Transcript Balancing Chemical Equations

Introduction
to:
Balancing Chemical Equations
Outline
Information in Chemical Equations
Why do we have to balance equations??
Human Balancing Act
Balancing Chemical Equations - Easy as 1,2,3...4!
Practice makes perfect or something along those
lines...
BIG IDEA of Unit
Chemicals react with each other in predictable ways!
Chocolate Chip Oatmeal
Cookies
Turn on oven to 170 degrees C
Place first 5 ingredients in mixing bowl, mixing well after each addition
In cup, dissolve baking soda and hot water, then add in mixing bowl
Add flour, oats, & chocolate chips. Mix well
Drop in spoonfuls onto cookie sheets
Bake in oven until starting to turn golden.
Makes about 50 cookies
Ingredients:
1 cup butter at room temperature
1 1/2 cups brown sugar
Balanced Chemical
Equation
2 H2 (g) + O2 (g) → 2H2O (g)
Comparison
Information
Communicated
Recipe
Chemical Equation
Starting materials
Ingredient list
Reactants
Conditions of starting materials
Directions (Ie. butter at room temp)
State symbols (g), (s), (l), (aq)
Proportions of starting materials
Quantities in ingredients list (Ie. 1
cup)
Coefficients of reactants (Eg.
2H2(g))
Plus sign between formulas of
Directions (Ie. mixing well after each
Instructions for combining materials
reactants, indicating they come into
addition)
contact
Resulting product
Title (Ie. Chocolate Chip Oatmeal
Cookies)
Products
Proportions/Quantities of Products
Final sentence (Ie. 50 cookies)
Coefficients of products (Ie. 2H2O
(g))
Recall (from earlier this class!):
Law of conservation of mass
In a given chemical equation, the mass of the
reactants is equal to the mass of the products
Chemical equations obey this law - show all the
atoms of the reactants are still present in the
products
Coefficients are added before chemical formulas
in an equation to ensure that the number of atoms
on each side of the arrow are equal (balanced!)
Human Balancing Act 1
Word equation: sodium + chlorine → sodium chloride
What are the chemical formulas of the reactants and
product in this equation?
Human Balancing Act 1
Na + Cl2 → NaCl
Volunteers needed to balance this equation!
Human Balancing Act 1
Balanced Equation: 2Na + Cl2 → 2NaCl
Human Balancing Act 2
A little trickier!
Word equation: zinc + silver nitrate → zinc nitrate +
silver
What are the chemical formulas of the reactants and
products in this equation??
Human Balancing Act 2
Zn + AgNO3 → ZnNO3 + Ag
Volunteers to balance equation!
Hint: Because polyatomic ions generally stay intact,
you can count them the same way as you count
atoms!
Human Balancing Act
Balanced Equation: Zn + 2AgNO3 → Zn(NO3)2 + 2Ag
Steps for Balancing
Chemical Equations:
EXAMPLE: Write the balanced chemical reaction of
magnesium with oxygen.
STEP 1: Write word equation for the reaction
Eg. magnesium + oxygen → magnesium oxide
Steps for Balancing
(Continued)
STEP 2: Replace each chemical name with the
correct chemical formula. (This is called the
skeleton equation)
Eg. Mg + O2 → MgO
Steps for Balancing
(Continued)
STEP 3: Count the number of atoms of each type on
either side of the arrow.
Eg. Mg + O2 → MgO
1 Mg atom
1 Mg atom
2 O atoms
1 O atom
Steps for Balancing
(Continued)
STEP 4: Multiply the formulas by an appropriate
coefficient until all the atoms are balanced. Keep
checking whether the numbers of each type of atom
on both sides are balanced.
Eg. MgO (on right) must be multiplied by
coefficient 2 to balance oxygen atoms
Mg + O2 → 2MgO
Mg (on left) must be multiplied by coefficient 2 so
there are two Mg atoms on each side
2Mg + O2 → 2MgO
Balanced Chemical
Equation
The final balanced chemical equation is:
2Mg + O2 → 2MgO
Something a little more
complicated...
Write a balanced chemical equation for the reaction
between iron (111) nitrate and sodium hydroxide to
produce iron (111) hydroxide and sodium nitrate.
Step-by-Step
STEP 1: iron (111) nitrate + sodium hydroxide → iron
(111) hydroxide + sodium nitrate
STEP 2: Fe(NO3)3 + NaOH → Fe(OH)3 + NaNO3
STEP 3:
Reactants:
1 Fe atom, 3 NO3 - ions, 1 Na atom, 1 OH - ions
Products:
1 Fe atom, 1 NO3- ion, 1 Na atom, 3 OH - ions
Ta-da!
STEP 4: Multiply NaOH (reactants) by 3 and NaNO3
(products) by 3 to get final balanced equation
Fe(NO3 ) + 3NaOH → Fe(OH)3 + 3NaOH
Remember...
Or at least the key to chemical equations...
Balancing Chemical
Equations Summary:
STEP 1: The word equation
STEP 2: The skeleton equation
STEP 3: Count atoms and ions on reactants and
products side. Add coefficients!
STEP 4: The balanced equation!
For more assistance, consult pages 233-236 in text!
Time to Practice!
Worksheet on balancing chemical equations!