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ont.net/2sntkKsH2fmlZyYGtxNn
Jy30P1...
Chapter 7
 Chemical
Reactions
7.1 Describing Chemical Reactions
 What
is a chemical reaction?
 Demos: Silver and Copper
 Chemical Reaction: is when a
substance undergoes a chemical
change to produce a new substance or
substances
 7 different types of chemical reactions
We will learn five types
Chemical equations: are used to
represent a chemical change/reaction
– Ex.
2 H2 + O2  2 H2O
 Reactants: are the substances that
undergo chemical change (bonds
broken) (ingredients)
 Products: are the new substances
produced by the change (new bonds
made) (cookies)


Reading Chemical equations
+ sign = “reacts with”
 sign = “to produce”, yields
s = solid state
l = liquid state
g= gas state
Labeling the parts of a chemical equations
http://www.chem.wisc.edu/deptfiles/genchem/sstutorial/Text3/Tx34/tx34p1.GI
F
Counting the amounts:
– Mole
is an amount of a
substance that contains
6.02X1023 particles. So rather
than using atoms, molecules or
ions we use the MOLE
– Coefficient = # of moles
Mole continued
Shoes come in pairs,
 Eggs in a dozen.
 Particles to chemists come in MOLES
 Particles can be atoms, molecules, or
ions


Law of conservation of mass: states that
mass is neither created nor destroyed in
a chemical reaction

By ordinary means, we can’t make or
destroy matter

ONLY REARRANGE IT!!!!!!!!
Balancing Equations

When writing chemical equations, the
law of conservation of mass must be
followed.

We must have the same amount of
each element on both sides of the
equation
 Coefficients:
are used to show
proportions of reactants and
products)
 PERFECT
PRACTICE,
MAKES PERFECT!!!!!!
Balancing Chemical Equations
What goes in must
come out!
Discovery balancing reactions
Discovery balancing equations


Adapted from
file://localhost/.file/id=6571367.1926035
Balancing Chemical Equations

Balancing a chemical equation is much like
the work of an accountant who has to show
every penny that comes in and where it has
gone to.
Objectives
 Learn
the steps to balancing
chemical equations.
 Take notes to help you understand.
 Test yourself with a set of
equations to balance.
 Enter your own equations to see if
they balance.
Law of Conservation of Mass
You need to remember this law!


The Law of Conservation of Mass states:
that mass is neither created nor
destroyed in any chemical reaction.
Therefore balancing of equations requires
the same number of atoms on both sides of a
chemical reaction.
The number of atoms in the Reactants must
equal the Number of atoms in the Products
Chemical Equations
Because of the principle of the
Conservation of Matter,
an equation
must be
balanced.
It must have the same
number of atoms of the
same kind on both sides.
Lavoisier, 1788
Law of Conservation of Mass

The mass of all the reactants (the
substances going into a reaction) must equal
the mass of the products (the substances
produced by the reaction).

Reactant + Reactant = Product
A simple equation, such as the
synthesis of Iron (II) sulfide,



iron + sulfur
Fe + S
Iron (II) sulfide
FeS
Note that in a chemical equation, by
convention, we use the arrow “
"
instead of the equals “ = ".

The last stage is to put in state of matter
symbols, (s, l, g, aq), as appropriate (solid,
liquid, gas, aqueous or dissolved in water)

Fe(s) + S(s)
FeS(s)
Balancing
Equations
2
3
___ Al(s) + ___ Br2(l) ---> ___ Al2Br6(s)
Steps to Balancing a Chemical Equation
1.
Write all reactants
on the left and all
products on the
right side of the
equation arrow.
Make sure you
write the correct
formula for each
element
2. Use coefficients in
front of each formula
to balance the
number of atoms on
each side.
Steps to Balancing a Chemical Equation
3. Multiply the
coefficient of each
element by the
subscript of the
element to count the
atoms. Then list the
number of atoms of
each element on
each side.
4.
It is often easiest to start
balancing with an element
that appears only once on
each side of the arrow.
These elements must have
the same coefficient. Next
balance elements that
appear only once on each
side but have different
numbers of atoms. Finally
balance elements that are
in two formulas in the
same side.
Re-cap of steps from rule 4:
Balance elements that appear only
once on each side of the arrow.
 Next balance elements that appear only
once on each side but have different
numbers of atoms.
 Finally balance elements that are in two
formulas in the same side.

Balancing Chemical Equations
An easier way
First you need an equation with the correct “formulae”
………. You’ll probably be given this in the question
Just like this one
Mg + O2  MgO
Then all you do is list the atoms that are involved
on each side of the arrow
Mg + O2  MgO
Mg
O
Mg
O
Then start balancing:
[1] Just count up the atoms on each side
Mg + O2 
MgO
1
Mg
1
2
O
1
[2] The numbers aren’t balanced so then add “BIG”
numbers to make up for any shortages
Mg + O2  2 MgO
And adjust totals
1
Mg
1
2
2
O
1
2
But the numbers still aren’t equal, so add
another “BIG” number
2 Mg + O2  2 MgO
2
1
Mg
2
2
O
2
And adjust totals again
NOW BOTH SIDES HAVE EQUAL
NUMBERS OF ATOMS
WE SAY THAT THE
EQUATION IS BALANCED!!
Try to balance these equations using the same
method:
[1] Na + Cl2  NaCl
[2] CH4 + O2  CO2 + H2O
[3] Li + HNO3  LiNO3 + H2
[4] Al + O2  Al2O3
How did you get on??
Here are the answers:
[1] 2 Na + Cl2  2 NaCl
[2] CH4 + 2 O2  CO2 + 2 H2O
[3] 2 Li + 2 HNO3  2 LiNO3 + H2
[4] 4 Al + 3 O2  2 Al2O3
HOPE YOU’VE GOT THE IDEA…
REMEMBER TO CHECK THAT
YOU CAN DO ELECTROLYSIS
EQUATIONS TOO
Example

NH3 + O2
Reactants
NO + H2O
Products

N appears once on both sides in equal
numbers, so the coefficient for NH3 is the
same as for NO.
Example: NH3 + O2


NO + H2O
Next look at H which appears only once on
each side but has different numbers of
atoms, 3 on the left and 2 on the right. The
least common multiple of 3 and 2 is 6, so
rewrite the equation to get 6 atoms of H on
both sides:
2NH3 + O2
NO + 3H2O
Example: 2NH3 + O2
NO + 3H2O

There are 2 oxygen atoms on the left and 5 on
the right — the least common multiple of 2 and 5
is 10, so rewrite the equation as:

2NH3 + 5O2
4NO + 6H2O
Now count the atoms on each side:






2NH3 + 5O2
4NO + 6H2O
Write them out keeping them on the
appropriate side of the chemical equation
2 N (nitrogen atoms)
4 N (nitrogen atoms)
6 H (hydrogen atoms) 12 H (hydrogen atoms)
10 O (oxygen atoms) 10 O (oxygen atoms)
This shows the equation not to be balanced
“YET”
Check the number again:

If you double the N and H on the left the
equation will be balanced:

4NH3 + 5O2
4NO + 6H2O
Double-check:

4NH3 + 5O2


4 N (nitrogen atoms)
12 H (hydrogen atoms)
10 O (oxygen atoms)

The equation is Balanced

4NO + 6H2O
4 N (nitrogen atoms)
12 H (hydrogen atoms)
10 O (oxygen atoms)
Balancing Practice
 For more help go to:
http://richardbowles.tripod.com/chemistry/bal
ance.htm#part0
 For some fun balancing equations go to:
 http://www.mpcfaculty.net/mark_bishop/bala
ncing_equations_tutorial.htm