l)
Yes. 2 C
= 2C
6H
= 6H
1O
= 1O
Steps in Balancing an
Equation
28
To balance the following equation,
Fe3O4(s) + H2(g)  Fe(s) + H2O(l)
 Work
on one element at a time.
 Use only coefficients in front of formulas.
 Do not change any subscripts.
Fe: Fe3O4(s) + H2(g)  3Fe(s) + H2O(l)
O:
Fe3O4(s) + H2(g)
 3Fe(s)
+ 4H2O(l)
H:
Fe3O4(s) + 4H2(g)  3Fe(s)
+ 4H2O(l)
29
Balancing Chemical Equations
1. Write the equation with the correct formulas.
NH3(g) + O2(g)
 NO(g)
+ H2O(g)
2. Determine if the equation is balanced.
No, not all atoms are balanced.
3. Balance with coefficients in front of formulas.
4NH3(g) + 5O2(g)  4NO(g) + 6H2O(g)
4. Check that atoms of each element are equal in reactants
and products.
4 N (4 x 1 N)
=
4 N (4 x 1 N)
12 H (4 x 3 H)
=
12 H (6 x 2 H)
10 O (5 x 2 O)
=
10 O (4 O + 6 O)
30
Balancing with Polyatomic Ions
MgCl2(aq) + Na3PO4(aq)  NaCl(aq) + Mg3(PO4)2(s)
Balance PO43- as a single unit
MgCl2(aq) + 2Na3PO4(aq)  NaCl(aq) + Mg3(PO4)2(s)
2 PO43=
2 PO43Balance Mg and Cl
3MgCl2(aq) + 2Na3PO4(aq)  6NaCl(aq) + Mg3(PO4)2(s)
3 Mg2+
=
3 Mg2+
6 Na+
=
6 Na+
6 Cl=
6 Cl-
31
Balancing Chemical Equations
 When
balancing equations you must
follow a simple set of rules:
1. Atoms
cannot appear from nowhere nor
can they disappear. There must be the
same number of each atom of each
element on either side of the chemical
equation.
32
Balancing Chemical Equations
2.
3.
4.
You cannot change the small subscript
numbers in a formula.
For example, H2O is water but H2O2 is
hydrogen peroxide, a type of bleach that
would be incredibly dangerous to wash
with or drink.
Change the subscript and you change
the chemicals.
33
Balancing Chemical Equations
You can only change the number in front of
each chemical formula (the coefficient).
For example, if you want to double the
number of oxygen atoms in an equation,
do not change O2 into O4. O4 does not
exist in that form and you just can’t go
about creating things that don’t exist!
Instead, write 2O2.
34
Balancing Chemical Equations
If you place a coefficient in front of a
compound like Al2(CO3)3 then you have
multiplied all the atoms in the formula by
that number. For example:
2
2
2
2
Al2(CO3)3 contains:
X 2 = 4 Al atoms
X 1 X 3 = 6 C atoms
X 3 X 3 = 18 O atoms
35
Balancing Chemical
Equations

Complete Exercise 6 page 12 booklet

Read page 90-91 text

Complete Q1-3 page 91 text


Now try:
Balancing equations worksheet in class files
36
Text page 91
37
Online activities

Chembalancer

Creative chemistry

It's elemental

Tutorial 1

Tutorial 2
38
I want more… Chemistry!!!