Magnesium, Gas Laws and Exam Review

Magnesium and Gas Laws

“What are we doing to help kids learn?”

It was close to the end of the semester and we were covering gas laws. Students were stressed over the idea of finals, final projects due, tests before finals and the holidays. Since we were finishing up the topic and it was important to end with one last assessment and/or lab but the timing was not good and the stress level for everyone was at an all time high. A different course of action was needed.

Inspiration struck with the experiment proposed by Mike Morgan with magnesium and hydrochloric acid. At the start of the week before exams it was announced that the last grade was a type of “lab practical”. The experiment was going to happen, but with a little “tweak”. You can find my modified student and teacher documents below this post.

Students were given a preview of the experiment at the beginning of the week. They were given as little information as possible. Students were told that they would have to predict the exact amount of gas at the end of the experiment and turn in their written prediction before the experiment took place. They could ask for data they thought they needed at the beginning of the class. Demonstrating the experiment and stating the problem at the beginning of the week created a “need to know” basis to succeed on their last graded assignment. Students were given a review type handout that went over most concepts of first semester (see below). They asked questions all week in anticipation of the last day and the experiment. Here are some of my favorite questions that students asked.

Student: “What will the pressure be when we do the experiment?”

Me: Don’t know. I can’t predict the weather that well.

Student: “How do we find the pressure on the day of the experiment?”

This turned out to be a great question that opened a “Pandora Box” of answers. We have a weather station on the roof of our school that reports to our classroom. You would think the pressure would be accurate. Teachers using the pressure were getting high experimental errors with simple experiments. This put the data into question. The question was put to local meteorologist, Scott Dimmich. Scott explained that essentially, weather data is reported is collected all over the world and put out to the internet. People from all over the country and the world might use it. The problem is that if people examine the air pressure in Denver (high altitude) and Cincinnati (low altitude) there could be a big difference on a clear dry day due to the altitude and not the weather. Scientists adjust these pressures for sea level so they are uniform. The goal is to cancel out the altitude and just get a reading of pressure that could help predict dry or humid air regardless of altitude. Our station at school was adjusting for altitude and not giving us a true reading. So how could we get a “true” reading? Well...there is an app for that. One of my most extremely resourceful students who loves to take things apart (I have to keep an eye on him…) informed me that the newer iphones are equipped with really good barometers. Sure enough, we were able to get a “station reading”, an “altitude”, and an adjusted reading for “sea level”. The station reading did the trick.

Student: “What is the balanced equation?”

Me: Don’t know...that is not data.

Student: “How many moles of magnesium are going to be used?”

Me: Check out the answer to the last question.

Student: “What about water vapor mixed with the gas?”

Me: they were thinking. This lead to a great discussion about Dalton’s Law of Partial Pressure. We talked about how to account for the vapor pressure of water.

Students came in the day of the lab practical. They had twenty minutes to ask questions about data. Some of the answers they liked and some they did not (see above). They were told that everyone had to turn in some neat and clean data, some calculations and a prediction. I did not care who they talked to or who they consulted. It was a bit of a free for all. Some trusted their partners and some did not. Students were forced to justify their answers and think about their data and calculations. The good news is that most students predicted within three percent of the actual value. They were able to use the Ideal Gas law and Dalton's Law of Partial Pressures to actually DO something. In an ideal world it would have been great to have students do this experiment themselves. As we all know, the world of school is not always ideal and sometimes we have to do the best we can with a “plan B”. Do you have a “plan B” that you like? Please let me know….I would love to hear from you and share it with others...