WEDNESDAY, OCTOBER
ST
21
Please do not touch the materials on your desks
Take out your notebook and a writing utensil
and copy down the following:
The law of conservation of mass tells us that
matter is never created nor destroyed.
The number of atoms at the beginning of a
chemical reaction must equal the number of
atoms at the end
You can count the atoms present in a chemical
reaction to ensure that the amounts match
BALANCING EQUATIONS
Purpose of today:
To understand how the “recipe” for reactions
is created
To practice balancing chemical equations
CHEM CATALYST
Does this equation obey the law of conservation
of mass? Why or why not?
CuCl2 (aq) + Na2S (aq) → CuS(s) + NaCl (aq)
WRITING CHEMICAL
EQUATIONS
Kind of like a mathematical formula
X + Y → XY
XY → X + Y
Stuff to the left of the arrow: reactants
Stuff to the right of the arrow: products
Reactants react to form products!
SETUP
Work with your table groups
You should have
One set of worksheets
 1 “Block Molecules” sheet
 1 “Equation 1” sheet
 1 “Equation 3” sheet
Dry erase marker and rag
Beaker of colored blocks
Your notebook and a pen/pencil
TASK 1
Consider the following chemical equation:
Zn (s) + HCl (aq) → ZnCl2 (aq) + H2 (g)
This equation tells us that solid zinc atoms react with hydrochloric
acid molecules to produce zinc (II) chloride molecules dissolved in
water and hydrogen gas.
Using the colored blocks, create a surplus (lots) of each of the
reactants and the products
 Zinc = purple
 Hydrogen = yellow
 Chloride = blue
TASK 2
Display the # of atoms/molecules, according to the chemical
equation, on the “Block Molecules” worksheet
Using the “Atom Inventory Chart” on the Reaction 1 worksheet,
count the atoms of each type in the reactants and the products
 Fill in the chart using the dry erase marker
Does the reaction follow the Law of Conservation of Mass
(matter)?
 Is the reaction balanced?
 If not, add more atoms or molecules to the “Block Molecules” display
worksheet accordingly
Change the numbers in your inventory chart to reflect the
changes you made by adding atoms/molecules to the reactants
and/or products
TASK 3
Use coefficients (large numbers written in front of
molecular formulas) to show how many of each reactant is
need and how many of each product is made
__ Zn (s) + __ HCl (aq) → __ZnCl2 (aq) + __ H2 (g)
Coefficients go here!
BALANCE THE REMAINING EQUATIONS
Steps to follow using the blocks:
1. Create a surplus of the reactants and products
2. Organize the correct # of atoms/molecules onto the “Block
Molecules” display sheet (according to the chemical equation)
3. Fill in an atom inventory chart to determine if the reaction is
balanced
–
If not, add more atoms/molecules accordingly to the Block Molecules
worksheet to balance
4. Change the numbers on your inventory chart to reflect
addition of reactants or products
5. Fill in the coefficients to reflect number of reactant
atoms/molecules needed and number of product
atoms/molecules made
6. Get your work checked by me after each equation!!
CLEAN-UP
Separate ALL blocks and put back in beakers
Erase worksheets
Leave everything as you found it
Write the following reaction in your notebook
__ Ca (s) + __ O2 (g) → __ CaO (s)
Create an atom inventory chart
Balance the reaction
BALANCING WITHOUT BLOCKS!
Write the following reaction in your notebook
__ Ca (s) + __ O2 (g) → __ CaO (s)
Create an atom inventory chart
Balance the reaction
ATOM INVENTORY EXIT SLIP
Complete on your own and turn in
before you leave!!