Tuesday
January 8, 2013
(Balancing Chemical
Equations)
Bell Ringer
Tuesday, 1-8-13
Translate the following sentence first into a
word equation, then into a formula equation:
Solid aluminum metal reacts with aqueous zinc
chloride to produce solid zinc metal and aqueous
aluminum chloride.
aluminum + zinc chloride → zinc + aluminum chloride
Al(s) + ZnCl2(aq) → Zn(s) + AlCl3(aq)
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QUIZ 14
S4
12/12
12/12
12/18
1/16
WS – Chemical
Reactions and
Equations (Part 1)
F1
1/7
1/11
1/11
Balancing Chemical Equations
The Law of Conservation of Mass states that matter
(mass) cannot be created nor destroyed in physical or
chemical changes, only changed in form.
This means that in a chemical reaction, the amount of
mass that you start with in the reactants will be the
amount of mass that you end up with in the products.
In order to represent this in a chemical equation, the
number of each type of atom in the reactants must be
the same as the number of each type of atom in the
products.
To make this happen, the equation must be balanced.
We will now discuss the methods for balancing a chemical
equation.
Balancing Chemical Equations
Here’s how to write and balance a
chemical equation:
Example:
Water vapor decomposes to produce
hydrogen gas and oxygen gas.
Write a word equation.
water → hydrogen gas + oxygen gas
Write a formula equation.
H2O (g) → H2 (g) + O2 (g)
Balancing Chemical Equations
Now, balance the formula equation according to
the Law of Conservation of Mass.
H2O (g) → H2 (g) + O2 (g)
In the equation above, there are 2 hydrogen
atoms and 1 oxygen atom in the reactants
(what you start with to the left of the →.)
In the products, however, there are 2
hydrogen atoms and 2 oxygen atoms!
Where did that extra oxygen atom come
from?
Balancing Chemical Equations
Now, balance the formula equation according to
the Law of Conservation of Mass.
2 H2O (l) → 2 H2 (g) + O2 (g)
There must be the same number of hydrogen atoms and
oxygen atoms on both sides of the equation in order for us
to end up with the same mass we started with (the Law
of Conservation of Mass.
Therefore, it is up to us to balance the equation so that
what we produced is the same as what we stsrted with.
We do this by adding coefficients as necessary to balance
things out.
Here’s how it’s done!
Balancing Chemical Equations
Now, balance the formula equation according to
the Law of Conservation of Mass.
2 H2O (l) → 2 H2 (g) + O2 (g)
This now indicates that we started with 4
hydrogen atoms and 2 oxygen atoms in the
reactants.
We produced 4 oxygen atoms and 2 oxygen
atoms also.
The equation is now balanced!
Sample Problem
The reaction of solid zinc metal with
aqueous hydrochloric acid produces a
water solution of zinc chloride and
hydrogen gas.
Write a balanced chemical equation for the
reaction.
Zinc + hydrochloric acid → zinc chloride + hydrogen
Zn (s) +2 HCl (aq) → ZnCl2 (aq) + H2 (g)
The equation is said to balance
in a ratio of 1:2:1:1
Worksheet
Chemical Reactions and
Equations
(Part 2)