The idea of providing extra bonus credits proposed by Weaver to encourage engagement and thinking is an excellent motivator. I wonder if bonus marks are more acceptable in the USA than Canada. I recall the new Ontario curriculum frowned upon the use of bonus marks; however, things might have changed in the 8 years since I retired. Even if giving bonus marks is frowned upon in Ontario, I am an advocate for them.
I was particularly fond of my year-end review demo presented to grade 12 classes during the last week of school. The demo involves the decomposition of hydrogen peroxide (Elephant’s Toothpaste). It was staged as a review of the entire course — with the exception of the organic unit — and a chance to compete for bonus credits.
A 1 L graduated cylinder is placed on the teacher's desk. At the beginning of class 100 mL of 30% hydrogen peroxide is poured into the cylinder along with a few drops of soap solution — the soap will end up producing copious amounts of suds. You can get 30% v/v peroxide cheap at a beauty supply store.
The equation, 2 H2O2(aq) -> 2 H2O(l) + O2(g),
was written on the board, and the challenge begins. I asked for the values of ΔH°, ΔS°, ΔG°, Keq and E°cell for the written reaction. The values could be determined in any order. Students could use their notes and text — no googling. Students can work in small groups — sharing the bonus. For each value I offered 5 bonus marks to the first correct value with a full solution. Classes were told that spurious guesses could bring a penalty. As values were found, they were written on the board.
Values using their textbook1 as reference are
ΔH° = –195 kJ,
ΔS° = +125 J/K,
ΔG° = –233 kJ or –210 kJ (from E°cell),
Keq = 8.90 x 1036 (Nernst) and E°cell = 1.09 V
Note the textbook does not give the entropy value for elemental oxygen (205 J/(mol K)). You should verify in advance the values based on the references your students will be using.
Once all five values are determined the game continues with Socratic-style review questions centred on the reaction. I started with “what do the values suggest about the reaction?” Focusing on spontaneity, I then pointed to the graduated cylinder and noted that the reaction is anything but spontaneous. My questions led to a review on activation energy and how to overcome it — a catalyst. You are limited only by your imagination and available time.
You can offer up some different catalysts — blood is one, a small price to pay for five bonus marks! — but potassium iodide works nicely. Prior to adding the catalyst, you can ask how one might test for the gas produced. You could even ask the class to calculate the predicted volume of gas at SATP — or even what would you need to measure to make this calculation. With these types of Socratic questions, I spread the marks around better, selecting students from all the raised hands. Since there is no “first buzzer”, I decided who might have been first. Correct answers earned a bonus. The catalyst speeds up the reaction so the peroxide quickly decomposes with significant heat and evolution of gas. A glowing splint in the produced suds will indeed catch fire before the suds put it out.
As with all demos safety precautions are a must — gloves and goggles are needed. The 30% peroxide is corrosive to the skin and can be quite hot — note the ΔH°. There is potential for the suds that exit the cylinder to have some residual chemical. One precaution is to have the cylinder in a large basin to collect the suds. I covered my workspace with paper towel in advance. As you are dealing with grade 12 students there is little likelihood that any will swarm your desk to touch the cylinder or its contents.
In all the years I have done this for senior chemistry classes, there was significant engagement. It was rare that one or two students garner all the rewards and more often they were spread nicely throughout the class. On other occasions I had to limit time on some questions and give the solutions in order to press on with the demo.
For those who worry about offering bonus credits, by year's end, the few marks that may be obtained — even if one student should get three answers — do not significantly impact on the student's final mark. However, the high level of engagement achieved and the linking together of all the units around one reaction was definitely worth the effort.
1. Nelson Chemistry 12, 1st Edition, Jenkins et al., 2003.
