1. The BCA Method in Stoichiometry
Larry Dukerich
Modeling Instruction Program
Arizona State University

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. Balanced Chemical Equations
Atoms are conserved in chemical reactions: balanced chemical equations.
Reactants --> Products
Ratio of particles: mole ratios through coefficients
Ex. 2 Mg + 1 O2 --> 2MgO
two moles of magnesium burn with one mole of oxygen gas to produce two moles of MgO
2 moles Mg: 1 mole O2: 2 moles MgO

5. Making Predictions Using mole ratios we can identify:
How much is needed (before)
How much can be made (after)
To make predictions we need also to consider the changes that occur during the reaction.
BCA: Before-Change-After chart to keep track of what happens to particle/mole ratios during a chemical reaction

6. 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 2.4 moles of hydrogen sulfide? H2S O2 ----> 2 SO H2O
Before: Change After

7. 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

8. Focus on mole relationships
Step 3: use ratio of coefficients to determine change
2 H2S O2 ----> 2 SO H2O
Before: xs Change –2.4 – After
Reactants are consumed (-), products accumulate (+)

9. Emphasize that change and final state are not equivalent
Step 4: Complete the table
2 H2S O2 ----> 2 SO H2O
Before: xs Change –2.4 – After xs

10. 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

11. Mole Ratios: #1
Lead will react with hydrochloric acid to produce lead chloride (PbCl2) and hydrogen gas. How many moles of hydrochloric acid are needed to completely react with 4.0 mole of lead?
Equation:
Before:
Change:
____________________________________________________
After:

12. Equation:
Lead will react with hydrochloric acid to produce lead chloride and hydrogen gas. How many moles of hydrochloric acid are needed to completely react with 4.0 mole of lead?
Equation: __Pb + __ HCl --> __PbCl2 + __H2
Before:
Change:
____________________________________________________
After:

13. Balance & Before:
Lead will react with hydrochloric acid to produce lead chloride and hydrogen gas. How many moles of hydrochloric acid are needed to completely react with 4.0 mole of lead?
Equation: __Pb + _2_ HCl --> __PbCl2 + __H2
Before: moles excess 0 moles 0 moles
Change:
____________________________________________________
After:

14. Change
Lead will react with hydrochloric acid to produce lead chloride and hydrogen gas. How many moles of hydrochloric acid are needed to completely react with 4.0 mole of lead?
Equation: __Pb + _2_ HCl --> __PbCl2 + __H2
Before: moles excess 0 moles 0 moles
Change: -4 moles -8moles mole + 4 mole
____________________________________________________
After:

15. After
Lead will react with hydrochloric acid to produce lead chloride and hydrogen gas. How many moles of hydrochloric acid are needed to completely react with 4.0 mole of lead?
Equation: __Pb + _2_ HCl --> __PbCl2 + __H2
Before: moles excess 0 moles 0 moles
Change: -4 moles -8 moles mole + 4 mole
____________________________________________________
After: 0 mole excess 4 moles 4 moles
You Can Convert: moles to grams
(multiply moles x molar mass (g/mol))

16. Mole Rations: #2
How many moles of hydrogen gas will be produced if 2.5 moles of calcium hydride react according to the following equation?
CaH H2O --> Ca(OH) H2
Before
Change
___________________________________________
After

17. Before:
How many moles of hydrogen gas will be produced if 2.5 moles of calcium hydride react according to the following equation?
CaH H2O --> Ca(OH) H2
Before moles excess moles 0 moles
Change
___________________________________________
After

18. Change:
How many moles of hydrogen gas will be produced if 2.5 moles of calcium hydride react according to the following equation?
CaH H2O --> Ca(OH) H2
Before moles excess moles moles
Change moles - 5 moles moles moles
___________________________________________
After

19. After
How many moles of hydrogen gas will be produced if 2.5 moles of calcium hydride react according to the following equation?
CaH H2O --> Ca(OH) H2
Before moles excess moles moles
Change moles - 5 moles moles moles
___________________________________________
After moles excess moles moles

20. Mole Ratios: #3
How many moles of water will be produced if 0.45 moles of oxygen gas reacts according to the following equation?
2 C6H O > CO H2O
Before
Change
____________________________________________________
After

21. Before
How many moles of water will be produced if 0.45 moles of oxygen gas reacts according to the following equation?
2 C6H O > CO H2O
Before mol excess mol 0 mol
Change
____________________________________________________
After

22. Change
How many moles of water will be produced if 0.45 moles of oxygen gas reacts according to the following equation?
2 C6H O > CO H2O
Before mol excess mol 0 mol
Change mol mol mol mol
____________________________________________________
After

23. After
How many moles of water will be produced if 0.45 moles of oxygen gas reacts according to the following equation?
2 C6H O > CO H2O
Before mol excess mol 0 mol
Change mol mol mol mol
____________________________________________________
After mol excess mol mol

24. Only moles go in the BCA table
If given mass of reactants for products, convert to moles first, then use the table.

25. 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 The balanced equation deals with how many, not how much.
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

26. 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.

27. 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

28. 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