Intro to Balancing Equations

Q: How could you represent this reaction (rusting bicycle), besides just describing it in words?

A: Scientists use a standard method to represent a chemical reaction, called a chemical equation.

Fe + O2 Fe2O3 (Rust)

ANALOGY for a Chemical Equation

FRAMEWHEEL HANDLEBAR PEDAL

BYCICLE - New product

Chemical equations are used to represent

CHEMICAL REACTIONS

Reactants: Zn + I2 Product: Zn I2

Q: What is the general equation for the reaction in which iron rusts?

Iron + Oxygen → Iron Oxide

Or, using symbols:

Fe + O2 → Fe2O3 (rust)

Q: What are the reactants and products?

Fe + O2 → Fe2O3 (new product - rust)

Reactants Products

Because of the principle

of the conservation of matter, an equation must be balanced.

Lavoisier, 1788

Chemical Equations

When is an equation called balanced?

when an equation has the same number of atoms of the same kind on both sides.

Learning Check:A chemical reaction is balanced

whena. The reactants are the same as the products

b. The same atoms and molecules appear on each side of the equation

c. The same number of each type of atom appears on both sides of the arrow.

d. None of the above

The substances that begin a chemical reaction are called

REACTANTS

The substances that form in a chemical reaction are called

PRODUCTS

Chemical Equations

4 Al (s) + 3 O2 (g) ---> 2 Al2O3 (s)

The numbers in the front are called

The letters (s), (g), and (l) are the physical states of compounds. (aq) – aqueous solution

coefficients subscripts

The arrow means: to yield = to produce = to form

Learning Check – Cold Call

What does “2“ mean in front of LiOH?

2Li (s) + 2H2O (l) 2LiOH (aq) + H2 (g)

What does “2“ to the right of “H2” mean?

What does “(aq)“ to the right of LiOH mean?

Number of molecules

Number of atoms of H

Aqueous solution

What does ““ mean?Yield, form, produce

Q: What is the chemical equation for this reaction?

A: The chemical equation is:

C + O 2 → CO 2

1 min – answer in your IAN

Q: How have the atoms of the reactants been rearranged in the products of the reaction?

C + O 2 → CO 2

Classroom discussion

A: Carbon atom chemically bonds with two oxygen atoms.

HOW ? What bonds are broken and what new bonds have formed?

A: Bonds between the oxygen atoms in the oxygen molecule have been broken, and new bonds have formed between the carbon atom and the two oxygen atoms.

C + O=O →

Classroom discussion

C + O 2 → CO 2

O=C=O

Bonds Break New Bonds Form

Learning Check 1

30 sec - Turn to your elbow partner…..

Q: What needs to happen for a chemical reaction (change) to take place?

A: Bonds between atoms of the reactants

break and NEW bonds form, producing

new substance.

Learning Check 210 sec Cold Call

Q: Is this chemical reaction correct? Explain why.

A: No. There are no “pink atoms” on the

reactant side – NEW matter (pink) on the

product side. Matter canNOT be created.

+ +

Learning Check 2

Why is this reaction correct?

B/C atoms must REARRANGE for a chemical reaction to take place

+ +

NEW PRODUCTS

Count the number of atoms of each element below:

An ATOM is the smallest particle of an element.

O2

H2O

HCl

Mg(NO3)2

2 atoms of oxygen (O)2 atoms of hydrogen & 1 atom of oxygen (O)1 atom of hydrogen (H) & 1 atom of chlorine (Cl)1 atom of magnesium (Mg), 2 atoms of nitrogen (N) & 6 atoms of oxygen (O)2x3 =6

2x1 =2

Count the number of molecules

below:

A MOLECULE is a pure substance

composed of two or more atoms.

O2

2H2O

17HCl

One molecule of oxygen gas

Two molecules of water

Seventeen molecules of an acid

1

Chemical Equations

4 Al(s) + 3 O2(g) ---> 2 Al2O3(s)

4 Al atoms

+ 3 O2 molecules

--->2 molecules

of Al2O3

This equation means:

Learning CheckNaHCO3

# of “O” atoms?

3

Mg(OH)2

# of “H” atoms?

2

C6H12O6

# of “C” atoms?

6

Complete Practice – Counting Atoms

5 min

Q: Is this reaction Balanced? When called on, be ready to

justify your answer

C + O 2 → CO 2

Cold Call

Which of the following equations is balanced?

a. 3Fe + 3O2 3Fe2O3

b. 2Fe + O2 Fe2O3

c. 4Fe + 2O2 3Fe2O3

d. 4Fe + 3O2 2Fe2O3

When called on, be ready to

justify your answer

Does this equation obey the Law of Conservation of Mass?

(Is it balanced?)

2H2O2 2H2O + O2

What is the coefficient ratio?

2:2:1

YES

Does this equation obey the Law of Conservation of Mass?

(Is it balanced?)

2Al + O2 Al2O3

Why?2 atoms of Al of left side2 atoms of Al on right side

NO

Al is balanced

2 atoms of O on left side3 atoms of O on right side

O is NOT balanced

REMEMBER:To balance chemical equation, you must add COEFFICIENTS (big numbers) in front of the chemical formulas in the equation. You cannot add or change subscripts!!! Why?

Because it will alter the identity of the substance otherwise.Ex. O2 – oxygen, if a subscript changed to 3, O3 = ozone

Balancing using shapes

____H2 + ____O2 ____H2O

Is this reaction balanced?

Now fix the H on the left.

Coefficient ratio: 2:1:2 BALANCED !

Double Hydrogen too.

2 21

Balancing using shapesYou try!

__Al2O3 ___Al + ___O2

Now fix the Al on the right.

Coefficient ratio: 2:4:3 BALANCED !

2 34

+

Balancing Equations using T-chart

___ H2(g) + ___ O2(g) ---> ___ H2O(l)

What Happened to the Other Oxygen Atom?????

This equation is NOT balanced!

1. Count the number of atoms of each element found in the reaction.

R P

2 H 2 H

2 O 1 O

Balancing Equations using T-chart

___ H2(g) + ___ O2(g) ---> ___ H2O(l)2 2

3. Don’t forget to

recount.

2. Add coefficients

to balance the atoms

1

2

4 4

Now fix the H on the left.

R P

2 H 2 H

2 O 1 O

Don’t forget there is a 1 in front of O2.

Now it is balanced!

Easy Balancing

Al + O2 Al2O33

RULE: Balance H and O last

R P

1 Al 2 Al

2 O 3 O

2

2

3 x 2 = 6

2:3 – find a common multiple

2

2 x 2 = 44

1

2 x 3 = 6

Coefficient ratio: 4:3:2

The charcoal used in a grill is basically carbon. The carbon reacts with oxygen to yield carbon dioxide. The chemical equation for this reaction,

C + O2 CO2,

Is this a balanced equation?

YES

R PCount the number of atoms of each element found in the reaction.

1 C

1 C2 O 2 O

You Try!Use shapes or T-cart method

Fe + O2 Fe2O3

_____Fe + ____O2 ____Fe2O32 2

3x2 = 6

Common multiple

3

2x2 = 4

Coefficient ratio: 4:3:2