Balancing Chemical Equations
Making the seemingly
impossible be easy….maybe
Balancing Chemical Equations
A chemical reaction is about to take place.
H3PO4 + Ca
You know you should look for (1) colour
changes, (2) production of gas (bubbles),
(3) heat or light and/or (4) formation of a
solid precipitate from two liquids
Uh oh, you are asked to define the reaction
chemically. That is make a prediction about
type, outcome and products. Yikes…the
impossible…
Balancing Chemical Equations
H3PO4 + Ca
A little review….What does H3PO4 mean?
Hydrogen (H) is bonded with the polyatomic ion
phosphate (PO4). To make this compound, there are
THREE hydrogen atoms for ONE phosphate polyatomic.
The hydrogen-phosphate bond is an ionic bond. That is
hydrogen GIVES electrons and the phosphate TAKES
electrons. This makes the electron configuration in both
chemicals stable…whoa….okay tough…but always recall
stability is the goal
PO4 is a covalent bond. Phosphorus and oxygen SHARE
electrons. Recall, a polyatomic ALWAYS stays together.
Balancing Chemical Equations
H3PO4 + Ca
What is the type of reaction?
There are four types: (1) synthesis, (2) decomposition,
(3) single displacement and (2) double displacement
•
•
•
•
Synthesis = Two make one
Decomposition = One makes two
Single Displacement = One replacement
Double Displacement = Two replacements
Using these clues which type of reaction will we see?
Balancing Chemical Equations
H3PO4 + Ca
This is a SINGLE
DISPLACEMENT reaction.
The CALCIUM ion (Ca) will
replace the HYDROGEN
ion (H).
Why?
Balancing Chemical Equations
H3PO4 + Ca
In an Ionic Bond, the POSITIVE ION is
always written first. So in H3PO4,
hydrogen is positive (H+).
Using the Periodic Table, we determine
calcium (Ca) is also a POSITIVE ION. Its
charge is Ca2+
Since like charges repel, calcium will
replace hydrogen. This is the only
change…a single change. Thus, this is a
SINGLE DISPLACEMENT REACTION
Balancing Chemical Equations
Thus, we know this…
H3PO4 + Ca  CaPO4 + H
The calcium replaces the hydrogen to make a
new compound combining calcium and
phosphate.
Are we done?
HINT: Recall the Law of Conservation of Mass.
That is, the Mass of the Products must equal
the Mass of the Reactants. Has this occurred?
Balancing Chemical Equations
H3PO4 + Ca  CaPO4 + H
First, hydrogen will not exist in nature by itself.
Thus, hydrogen bonds covalently with another
hydrogen.
H3PO4 + Ca  CaPO4 + H2
Are we done now?
There are three H ions in the reactants and only
two hydrogen in the products. The Law of
Conservation of Mass is not met? We are
required to BALANCE this equation…(rats)
Balancing Chemical Equations
H3PO4 + Ca  CaPO4 + H2
Balancing means there must be equal amounts of
products and reactants.
Where to start? …Let’s look at the charges of the ions.
What? Okay, this is hard. Recall, elements have specific
structures. Protons and neutrons reside in the centre (i.e.,
nucleus) of the atom, while electrons occur in various
shells or orbits around the nucleus. To have a structure
that is electrically stable, the outer shell of electrons
(…called VALENCE SHELL) must be full. The full
shells…presently…are 2,8.8. That is, Shell #1 is full when
it has two electrons and Shell #2 is full with eight
electrons. If the Valence Shell is not full, the element will
either GIVE / TAKE electrons or SHARE electrons until the
outer shell is full. This is the first part of chemistry.
Balancing Chemical Equations
H3PO4 + Ca  CaPO4 + H2
Calcium is element #20. That means it has 20 protons
and 20 electrons (20+ and 20- respectively). The protons
are in the nucleus. The number of protons CANNOT
change (e.g., calcium always has 20 protons).
Yet, the Valence Shell of electrons of calcium is NOT full.
How can calcium get a full outer shell? It can get more
electrons by taking or giving away electrons for its outer
shell. Calcium has an attractive force of 20. It is hard for
calcium to attract and hold enough electrons to fill the
outer shell…in this case, six. So, calcium gives away the
two electrons in its outer shell. The calcium ion has 20+
and 18- for a charge of 2+
H3PO4 + Ca2+  CaPO4 + H2
Balancing Chemical Equations
H3PO4 + Ca2+  CaPO4 + H2
Does the charge for calcium also occur in
the products? Yes, it must. Why?
H3PO4 + Ca2+  Ca2+PO4 + H2
Looking at the Periodic Table, hydrogen has
a charge of 1+
Thus….
(H3)1+PO4 + Ca2+  Ca2+PO4 + (H2)1+
Balancing Chemical Equations
(H3)1+PO4 + Ca2+  Ca2+PO4 + (H2)1+
Finally, the PO4 is a polyatomic. It STAYS TOGETHER in a
chemical reaction. This is the hard one. How many PO4
occur in this reaction? There is only one PO4. The
subscript one is NOT written; rather, it is implied.
(H3)1+(PO4)1 + Ca2+  Ca2++ (PO4)1 + (H2)1+
What is the ionic charge of the phosphate polyatomic ion?
Recall, the CRISSCROSS RULE. In this case, the rule is
applied backwards (i.e., cross upwards). Since it is H3, the
PO4 must have a charge of 3- (i.e., three hydrogen giving
one electron each to bond with one PO4 that needs three
electrons).
(H3)1+(PO4)13- + Ca2+  Ca2++ (PO4)13- + (H2)1+
Balancing Chemical Equations
Now, apply the Crisscross Rule to make sure the
right number of atoms are used to make the new
compounds
(H3)1+(PO4)13- + Ca2+  Ca32+(PO4)23- + (H2)1+
Finally, eliminate all the charges (…superscripts)
H3PO4 + Ca  Ca3(PO4)2 + H2
Holy crap, Batman…we did it. All the chemicals
are present AND in their right proportions.
Wait…look again…it is not chemically balanced.
For example, there are more hydrogen reactants
than hydrogen products. Can this be? No. Why?
Balancing Chemical Equations
Let’s balance…
Pick any chemical that is not balanced and start.
So, hydrogen….three reactants and two products.
What value makes the same number of both
sides? SIX.
Yet, how do you make six hydrogen on both
sides? If I have 2(H3PO4), I have six hydrogen in
the reactants (2 X 3). As well, I have 3H2 which is
3 X 2 or six hydrogen in the products. Hydrogen
is balanced.
2(H3PO4) + Ca  Ca3(PO4)2 + 3H2
If I look closely, I also have two PO4 on both sides:
2PO4 and (PO4)2
Balancing Chemical Equations
Almost done…look closely…look at calcium and
decide what to do
2(H3PO4) + 3Ca  Ca3(PO4)2 + 3H2
To check, add it all up –
• Reactants = 6H, 2PO4 and 3Ca
• Products = 6H, 2PO4 and 3Ca
The correct, balanced chemical formula for this
reaction is
2(H3PO4) + 3Ca  Ca3(PO4)2 + 3H2
In other words, I must mix 2 parts of H3PO4 and
three parts Ca to get one Ca3(PO4)2 and three H2
Hooray…you are starting to understand…learning
is fun