Section 2.7—Balancing Equations

Objectives:•Use the Law of Conservation of Mass to explain the need for balanced equations•Convert word equations to balanced chemical equations

Review

1. How to write chemical formulasa. For ionic compounds:

1) write the symbol and charge for the cation

2) write the symbol and charge for the anion

3) criss-cross the NUMBER only of the charge

to obtain a subscript

4) Reduce the subscripts, if necessary

5) Use parentheses when there is more than

one polyatomic ion needed.

Review

b. For binary covalent compounds:

1) Write the symbols of the first and

second elements

2) Translate any PREFIXES into a

subscript of that element

Example: dinitrogen pentoxide

N2O5

Review

c. For acids without oxygen (the acid will be named with the prefix “hydro”): 1) The cation will be H+

2) The anion will be found in the root of the acid’s name (following the “hydro”) – determine its symbol & charge 3) Criss-cross the numbers of the charges to get the formula

Review d. For acids with oxygen (acid name will end in “ic” or “ous”): 1) The cation is H+

2) If the acid ends in “ic”, look for a polyatomic anion that ends in “ate” 3) If the acid ends in “ous”, look for a polyatomic anion that ends in “ite” 4) Criss-cross the numbers of the charges to get the formula

REMEMBER: Bases are just ionic compounds with the anion “hydroxide”. NH3 (ammonia) is an exception.

Review

e. There are seven elements that are found in nature as diatomic molecules. They must be written that way in a formula.

H2 N2 O2 F2 Cl2 Br2 I2

Review

2. How to write formula equations

a. Locate each name of a compound and

write its formula (using those rules

reviewed in previous slides.)

b. Separate all reactants and products

with a “+” sign

c. Separate reactants ffrom products with

an

Practice

Hydrogen and oxygen react to form water.

Law of Conservation of Matter/Mass

Notice that, in this practice equation, the number of atoms that we start with does not equal the number of atoms we finish with.

2 H atoms + 2 O atoms 2 H atoms + 1 O atom

This violates a scientific law that says matter cannot be destroyed (or created.)

Law of Conservation of Matter/Mass

Law of Conservation of Matter – Matter cannot be created nor destroyed during chemical or physical changes

Also called the Law of Conservation of Mass (since all matter has mass)

How Does the Law Lead to Balancing?

Law of Conservation

of MatterTherefore… So we must…

Matter cannot be created nor destroyed during a chemical or physical change

The matter on the reactants side and the matter on the products side must be the same

Ensure the numbers of each type of atom are the same on both sides of the equation…by balancing!

How do we Balance Equations?

2 H2 + O2 2 H2O

Subscripts

Coefficients

# of atoms in a compound

Number of compounds in the reaction

Subscripts balance charges within a compound.

Coefficients balance atoms in an equation

What do Coefficients Really Mean?

CH4 + 2 O2 CO2 + 2 H2O

Total:1 C4 H4 O

Total:1 C4 H4 O

The equation is balanced.

H

C

H

HH

O O

O O

CO O HO

H

HO

H

H

CC

H

HH

O O

O O

CCO O HO

H

HO

H

How to Balance Chemical Equations

Make a table of elements

_____ __________

How to Balance By Inspection:

1

Reactants Products

CH4 + O2 H2 O CO2+_____

C CH HO O

2 Count the number of each element on the reactants and products side.

_____ __________

How to Balance By Inspection:

Reactants Products

4 H

2 O 3 O

2 H

1 C 1 C

CH4 + O2 H2 O CO2+_____

Don’t forget to add all the atoms of the same element together—even if it appears in more than one compound!

3Each time you add a coefficient, update your table with the new quantities of each atom.

ADD COEFFICIENTS TO BALANCE THE NUMBERS

_____ __________2

How to Balance By Inspection:

Reactants Products

4 H

2 4 O 3 4 O

2 4 H

1 C 1 C

2CH4 + O2 _____ H2 O CO2+

Let’s Practice #1

Example:Balance the

following equation

__ H2 + __ O2 __ H2O

Let’s Practice #2

Example:Balance the

following equation

__ H2 + __ O2 __ H2O2 2

Let’s Practice #2

Example:Balance the

following equation

__ HCl + __ Ca(OH)2 __ CaCl2 + __ H2O

Let’s Practice #1

Example:Balance the

following equation

__ HCl + __ Ca(OH)2 __ CaCl2 + __ H2O2 2

Did you see the “OH” polyatomic ion & change H2O to HOH?

Let’s Practice #3

Example:Write and

balance the following equation

Iron and oxygen combine to produce iron (III) oxide.

Let’s Practice #3

Example:Write and

balance the following equation

__ Fe + __ O2 ___ Fe2O34 3 2