The Case of the Barking Dog

To grasp the concept of oxidation and reduction reactions, I have my high school students write half reactions to show the loss and gain of electrons by the substances being oxidized and reduced. To help with this concept, I developed a quick lab activity involving the reaction between magnesium metal and dilute hydrochloric acid, which in turn led to the students collecting the hydrogen gas and then testing for its presence. The lab was a success as I watched startled students produce the so-called barking dog sound as they combusted the hydrogen gas in their inverted test tubes.

 However, When it came time to write the half reactions, particularly for the combustion of the hydrogen gas with the oxygen gas to produce water, my students were confused regarding when to write subscripts vs coefficients. 

            Balanced equation:   2H₂  +  O₂ à 2H₂O

            Student example:

            Oxidation half reaction: 2 H₂⁰  à 2H₂⁺ + 4e^- incorrect

            compared to

            Oxidation half reaction: 2H₂⁰  à 4H⁺ + 4e^-   correct

The same was true with the original reaction showing the H⁺ of the hydrochloric acid vs the H₂ of the hydrogen gas produced. 

This time the students were not copying the equation but just did not understand when to use a coefficient or a subscript.

            Mg_(s)  +  2HCl_(aq)  à MgCl_2(aq)  +  H_2(g)   

            Reduction half reaction: 2H⁺ +  2e^- à  H₂⁰    correct

            compared to 

            Student example:

            Reduction half reaction: 2H⁺  + 2e^- à  2H⁰  incorrect

What was happening was my students were copying down exactly what was written in the original equation without thinking about what this looked like on the particulate level. This required me to get a little creative and I had remembered Michelle Okroy’s blog about the program molview. I thought that if the students could build and see the structure of the hydrogen molecule vs the water molecule then they may gain a better understanding of why 4H⁺ was correct vs the 2H₂⁺ and when to use it. I hoped that by looking at the structure of these two molecules that maybe they would see the difference between the two substances. With a class set of iPads we went to the http://molview.org/ site and began to explore.

After a quick tutorial, I had my students build hydrogen gas and then build the water molecule and compare them. The program works seamlessly on the iPads and my students loved being creative with it. 

When I questioned my students regarding what was different between the two pictures they responded by saying that in the hydrogen gas picture the two hydrogen atoms were connected to one another while in the water molecule picture they were not.  My students were familiar with hydrogen existing as a diatomic molecule and once we assigned oxidation numbers they recognized hydrogen now as an ion and as H⁺. We still had a learning curve with MgCl₂ in the original equation with some students writing Cl₂ but they developed a somewhat rule of how to write them when diatomic vs as when they are shown as an ion.  Luckily, chlorine is a spectator ion in that equation but they applied their rule to the hydrogen and gained success with more practice when asked to write the half reaction on their assessment.

If you log in to your ChemEd X account, you can download the student and teacher versions of the activity.