Balancing Chemical Equations

Essential Standard 2.2Understand chemical bonding and

chemical interactions.

Learning Objective 2.2.4

Exemplify the law of conservation of

mass by balancing chemical

equations.

I Can StatementsAt the end of this lesson, you should be

able to say, with confidence:

• I can state what the Law of Conservation

of Mass is and explain what it means.

• I can balance chemical equations.

Glucose & Oxygen

Chemical ReactionsChemical Reactions are a chemical change

in which one or more substances are

converted into new substances

Reactants and Products

Reactants are the substances that

react and products are the substances

that are produced.

CO2 + H2O C6H12O6 + O2

Reactants Products

Chemical reactions are expressed as

chemical equations.

Conservation of MassThe Law of Conservation

of Mass states that in a

chemical reaction, matter

is not created or destroyed,

only transformed.

In this example, the mass of the

candle and oxygen gas before

the candle is burned is the same

as the mass of the candle and

carbon dioxide gas, after the

candle is burned.

Rearrangement of AtomsDuring a chemical reaction, no new atoms

are added, they are only rearranged.

CO2 + H2O C6H12O6 + O2

1 Carbon

2 Hydrogen

3 Oxygen

The above reaction could not take place

because you have more carbon, hydrogen,

and oxygen atoms in the products than were

in the reactants.

6 Carbon

12 Hydrogen

8 Oxygen

Balanced ReactionsIn order to know which reactions would take

place, we have to balance the equations.

The number of each type of atom on the

reactant side must equal the number for each

type of atom on the produce side.

6CO2 + 6H2O C6H12O6 + 6O2

6 Carbon

12 Hydrogen

18 Oxygen

6 Carbon

12 Hydrogen

18 Oxygen

Balanced ReactionsWhat this means is that in order for the

photosynthesis reaction to occur, there

must be 6 CO2 molecules and 6 H2O

molecules, in order to produce 1 glucose

molecule and 6 O2 molecules.

6CO2 + 6H2O C6H12O6 + 6O2

6 Carbon

12 Hydrogen

18 Oxygen

6 Carbon

12 Hydrogen

18 Oxygen

CoefficientsThe numbers written in front of molecule are

called coefficients.

Coefficients are used to balance equations.

6CO2 + 6H2O C6H12O6 + 6O2

Coefficients & SubscriptsWhen balancing chemical equations, you can

change the coefficients, but you cannot

change the subscripts.

6CO2

The 6 indicates that there are 6 molecules

of CO2

CO2 CO2 CO2 CO2 CO2 CO2

SubscriptsThe subscript, 2, signifies that within a CO2,

molecule there will always be 2 oxygen atoms

for every carbon atom. (it’s a ratio of 1:2)

6CO2

Even with the 6 coefficient added, the ratio of

atoms within the molecule does not change.

6 C 12 O

It is still a 1:2 ratio.

Balancing Equations

1. List the elements in alphabetical order

2. Write down the number of atoms per element

2. Find a common numerator and write it into the

reaction as a coefficient

3. Change the number for all elements that are

affected

4. Do the same for the remaining elements

There are five basic steps to balance any

chemical equation:

Balancing EquationsThis chemical reaction occurs when propane

is burned in the presence of oxygen and water

and carbon dioxide is produce.

Propane Oxygen Water Carbon

Dioxide

First StepList the elements for the reactants and the

products in alphabetical order.

Second StepWrite down the number of atoms per element.

Notice that the oxygen atoms from the H2O and CO2

were added together on the product side.

Third StepFind a common numerator and add it to the

reaction as a coefficient.

Beginning with the first element, C, a common

numerator for 3 and 1 would be 3.

Third StepFind a common numerator and write it into the

reaction as a coefficient.

After the 3 is written as a coefficient, change the number

of carbons on the product side to 3.

Fourth StepChange the number for all the elements that

are affected.

The oxygen atom was also affected, so now there are

7 oxygen atoms on the product side.

Fifth StepDo the same for the rest of the elements.

A common numerator of 8 and 2 is 4, so a 4 is written

on the product side as a coefficient in front of H2O.

Fifth StepDo the same for the rest of the elements

The oxygen atom in the H2O was also affected, so

now there are 10 oxygen atoms on the product side.

Fifth StepDo the same for the rest of the elements

A common numerator of 10 and 2 is 5, so a 5 is written

on the reactant side as a coefficient in front of O2.

Check Your WorkThe equation is balanced when the number of

each type of atom on the reactant side equals the

number of each type of atom on the product side.

Practice

HgO Hg + O2

Balance the following equation:

Seriously, try it.

Then, check your answer.

Practice

HgO Hg + O2

1 Hg

1 O

1. List the elements in alphabetical order

1 Hg

2 O

2. Write down the number of atoms per element

Practice

HgO Hg + O2

1 Hg

1 O

1 Hg

2 O

2 is a common numerator of 1 and 2, so write in

a 2 as a coefficient for HgO on the reactant side.

Hg is already

balanced, so move

on to O.

3. Find a common numerator and add it to the

equation as a coefficient

Practice

2 HgO Hg + O2

4. Change the number for all the elements

affected

2 Hg

2 O1 Hg

2 OOn the reactant side, both the Hg and the O

were affected.

The equation is now balanced.

Last ThingIf there is an odd number of any of the atoms,

multiply them together to find the common

numerator.

3H + O H2O

3 H

1 O2 H

1 O

3 x 2 = 6.

Last ThingBut to get the numerator of 6, you multiply the

3 on the reactant side by 2.

6H + O 3H2O

(2) 3 H

1 O

(3) 2 H

1 O

Then, multiple the 2 on the product side by 3.

Then, Fix the RestBut to get the numerator of 6, you multiply the

3 by 2 on the reactant side.

6H + 3O 3H2O

6 H

3 O

6 H

3 O

The equation is now balanced.

The End