Balancing Chemical Equations

Objective: Learn how to Balance Chemical Equations

Warm Up: Balance the chemical equation Na + Cl –> NaCl
Please show all your work.

Today’s topic 4 (really 5) types of chemical reactions.

Draw a Quad Chart showing the 5 types of reactions. Note two of these are naturally combined in one box pf the quad chart.

From your reading: Prentice Chemistry pg 330-336 you already know this so if you do consider this a review.

4 Types of Chemical Reactions
Synthesis
A + B -> AB
Combustion (Oxidation)
A + O -> AO
Decomposition
AB -> A + B
Replacement ( Single or Double)
AX + B -> A + BX

AX + BY -> AY + BX

Here’s an example of an Oxidation Reaction


Introduction / motivation: Every time I speak to a college student and they find out I teach Chemistry, they always tell me the one thing they have the most trouble with is balancing chemical equations. In this lesson, I’m going to teach you how to balance chemical equations one step at a time.

Definitions: How is balancing a chemical equation different than balancing  a mass equation or converting using Avogadro’s Number?

Review last class’ demonstration – burning steel wool. What kind of reaction is this? (combustion – oxidation)

Oxidation numbers:

Find Iron on the periodic table. Fe is a metal so it gives up electrons.

Fe(0) -> Fe(+1) + e- {loses electron}

Fe(0) -> Fe(+2) + e- + e- {loses 2 electrons}

Fe(0) -> Fe(+3) + e- + e- + e- {loses 3 electrons}

Find Oxygen on the periodic table, how many valence electrons does Oxygen have? (6).  Class answers vary – students call out 2, 6 and 8.  these are all correct especially from memory but should be put together in a sentence in the following way.

“Oxygen has 6 valence electrons and needs 2 more to have 8 outer shell electrons to completely fill its outer shell and to satisfy “The Octet Rule”.

O(0) -> O(-1)  {gains one electron}

O(0) -> O(-2)  {gains two electrons}

Apply concept to Prior Knowledge.

Review last class’ demonstration – burning steel wool. What kind of reaction occurred? (Combustion – oxidation) . Explain that the fast reaction of burning requires less energy to happen so it involves smaller oxidation state numbers as compared with rusting so burnt iron os Fe(+1) and rust is Fe(+3).  Also Burnt iron is blue and rusted iron is red or brown.

Essential idea: Balancing chemical equations is like cooking!

Ask the class who likes to cook.  Jonathan volunteers that he likes to cook Chile Rellenos.;  He offers the recipe – get out all the ingredients, then start cooking.

How to Balance Chemical Equations

1] decide on the ingredients, what is the cooked dish?

Reactants (reagents) —> Products

chiles + eggs + cheese + tomatoes  –> cooking —> Rellenos

2] how much of each ingredient.

Balancing chemical