Objective
I
can write balanced
equations describing
chemical reactions using
the law of conservation of
matter
What do all of these have
in common?
What do all of these have
in common?
How do you write a chemical reaction?
How do you write a chemical reaction?
Reactants

Products
VERY IMPORTANT!
Please note that the atoms themselves are
neither created nor destroyed
…they are merely rearranged.
VERY IMPORTANT!
Please note that the atoms themselves are
neither created nor destroyed
…they are merely rearranged.
The Law of Conservation of Mass—mass is
neither created nor destroyed.
VERY IMPORTANT!
Please note that the atoms themselves are
neither created nor destroyed
…they are merely rearranged.
The Law of Conservation of Mass—mass is
neither created nor destroyed.
Every atom that appears in the reactants
must also appear in the products
Describing Chemical Reactions
with Chemical Equations

You can use a word description…



Propane gas reacts with oxygen in a Bunsen burner to produce
carbon dioxide and water vapor.
Using chemical formulas in an equation is more efficient

A skeleton equation is not balanced

C3H8 + O2  CO2 + H2O
A balanced equation obeys the law of conservation of mass &
shows the relative amounts of the molecules of reactants and
products.

C3H8 + 5 O2  3 CO2 + 4 H2O
You can indicate the physical state of a substance in
the equation by putting a symbol after each
formula.
(s) —solid
(l) —liquid
(g) —gas
(aq) —aqueous solution = (dissolves in water)
“Hydrogen gas and oxygen gas can combine to
produce liquid water.”
2H2(g) + O2(g)

2H2O(l)
Example #1: Rusting
“solid iron reacts with oxygen gas to produce iron(III) oxide
(rust)”
iron + oxygen  iron(III) oxide
Example #1: Rusting
“Iron reacts with oxygen to produce iron(III) oxide (rust)”
iron + oxygen  iron(III) oxide
Fe3+
Fe(s) +
O2-
O2(g)
Fe2O3(s)
4 Fe(s) + 3O2(g)
2Fe2O3(s)
Some chemical reactions require the use of
a catalyst.
Catalyst—a substance that speeds up the
rate of a reaction but that is not used up
in the reaction. It is neither a reactant or
product.
Catalyst example:
The decomposition of hydrogen
peroxide when poured onto a cut is
an example of a catalyst at work.
H2O2(l)  H2O(l) + O2(g)
The catalyst in this case is iron in
blood that speeds up the reaction
and causes oxygen gas to be
released. This gas is observed as
bubbles forming on the wound.
Example #2:
Hydrogen Peroxide H2O2
“Hydrogen peroxide
decomposes to form water and
oxygen gas.”
Hydrogen Peroxide  water + oxygen
Example #2:
Hydrogen Peroxide H2O2
“Hydrogen peroxide
decomposes to form water and
oxygen gas.”
Hydrogen Peroxide  water + oxygen
H2O2(aq)
H2O(l) + O2(g)
2H2O2(aq) 2H2O(l) + O2(g)
Balancing Chemical Equations
Balancing a chemical equation is necessary
so that it obeys the law of conservation of
mass.
In every balanced equation, each side of the
equation has the same number of atoms
of each element.
Rules for Balancing Equations
1. Determine correct chemical formulas for all the reactants
2.
3.
4.
5.
and products in the reaction.
Put reactants on the left and products on the right with
a yield sign () in between. Use plus signs to separate
multiple reactants or products.
Count the number of atoms of each element in the
reactants and products.
--a polyatomic ion appearing unchanged on both sides of
the equation counts as a single unit.
Balance the elements one at a time by adding
coefficients. If no coefficient is written, it is assumed to
be 1. --Do not try to change the subscripts!!!
Finally, make sure all the coefficients are in the lowest
possible ratio that balances.
practice: “Hydrogen gas and
oxygen gas combine to produce
liquid water”
_____ H2(g) + _____ O2(g) 
O ________
H ________

_____ H2O(l)
O_______
H_______
“Hydrogen gas and oxygen gas combine
to produce liquid water”
Skeleton
Equation
H2(g) +
O2(g)

H2O(l)
Balanced
Equation
2H2(g) +
O2(g)

2H2O(l)
“Silver nitrate reacts with hydrogen sulfide to
form silver sulfide and nitric acid – HNO3.”
____Ag(NO3) + ____H2S  ____Ag2S + ____H(NO3)
Ag______
(NO3) ______
Ag______

(NO3) ______
S ______
S ______
H ______
H ______
Desk Work
• Finish the desk practice balancing
problems & get them checked