1. The BCA Method in Stoichiometry

2. Typical Approach to Stoichiometry
Very algorithmic
grams A -->moles A-->moles B-->grams B
Fosters plug-n-chug solution
Disconnected from balanced equation
Especially poor for limiting reactant problems

3. BCA Approach
Stresses mole relationships based on coefficients in balanced chemical equation
Sets up equilibrium calculations later (ICE tables)
Clearly shows limiting reactants

4. Emphasis on balanced equation
Step 1- Balance the equation
Hydrogen sulfide gas, which smells like rotten eggs, burns in air to produce sulfur dioxide and water. How many moles of oxygen gas would be needed to completely burn 8 moles of hydrogen sulfide? __ H2S + __ O2 ----> __ SO __ H2O
Before: Change
After

5. Emphasis on balanced equation
Step 1- Balance the equation
Hydrogen sulfide gas, which smells like rotten eggs, burns in air to produce sulfur dioxide and water. How many moles of oxygen gas would be needed to completely burn 8 moles of hydrogen sulfide? H2S O2 ----> 2 SO H2O
Before: Change
After

6. Focus on mole relationships
Step 2: fill in the before line
2 H2S O2 ----> 2 SO H2O
Before: xs Change
After
Assume more than enough O2 to react
Products are always ZERO before the reaction starts

7. Focus on mole relationships
Step 3: Set up and solve your proportions
2 H2S O2 ----> 2 SO H2O
Before: xs Change X Y Z
After
Put a variable in for every unknown.
Use a proportion to solve for the unknown value!

8. Focus on mole relationships
Step 3: Set up and solve your proportions
H2S O2 SO2 H2O
8 X Y Z
8 X Y Z
2X = 24 2Y = 16 2Z = 16
X = 12 Y = 8 Z = 8 =

9. Focus on mole relationships
Step 3: use ratio of coefficients to determine change
2 H2S O2 ----> 2 SO H2O
Before: 8 xs Change
After
Reactants are consumed (-), products accumulate (+)

10. Emphasize that change and final state are not equivalent
Step 4: Complete the table
2 H2S O2 ----> 2 SO H2O
Before: xs Change
After 0 xs 8 8
After can be calculated by subtracting Change values from Before.
One reactant must COMPLETELY be consumed. You must figure out which one that is!

11. Complete calculations on the side
In this case, desired answer is in moles
If mass is required, convert moles to grams in the usual way

12. Only moles go in the BCA table
The balanced equation deals with how many, not how much.
If given mass of reactants for products, convert to moles first, then use the table.

13. Limiting reactant problems
BCA approach distinguishes between what you start with and what reacts.
When 0.50 mole of aluminum reacts with 0.72 mole of iodine to form aluminum iodide, how many moles of the excess reactant will remain? How many moles of aluminum iodide will be formed?
2 Al I2 ----> 2 AlI3
Before: Change
After

14. Limiting reactant problems
Guess which reactant is used up first, then check
2 Al I2 ----> 2 AlI3
Before: Change
After
It’s clear to students that there’s not enough I2 to react with all the Al.

15. Limiting reactant problems
Now that you have determined the limiting reactant, complete the table, then solve for the desired answer.
2 Al I2 ----> 2 AlI3
Before: Change
After

16. BCA a versatile tool
It doesn’t matter what are the units of the initial quantities
Mass - use molar mass
Gas volume - use molar volume
Solution volume - use molarity
Convert to moles, then use the BCA table
Solve for how many, then for how much