Balancing Chemical Equations

Compounds and Molecules

Before we can balance equations, we need to be able to count the atoms in a compound or molecule. Example:

O2 There are 2 atoms of oxygen in this molecule.MgCl2 There is 1 atom of Magnesium and 2 atoms of Chlorine.

Compounds/Molecules are bonded together. They CANNOT be separated!!

You try!!

Count the atoms in the compounds/molecules below.

Compound/Molecule

CaCl2 Ca = Cl =

K2O K = O =

CO3 C = O =

Coefficients and Subscripts

Coefficients in front of the compound/molecule means that there are multiple compounds/molecules.Subscripts are after an atom and show how many of that particular element are in a compound.For example:

2 H2O means that there are two water moleculesThis is one water molecule. There are 2

Hydrogen and 1 Oxygen.If I have two water molecules, there are 4 Hydrogen and 2 Oxygen

You try!!Given the compounds/molecules, fill in the chart

Picture Compound/Molecule

2 NO2 N = O =

3 NH3 N = H =

What did you notice about the coefficients and subscripts?

Look back at our examples and write a short response to the title of this slide.

Balancing EquationsImagine that your equation is on a scale. The left side and the right side must contain the same number of atoms.The left side of our equation is called the reactants. The is like an equal sign. When you see it, say “yields”.The right side of our equation is called the products.Example: 2 H2 + O2 yields 2 H2O

2 H2 + O2 2 H2O

Step 1: List your elements

H2 + O2 = H2O

H = H =O = O =

List the reactants side, then list the products side.

Step 2: Count your atoms

H2 + O2 = H2O

H = H =O = O =

Count the atoms on the reactants side, then count the atoms on the products side.

Step 3: Find where it is unbalanced

H2 + O2 = H2O

H = 2 H = 2O = 2 O = 1

• O on the reactants side has 2 Oxygen atoms, but O on the products side only has 1. Let’s add a COEFFICIENT of 2 in front of the products side. So now, it will look like this:

H2 + O2 = 2 H2O

Step 4: Recount your atomsH2 + O2 = 2 H2O

H = H =O = O =

After you recount your atoms, is it balanced now?H = 2 H = 4O = 2 O = 2

Oxygen is balanced, but now Hydrogen is not. Find the low side, and add a coefficient of 2.

Step 5: Add a coefficient and recount

2 H2 + O2 = 2 H2O

H = H =O = O =

Count your atoms on the reactants side, then count your atoms on the products side. Are they balanced?

H = 4 H = 4O = 2 O = 2

Yes! They are balanced! We have the same number of atoms on the reactants as products side.

Recap

Step 1: List your elementsStep 2: Count your atomsStep 3: Find where it is unbalancedStep 4: Add a coefficient and recount your atomsStep 5: Add a coefficient (if necessary) and recount

DON’T FORGET!! You can only add coefficients! Never change the subscripts!!