section 2.7—balancing equations

Section 2.7—Balancing Equations

We need to finish writing those equations we started!

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

H

O

C

CH4 + O2 H2 O CO2+_____

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

_____ __________

How to Balance By Inspection:

Reactants Products

H

O

4

2 3

2

C 1 1

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

H

O

4

2 3

2

C 1 1

2

4

44

CH4 + O2 _____ H2 O CO2+

4

Filling each coefficient location lets you and the grader know that you finished the problem rather than you left some blank because you weren’t done!

Place a “1” in any empty coefficient location

_____ __________2

How to Balance By Inspection:

Reactants Products

H

O

4

2 3

2

C 1 1

2

4

44

1 1CH4 + O2 _____ H2 O CO2+

Choosing the Order of Balancing

Save for laterElements that are uncombined

Save for laterElements that appear more than 1 time per side

StartElements that appear

only 1 time per side

StartElements in most

complicated molecules

How do you know what order to balance in?

Pb + PbO2 + H+ Pb2+ + H2O

To balance this equation, use the order: O, H, Pb

_____

How is Balancing Affected by Order?

What happens if we balance in the order determined in the last slide?

Reactants Products

H

Pb

1

2 1

2

O 2 1

4

2

_____ __________1 21 2Pb Pb+ O2 H2 O Pb2++H+_____4+

2

4

O, H, Pb

2

_____

What about a different order?

How is it different if we balance in a different order?

Reactants Products

O

Pb

2

2 1

1

H 1

2

2

_____ __________1 21 2Pb Pb+ O2 H2 O Pb2++H+_____2+

2 4

4

4

You’ll still get to the correct answer, but it will take longer and be more complicated!

H, O, Pb

Polyatomic Ions

Polyatomic ion – Group of atoms that together has a net charge

e.g. Nitrate NO31-

Carbonate CO32-

OH is a polyatomic ion that is sometimes “hidden” in H2O.Re-write H2O as HOH to “see” the OH polyatomic ion.

Make a table of elementsYou may leave polyatomic ions together—IF they appear intact on both sides of the reaction.

__________ __________

Balancing with Polyatomic Ions:

Reactants Products

PO4

Ca

H

Ca (OH)2+H3 Ca3 (PO4)2 H2O+PO4

OH

HOH

1

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

__________ __________

Balancing with Polyatomic Ions:

Reactants Products

PO4

Ca

1

1 3

2

H 3 1

Ca (OH)2+H3 Ca3 (PO4)2 H2O+PO4

OH 2 1

HOH

3 Add coefficients to balance the numbers

__________ __________2

Balancing with Polyatomic Ions:

Reactants Products

PO4

Ca

1

1 3

2

H 3 1 6

3

3 6Ca (OH)2+H3 Ca3 (PO4)2 H2O+PO4

OH 2 1

HOH

6

2

6 6

Let’s Practice #1

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 1 1 2

HOH

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

Let’s Practice #2

Example:Balance the

following equation

__ H2 + __ O2 __ H2O

Let’s Practice #2

Example:Balance the

following equation

__ H2 + __ O2 __ H2O2 1 2

Let’s Practice #3

Example:Balance the

following equation

__ Fe + __ O2 ___ Fe2O3

Let’s Practice #3

Example:Balance the

following equation

__ Fe + __ O2 ___ Fe2O34 3 2