Balancing Chemical Equations

TEKS 8.5D recognize that chemical formulas are used to identify substances and determine the number of atoms of each element in chemical formulas containing subscriptsTEKS 8.5F recognize whether a chemical equation containing coefficients is balanced or not and how that relates to the law of conservation of mass

occurs when the chemical bonds between elements are formed or broken.

A chemical reaction

This requires a gain or loss of

energy.

is a short, simple way to describe a chemical reactions using symbols and formulas instead of words.

A chemical equation

are the substances you begin with in a chemical reaction.

Reactants

These are shown to the left side of an arrow called the yield sign.

Fe + 2 HCl FeCl2 + H2

reactants products

yield sign

During a chemical reaction, the atoms rearrange to form new bonds between

atoms.

Productsare the substances you end with in a chemical reaction.

Products are shown to the right of the yield

sign arrow.

Fe + 2 HCl FeCl2 + H2

reactants products

yield sign

The products formed in a chemical reaction have

different properties from the reactants.

A yield sign

Fe + 2 HCl FeCl 2 + H 2

is used to show the direction of the

chemical reaction.

A chemical equation is read like this:

1 atom of iron

reacts with

2 molecules of

hydrochloric acid

to yield1

molecule of Iron

chloride

and

1 molecule

of hydrogen

Fe + 2 HCl FeCl2 + H2

During a chemical reaction, atoms cannot be created or destroyed, so the number of atoms at the beginning of a reaction MUST equal the number at the end of the reaction!

This is called the Law of Conservation of Mass.

This law also means that the mass of the reactants must equal the mass of the products.

What if the number of atoms in the reactants does not match the number in the products?

C3H8 + O2 CO2 + H2O

mass =76 gramsmass =76 grams mass = 62 mass = 62 gramsgrams

3-carbon3-carbon 1-carbon1-carbon

8-hydrogen8-hydrogen 2-hydrogen2-hydrogen

2-oxygen2-oxygen 3-oxygen3-oxygen

The Law of Conservation of Mass says that the number of each atom must be equal on both sides of the yield sign along with the masses. Let’s balance the equation using some basic rules.

Basic Rules for Balanced Equations

Chemical formulas

CANNOT be altered!

When balancing an equation, the subscript

cannot be changed.

H2O

Don’t change me!

Coefficients can be added before

an element or molecule.

C3H8 + O2 => CO2 + H2O5 3 4

Add coefficients to the equation to

balance the number of atoms

C3H8 + 5O2 => 3CO2 + 4H2O3-carbon 3-carbon8-hydrogen 8-

hydrogen10-oxygen 10-

oxygen

Atoms on each side

are equal!!

Coefficients were used to balance the number of atoms on

each side of the chemical equation.

If we mass the reactants and products before

and after the reaction, the masses are equal. We have followed the

law!

mass = 204 grams =>mass = 204 grams

C3H8 + 5O2 => 3CO2 + 4H2O