Rethinking Stoichiometry

BCA table

Stoichiometry is arguably one of the most difficult concepts for students to grasp in a general chemistry class. Stoichiometry requires students to synthesize their knowledge of moles, balanced equations and proportional reasoning to describe a process that is too small to see. Many times teachers default to an algorithmic approach to solving stoichiometry problems, which may prevent students from gaining a full conceptual understanding of the reaction they are describing. 

Modeling Instruction, developed at Arizona State University from the American Modeling Teachers Association (AMTA) has a problem-solving framework that helps students apply proportional reasoning to see the big picture of the reaction they are analyzing. This framework uses a table to organize mole data for the whole reaction instead of just isolating one part of the problem.

Let’s use the reaction between 5.0 moles of magnesium and excess oxygen gas to produce magnesium oxide. Students start by writing the balanced equation:

2Mg   +        O2    ==>      2MgO

Once students have the balanced equation, they can begin putting information into the BCA table. BCA stands for before-change-after. Students start by filling in the quantity of reactants that are present before the chemical reaction happens. If no starting amount of reactant is specified in the question, students write “XS” to show there is more than enough reactant for the reaction to proceed to completion. Before the reaction has started, no products have formed yet so students write in zero. 

 

2Mg   +        O2     =>       2MgO

 

B

5.0 mol

XS

0 mol

C

 

 

 

A

 

 

 

 

The “change” line is where the proportional reasoning comes in. In this problem, there is no limiting reactant so all 5.0 moles of magnesium gets used. Students then use the coefficients from their balanced equation to fill in the rest of the change line. A student would use the reasoning, “for every 2 moles of magnesium there is 1 mole of oxygen, therefore the reaction must need half as much oxygen as magnesium.” To find the product, a student might reason, “for every 2 moles of magnesium, 2 moles of  magnesium oxide are formed. That is a 1:1 ratio so the moles of magnesium used is equal to the moles of magnesium oxide produced.”

 

2Mg   +        O2     =>       2MgO

 

B

5.0 mol

XS

0 mol

C

-5.0 mol

-2.5 mol

+5.0 mol

A

 

 

 

 

To fill in the “after” line, the student performs some simple arithmetic. If all 5.0 moles of the magnesium are used, after the reaction there is no magnesium left. If there was excess oxygen to start with, there will still be excess oxygen after the reaction. If 5.0 moles of magnesium oxide are formed and there was no magnesium oxide to begin with, after the reaction there are 5.0 moles of magnesium oxide. 

The student now has a full picture of what is going on during this reaction, no matter what value the question asked for. This is especially useful for limiting reactant problems.

 

2Mg   +        O2      -->      2MgO

 

B

5.0 mol

XS

0 mol

C

-5.0 mol

-2.5 mol

+5.0 mol

A

0 mol

XS

5.0 mol

 

In the example below, the student can easily see which reactant is limiting and how much excess reactant is left without doing separate, seemingly unrelated calculations. 

 

CH4    +        2O2     -->       CO2      +         2H2O

 

B

10.0 mol

15.0 mol

0 mol

0 mol

C

-7.5 mol

-15.0 mol

+7.5 mol

+15 mol

A

2.5 mol

0 mol

7.5 mol

15 mol

 

The BCA table also helps students quickly pick out the limiting reactant because they convert mass values to mole values in a separate step from applying the mole ratios. Since all stoichiometry is done in the BCA table, it is easy for students to convert between mass, molarity, gas volume and moles outside of the table, making it a versatile tool.

Larry Dukerich published a blog post, Conceptual Chemistry, about BCA tables in 2014 that you will want to refer to for more information. 

 

Join the conversation.

Comments 5

Tracy.Schloemer's picture
Tracy.Schloemer | Sat, 02/06/2016 - 13:28

Hi Lauren-

This is a cool scaffold I've never used, but might incorporate into my unit this year.

Do you also teach AP chemistry? Do you find that this scaffold helps students think about equilibrium ideas (and ultimately the logic behind RICE tables often used to teach the mathematics behind the idea that every reaction at a constant temperature is goverened by an equilibrium constant)?

Deanna Cullen's picture
Deanna Cullen | Sat, 02/06/2016 - 14:46

Hi Tracy, 

My students grasped the idea of ratios in stoichiometry using this method very quickly last year and then when we covered RICE/ICE table this year in AP Chemistry, they had no hesitation. I will not be going back to my old methods.

Michelle Okroy's picture
Michelle Okroy | Mon, 02/08/2016 - 22:05

I agree Deanna, my AP Chemistry students were very comfortable describing ratios, whether it was BCA tables or creating 'for every statements', and it led to a better conceptual understanding of equilibrium.

Lauren Stewart's picture
Lauren Stewart | Sun, 02/07/2016 - 17:20

Thanks for sharing your AP experience Deanna! I do not teach AP but I do use the idea of proportional reasoning based problem solving often in my class. I use a similar method from the Modeling Instruction curriculum to solve gas law problems. I will make sure to post on that as well! 

Lowell Thomson's picture
Lowell Thomson | Sat, 04/02/2016 - 08:36

I was looking online for some resources to teach stoichiometry using BCA on Monday and recalled some posts here on ChemEdX. 

This may not be what is prescribed by modeling, but I've already taught the typical algorithmic method to my Chem II class (grades 9 and 10 intro class, basically). I realized I had wanted to try BCA with this group, so I'll be giving it a go on Monday.

I'll report back on the results of dipping my toes into this method at some point down the road.

Thanks for the post.

Lowell