Transcript Lesson 4.02 Balancing Equations Lesson Standards & Objectives • SC.912.P.8.8—Apply the mole concept and the law of conservation of mass to calculate quantities of.

Lesson 4.02
Balancing Equations
Lesson Standards & Objectives
• SC.912.P.8.8—Apply the mole concept and
the law of conservation of mass to calculate
quantities of chemicals participating in
reactions.
• Describe how mass and atoms are always
conserved in a reaction.
• Demonstrate the use of coefficients to
balance equations.
Vocabulary (Literacy Strategy)
• Chemical Equation: A representation, using formulas and
symbols, of a chemical reaction.
• Reactants: The substances that are present at the
beginning of a reaction. These are located on the left side
of the arrow.
• Products: The new substances that are formed or
produced in a chemical reaction. These are located on the
right side of the arrow.
Sample equation:
A + B  AB
“A” and “B” are both reactants. “AB” is the product
You will learn about different types of reactions and in
lessons 4.03 – 4.05
• Law of Conservation of Mass: Matter is neither
created nor destroyed in a chemical reaction.
• So, what does this have to do with
equations?
It is important that chemical equations apply this
law by having the same number of atoms of each
element on both sides of the equation; chemists
call this a “balanced” equation.
So, why do we need to balance?
• Consider the following reaction between
hydrogen gas and oxygen gas:
N2 + H2  NH3
• Both hydrogen and nitrogen are diatomic
elements. When they react, they will
always produce ammonia, NH3. Notice how
it appears as though we lost a nitrogen
atom and gained a hydrogen atom?
• We really didn’t. We just need to balance
this equation! Let’s get to it.
Diatomic Elements
• Please make sure that you know the 7
diatomic elements.
H2, N2, O2, F2, Br2, Cl2, I2
• When these elements are by themselves in
a reaction, they will always have a subscript
of a two. (ex: N2 + H2  NH3)
• If these elements are in an ionic bond, you
must look at their charges (Recall lesson
3.05/3.08)
What do we do to balance an equation?
• When we balance equations, we CANNOT
change subscripts. Subscripts are what
make the chemical. If we change the
subscripts, we change the chemical and
thus change the reaction.
H2O is water
H2O2 is hydrogen peroxide.
• So, what do we do to balance an equation?
– We add a coefficient in front of the chemical.
– Let’s work through an example.
Basic Steps in Balancing Equations:
1. Make a table of total number of atoms of
each element for the reactants and for the
products.
N2 + H2  NH3
Element
N
H
Reactants
Products
Literacy strategy
• Why should we make a table?
– Tables help organize challenging or
complicated material. This will aid in
understanding and mastering the concept.
– In balancing equations, a table can help us
quickly see how many atoms are on both sides
of the equation. This will enable us to balance
the equation and check our answer easier.
Basic Steps in Balancing Equations:
2. Determine which elements are not
balanced. Let’s add a coefficient in front of
the chemical to get them balanced. Update
table with new TOTALS for each element.
N2 + H2  NH3
Element
Reactants
Products
N
2
1
H
2
3
Basic Steps in Balancing Equations:
3. Determine if any other element needs to
be balanced. If yes, place a coefficient to
balance it. Update table with new TOTALS.
Repeat for all remaining elements.
N2 + H2  2NH3
Element
Reactants
Products
N
2
2
H
2
6
Basic Steps in Balancing Equations:
4. Check your final equation. Double check
that you have not made an error. It is very
easy to make silly mistakes in balancing.
N2
Element
N
H
+ 3H2  2NH3
Reactants
Products
Steps in Balancing Equations Summary
Summary of the steps:
1. Make a table with the total number of atoms of each
element for the reactants and for the products.
2. Determine which elements are not balanced. Add a
coefficient in front of the chemical to balance the
element. Update table with new totals.
3. Continue for all elements until they are al balanced
Update table with new totals.
4. Once all elements are balanced, double check your totals
to catch any silly mistakes.
Let’s do Some Practice
• Now that we’ve seen a easy way to balance
equations by using the table, let’s do some
practice.
• There are 4 practice problems.
• Watch all, or stop if you feel comfortable
balancing equations.
Practice #1
Fe2O3
Element
Fe
O
H
+
Reactants
H2
→
Fe + H2O
Products
Practice #2
H2S
Element
H
S
O
+
O2 →
Reactants
H2O
+
SO2
Products
Practice #3
C8H18
Element
C
H
O
+
O2 
Reactants
CO2
+
Products
H2O
Practice #4
Al2(SO4)3
Element
Al
S
O
K
H
+
KOH →
Reactants
Al(OH)3 +
Products
K2SO4