Conceptual Physics- Chemical Equations
Day 1 | Day 2 | Day 3 - 4 | Lab | Vocabulary Test

This Unit's	Process Standards: 1.1, 2.1, 3.5, 4.2, 4.5, 5.1, 5.3, 6.4 Content Standards: 1.1, 1.2 Instructional Technology Standards: 2.1, 4.2

Chemical equation Balanced equation	Subscript Coefficient	Balancing by inspection Diatomic molecules

There will be a formal test before Lab Day. You will be asked to use the seven steps below to balance chemical equations by inspection. The vocabulary test for this week is similar to the test you will take. A pencil and one hand written page of notes may be used during the test, but nothing else.
How many hydrogen atoms are represented in this drawing?

Chemical equations tell you the following:

The substances that react together.
The substances that are formed.
The number of atoms and molecules of each substance involved.

• The arrow () is read as "yields".
• The arrow always points from reactants to products.
  • Most equations are written with the arrow pointing from left to right like this .
  • There are special situations that call for the arrow to be written pointing from right to left like this . Since the arrow always points from reactants to products, the reactants would be on the right if the arrow is pointing this way.
  • There are chemical reactions that begin to form products and the products break down into the original reactants. These reactions are known as "reversible reactions" and are indicated with arrows pointing in both directions like this .

Equations must be balanced because:

Atoms can be neither created nor destroyed by ordinary chemical means,
so there must be the same number of atoms on both sides of the equation.

These numbers are found in a chemical equation:

Subscripts:

The small numbers to the lower right of chemical symbols. Subscripts represent the number of atoms of each element in the molecule.

Coefficients:

The large numbers in front of chemical formulas. Coefficients represent the number of molecules of the substance in the reaction.

Just as subscripts of 1 are never written, coefficients of 1 are not written either. Both are "understood".

The balanced equation S8 + 12O2 8SO3 tells us that one molecule of S8 reacts with twelve molecules of O2 to produce eight molecules of SO3.

Using coefficients and subscripts to count atoms in equations:

Multiple the coefficient in front of the chemical formula by the subscript after the atom.
number of atoms = coefficient X subscript

Example: How many atoms of hydrogen and oxygen are represented in the graphic at the top of this page? # of H atoms = coefficient 2 X subscript 2 = 4 # of O atoms = coefficient 2 X subscript 1 = 2

Atoms inside parenthesis in a formula have two subscripts.

# of atoms = coefficient X subscript inside ( ) X subscript outside ( )

Day 2

Why must chemical equations be balanced?

The order in which these 7 steps are performed is important.

While there are some shortcuts that can be used, following these steps in the order given below is the best way to be sure your equation is correct.

1. Check for Diatomic Molecules - H2 - N2 - O2 - F2 - Cl2 - Br2 - I2 If these elements appear by themselves in an equation, they must be written with a subscript of 2 2. Balance Metals 3. Balance Nonmetals 4. Balance Oxygen 5. Balance Hydrogen 6. Recount All Atoms Balance equations by changing coefficients, never by changing subscripts in formulas. If the atoms are not balanced at this point, there is a problem somewhere. Work your way back up the steps until you find the problem, and correct it. 7. If every coefficient will reduce, rewrite in the simplest whole-number ratio. An equation is not properly balanced if it is not written in its lowest whole-number ratio.

Play a game balancing equations.

Day 3 - 4

What does this arrow mean in an equation?

Indian Health Services

Use the next two class days to practice balancing these equations.

Research Links:

• Equation Balancing - Diamond Bar High School ChemTeam

Physical Science

#1 - "Counting Atoms" answers:

1. • Ca - 3
   • P - 2
   • O - 8
2. • H - 2
   • P - 1
   • O - 4
3. • H - 8
   • C - 6
   • O - 6
4. • Pb - 2
   • N - 4
   • O - 12
5. • C - 3
   • F - 3
   • Cs - 3
6. • N - 2
   • H - 8
   • S - 1
   • O - 4
7. • K - 6
   • Cl - 6
   • O - 18
8. • Al - 2
   • S - 3
   • O - 12

#2 - same number on both sides:

1. no
2. no
3. yes
4. no
5. no

"Balancing Equations" answers:

1. 2 NaOH Na₂O + H₂O
2. 4 Fe + 3O₂ 2 Fe₂O₃
3. 6 CO₂ + 6 H₂O C₆H₁₂O₆ + 6 O₂
4. FeS + 2 HCl FeCl₂ + H₂S
5. O₂ + 2 H₂ 2 H₂O
6. Cl₂ + 2 NaI 2 NaCl + I₂
7. 2 Al(NO₃)₃ + 3 H₂SO₄ Al₂(SO₄)₃ + 6 HNO₃

The coefficient multiplied by the subscript indicates a total of 4 hydrogen atoms.

A chemical equation must be balanced because of the Law of Conservation of Matter and Energy.
Chemical reactions cannot create or destroy atoms. For this reason, there must be the same number of atoms after a reaction as before.

An arrow pointing in both directions in a chemical equation means the reaction is reversible.
The reaction begins going from left to right as usual. As more and more of products form on the right, some of them begin to break down into the original reactants. Part of the reaction is now going from left to right while some is going from right to left.