Download Balancing Chemical Equations

Equations
Law of Conservation of Matter
Balancing Chemical Equations
3-1
Stoichiometry
Stoichiometry
The study of quantitative relationships
between substances undergoing chemical
changes.
Law of Conservation of Matter
In chemical reactions, the quantity of
matter does not change.
The total mass of the products must equal
that of the reactants.
3-2
Chemical equations
Chemist’s shorthand to describe a reaction.
It shows:
• All reactants and products
• The physical state of all substances
• Any conditions used in the reaction
CaCO3 (s)
Reactant

CaO (s) + CO2 (g)
Products
A balanced equation shows the relationship
between the quantities of all reactants and products.
3-3
Balancing chemical equations
Each side of a chemical equation must have the
same number of each type of atom.
CaCO3 (s)
Reactants
1 Ca
1C
3O
CaO (s) + CO2 (g)
Products
1 Ca
1C
3O
3-4
Balancing chemical
equations
Step 1
Count the number of atoms of each
element on each side of the equation.
Step 2
Determine which atom numbers are
not balanced.
Step 3
Balance one atom at a time by
using coefficients in front of one or
more substances.
Step 4
Repeat steps 1-3 until everything is
balanced.
3-5
Balance the following
HCl + Ca _____> CaCl2 + H2
Step 1 & 2 1 H
2 H - not balanced
1 Cl
2 Cl - not balanced
1 Ca
1 Ca
Step 3 2 HCl + Ca _____> CaCl2 + H2
Step 4
2H
2 Cl
1 Ca
2H
2 Cl
1 Ca
The balanced equation
2 HCl + Ca _____> CaCl2 + H2
3-6
Balance the following
C2H6 + O2 _____> CO2 + H2O
Step 1 & 2
2C
6H
2O
1 C - not balanced
2 H - not balanced
3 O - not balanced
Step 3 Balance biggest molecule first, C2H6
C2H6 + O2 _____> 2 CO2 + 3 H2O
Step 4
2C
2C
6H
6H
2O
7 O - not balanced
3-7
Balance the following
C2H6 + O2
_____
> 2 CO2 + 3 H2O
Step 3a Now let’s balance O2
C2H6 + 3.5 O2 _____> 2 CO2 + 3 H2O
Step 4a
2C
2C
6H
6H
7O
7O
You can’t have 3.5 O2 so double the equation!
Balanced Equation
2 C2H6 + 7 O2 _____> 4 CO2 + 6 H2O
3-8
Balance the following
Sometimes equation are too difficult to balance simply
by inspection. Try this one:
Cu + HNO3  Cu(NO3)2 + H2O + NO
Step 1 & 2
1 Cu
1H
1N
3O
1 Cu - balanced
2 H - not balanced
3 N - not balanced
8 O - not balanced
Step 3 The N and O elements appear in more
than just one react and one product, which
means they must be balanced simultaneously.
3-9
Balancing chemical equations with Algebra
Step 1 Write correct chemical formulas for
all reactants and products.
Step 2 Assign a variable (a, b, c, etc.) for the
coefficient of each reactant and product.
Step 3 Write an expression for each element by
counting the number of atoms present in
each reactant and multiplying by the
coefficient, then repeat for the elements
on the product side.
3 - 10
Balancing chemical equations with Algebra
Step 4 The sum of the reactant values must
equal the sum of the product values.
Step 5 Let one variable have a value of 1 and
solve for the other variables.
Step 6 Express all of the coefficients as a
whole-number ratio.
Step 7 Recheck by counting the total number of
atoms of each element in the reactants as
compared to those in the products. The
must be equal.
3 - 11
Balance an Equation Using Algebra
a Cu + b HNO3  c Cu(NO3)2 + d H2O + e
NO
For Cu a = c
For H
b = 2d
For N
b = 2c + e
For O
3b = 6c + d + e
Let d = 1  b = 2(1) = 2
2 = 2c + e  e = 2 - 2c
3(2) = 6c + 1 + (2 - 2c)  c = 3/4 = a
e = 2 - 2(3/4)  e = 2/4 or 1/2
3/4Cu + 2HNO3 3/4Cu(NO3)2 + 1H2O +
1/2NO