L10 - Balancing chemical equations

Learning Goal:

Success
Criteria:

Why are we
learning this?

• Understand chemical equations

I can:
• identify the reactants and products.
• write word equations.
• balance chemical equations.

Balancing chemical equations is
important to obey the law of conservation
of mass. It is important in many industrial
processes, e.g. to work out how much
materials are needed.

• Law of conservation of mass – matter is not created or

destroyed in a chemical reaction. Atoms are just
rearranged.

• Number of atoms on the left = number of atoms on the right.
• Example:

Balancing Chemical Equations

​Terms:

Multiple Choice

The substances that go into a reaction are called the _______.

• Reactants

• Products

Multiple Choice

The products are on the ______ side of the equation.

• Left

• Right

• Write the equation.
• Count the atoms of each element in the reactants and the products.
• Choose an element and change the coefficients to make it equal on

both sides of the equation.

• Balance the remaining reactants and products.
• Check that the equation is correct and the coefficients are in their

lowest possible ratio.

• The coefficients are the only numbers that can be changed –

you can’t change chemical formulas.

Rules for Balancing Equations

• The small numbers (subscripts) are part of the formula of

the compound and cannot be changed. The subscript
only applies to the element it is written at the end of.

E.g. CO2 is 1 carbon atom and 2 oxygen atoms

Balancing Chemical Equations

Multiple Choice

What does the Law of Conservation of Mass say? It says,

"During a chemical reaction, mass is neither _______ or destroyed."

• burned

• created

• melted

Multiple Choice

The Law of Conservation of Mass:

The mass of all REACTANTS must be __________ the mass of all PRODUCTS in a chemical reaction.

• equal

• greater than

• less than

• The large numbers at the front of the formula

(coefficients) can be changed to balance the equation.
The coefficient applies to all parts of the compound.

E.g. 2MgO is 2 magnesium atoms and 2 oxygen atoms.

If an element has a coefficient and a subscript, we
multiply these numbers.
E.g. 2CO2 is 2 x 2 = 4 oxygen atoms

Balancing Chemical Equations

Multiple Choice

How many Cl atoms in $$2Cl_2$$ ?

• 1

• 2

• 3

• 4

Multiple Choice

How many H atoms are in $$H_2O$$ ?

• 1

• 2

• 3

Balancing equations

Hydrogen + Oxygen → Water

H2 + O2 →

H2O

???????

HH

O O

Remember we need to balance it!

What do you notice??

Balancing equations

HH

O O

Why is this wrong??

We haven't used both of the oxygen atoms.

Remember we need to balance it!

O

H H

Hydrogen + Oxygen → Water

H2 + O2 →

H2O

Balancing equations

O O

Remember we need to balance it, BOTH SIDES

Is this correct??

YES, There are now 4 hydrogen atoms and two oxygen atoms on the
left AND the same amount on the right ☺

O

H H

O O O

H H

O

H H
H H

H H

Multiple Choice

Who sang: You're the Voice?

• Jimmy Barnes

• John Farnham

• Meatloaf

• ACDC

C + O2 → CO2

C

O

O

O

C

O

How Will Our See-Saw Look?

C + Cl2 → CCl4

Cl

C

Cl

Cl Cl

Cl

Cl

C

It’s a Bit Product Heavy

Cl

C

Cl

Cl

Cl

C

C + 2Cl2 → CCl4

Now everything balances!

C + 2Cl2 → CCl4

Cl

C

Cl

Cl

Cl

Cl

Cl

C

Cl

Cl

Video:

◦ Professor Dave Explains - Balancing chemical equations

(4:17)

Link to Prof. Dave video

Link:

https://www.youtube.com/watch?v=X7ckfeRjjvI&t=34s

Balancing Chemical Equations

Coefficient: number written in front of a chemical formula.

Example

Multiple Choice

How many O atoms are on the left?

$$CO_2\ +\ H_2O\ \rightarrow\ C_6H_{12}O_6\ +\ O_2$$

• 1

• 2

• 3

• 4

Multiple Choice

Is this equation balanced?

$$CO_2\ +\ H_2O\ \rightarrow\ C_6H_{12}O_6\ +\ O_2$$

• Yes

• No

PhET simulation
Simulation

Google:
phet simulation
balancing chemical
reactions

Link to PhET interactive:

​https://phet.colorado.edu/sims/html/balancing-chemical-equations/latest/balancing-chemical-equations_en.html

Worksheet

Element

Reactants

Products

You can draw a table in your book to help
balance the equations

Multiple Choice

Where is this?

• U.S.A.

• Italy

• France

• Tokya

Answers

Examples:

1.

____ KClO3
____ KCl + ____ O2

2. ____ C3H8 + ____ O2
____ CO2 + ____ H2O

Balancing Chemical Equations