Balancing Equations with Molecular Models

molecular model kit

This year I received a Hach Grant to purchase new molecular model sets for my classroom. I have been using the same old Sargent Welch model kits made of wood for many years. While they did a good job for organic molecules they were lacking for any geometry that had more than four ligands. I applied for this grant because I have been expanding what I am doing with model kits in my classroom and was fortunate enough to get it. I chose a kit from Carolina.

I am a very firm believer that the world of physical science can be visualized and is an excellent medium for teaching students to model and to picture what happens at the molecular level.  The first topic we decided to explore was balancing chemical equations. This seems like such a simple topic to chemistry teachers but I have found that it can be quite challenging for many of my inner city students. The first thing they ask me for is a list of rules that they can follow.  We can discuss the problems of algorithmic teaching in a later post! For the time being let’s talk about how to get students to understand why they need to balance equations and discuss what we can call “Conservation of Atoms”. My colleagues and I came up with an activity where we have several simple chemical reactions that occur in different ratios.

Concepts: 

Balancing equations

Time required: 

50 minutes

Procedure: 

Using the student document provided, we start with an example of a reaction like

H2 + Br2 --> 2 HBr

I have the student build simple models of H2 and Br2. Once complete I ask them to close the model kit and not reopen it until we give them the okay. I want them to take the model pieces they have and turn them into the product HBr. We can all see where this is going! They end up with HBr and “leftovers”. Many of them will catch on quickly at this point to make another HBr. We have started to explore the idea of balanced equations now. It is misleading to some of the students because they believe at first that all reactants are balanced to begin with. So we need to find a reaction where the reactants need balancing also.

I then have them take over with the formation of methane.

C + 2 H2 --> CH4

This works well for their first try on their own. They will spot quickly once the box is closed they can’t build CH4without two H2 molecules. The next reaction will be something like

N2 + 3 H2 --> 2 NH3

I really like this because of how odd the numbers turn out to be. There are not enough atoms to build even one NH3. So the activity now goes into the idea of balancing before you start building. The same idea applies here, build the reactants in the balanced ratio, close the box and convert them to products without leftovers. It is often helpful to have extra sticks out for bonds. We can discuss valence and bonding later in the year.

I think you can picture going through a few more of these for practice and then extending it to the idea of reaction prediction. I ask them to assemble a model of Zinc and two of HCl. They then get to try to figure out what it can be changed into.

I have a different way of grading this lab. I walk around the room while they are doing it with a rubber stamp. Once I see a properly built and balanced set of reactants I give them a stamp next to the question. I come back again in a few minutes and stamp a second time if they have the properly built products. All I have to do is look at their paper and see how many stamps they have and that determines their score. It provides very quick feedback to the students and saves me time. Some of my colleagues who are doing this also have come up with their own stamps and we can allow students to make up the lab or finish an incomplete lab with another teacher and we know where they did it based on what stamps are on their paper.

 

Preparation: 

The molecular model kits I use are from Carolina: Advanced Level Chemistry Set
Article Ref: MMS-002 - 64. 

Attribution: 

Thanks to my colleagues at Francisco Bravo Medical Magnet High School, Los Angeles, California.

Join the conversation.

Comments 6

Leslie Walker | Thu, 07/28/2016 - 08:46

Is there an accompanying student paper to go along with this?

Deanna Cullen's picture
Deanna Cullen | Thu, 07/28/2016 - 09:24

Hi Leslie,

Thanks for reading. You should find the Student Document near the bottom of the page...under Preparation and Attribution. It is in pdf format.

Deanna

 

Leslie Walker | Thu, 07/28/2016 - 12:05

There is nothing there, I've tried firefox also.

Deanna Cullen's picture
Deanna Cullen | Fri, 07/29/2016 - 08:05

Hi Leslie

After contacting the admin, they suggest you sign in to the site and try again. I apologize for the inconvenience. Thanks for your interest. 

Deanna

ALFREDO TIFI's picture
ALFREDO TIFI | Mon, 08/01/2016 - 02:10

I like that very much. By simple try and errors the learner constructs first the "sense" and meaning of such numbers that will then be called with a chemestrian term "stoichiometric coefficients", in such way to debunk the high consideration that most teachers deserve to balancing, to the point that balancing becomes more important of understanding the meaning of a chemical reaction.

Shahrokh Ghaffari | Thu, 08/04/2016 - 22:43

I have been using modeling for variety of topics in chemistry. When, more than 10 years ago, I started using models for balancing, stiochiometry, and etc no one doing it. I originated experiments for use in classroom or lab for the first time. My paper was published in 2006 in Journal of Chem Ed. Since then I am using them and made some revision too. This year I and my colleague presented a workshop on the same topic at 2016 CHEM ED conference in Greeley Colorado. I am sure those who are familair with Journal of Chem Ed will be able to search and find my paper. Here is the chem ed address

http://pubs.acs.org/doi/pdf/10.1021/ed083p1182

Intelectual and Academic Integrity!