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Balancing Chemical Equations

The Balanced Equation

Atoms can’t be created or destroyed. All the atoms we start with we must end

up with. A balanced equation has the same

number of each element on both sides of the equation.

Balanced equations show how mass and atoms are conserved.

The Balanced Equation

C + O2 ® CO

This equation is NOT balanced. There is one carbon atom on the left

and one on the right. There are two oxygen atoms on the left

and only one on the right.

C + O® COO

C + O2 ® CO We need one more oxygen atom in the

products. We can’t change the formula, because it

describes what it is.

C + O® COO

In order to have two oxygen atoms, another CO must be produced.

But where did the other carbon come from?

C +O

®C

OO

OC

We must have started with two carbon atoms.

C

+O

®C

OO

OC

C

The balanced chemical equation is

2 C + O2 ® 2 CO

C

+O

®C

OO

OC

C

Chemical Equations

Numbers and types of atoms must balance.

Rules for Balancing

1. Write the correct formulas for all the reactants and products.

2. Count the number of atoms of each type appearing on both sides.

3. Balance the elements one at a time by adding coefficients (the numbers in front).

4. Check to make sure it is balanced.

Never

Never change a subscript to balance an equation.

If you change the formula you are describing a different reaction.

Never

Never put a coefficient in the middle of a formula.

2 NaCl is okay; Na2Cl is not.

Example

H2 + H2OO2 ®

Make a table to keep track of where youare in the balancing process.

Example

H2 + H2OO2®

We need twice as much O in the product.

Reactants

ProductsH

O

2

2

2

1

Example

H2 + H2

OO2

®

We need 2 oxygen atoms on the product side, so a coefficient of 2 should be placed in front of water.

2

Example

H2 + H2

OO2

®

We must recalculate the number of hydrogen and oxygen atoms on the right. Coefficients are used as multipliers. R P

H

O

2

2

2

1

2

4

2

Example

H2 + H2

OO2

®

Now we need 4 hydrogen atoms on the reactant side.

R P

H

O

2

2

4

2

2

Example

H2 + H2

OO2

®

A coefficient of 2 should be placed in front of hydrogen gas.

R P

H

O

2

2

4

2

22

4

Example

H2 + H2

OO2

®

The equation is balanced; it has the same number of each kind of atom on both sides!

R P

H

O

4

2

4

2

22

Example

H2 + H2OO2®

This is the answer,

R PH

O

2

2

2

1

2

2

4

2

4

not this.

Balancing Hints

Balance elements in the following order: metals nonmetals hydrogen oxygen

Balancing Hints

If an atom appears more than once on a side, balance it last.

If you fix everything except one element, and it is even on one side and odd on the other, double the first number, then move on from there.

Problem

Balance the following equation.

CH4 + O2 ® CO2 + H2O

CH4 + 2 O2 ® CO2 + 2 H2O

Problem

2 AgNO3 + Cu ® Cu(NO3)2 + 2 Ag

Balance the following equation.

AgNO3 + Cu ® Cu(NO3)2 + Ag

Problem

Balance the following equation.

Mg + N2 ® Mg3N2

3 Mg + N2 ® Mg3N2

Problem

Balance the following equation.

P + O2 ® P4O10

4 P + 5 O2 ® P4O10

Problem

Balance the following equation.

Na + H2O ® H2 + NaOH

2 Na + 2 H2O ® H2 + 2 NaOH

Objectives

PSc.2.2.5–Classify types of reactions such as synthesis, decomposition, single replacement, or double replacement.