I always make my students a promise: that I’ll never give them “busy work”—that I’ll only give as much homework as I think it takes to master a particular skill. Of course, I point out that this means that any question they don’t attempt risks that they won’t master the relevant skill.
With that promise in mind, I do give summer homework, but it’s not just a review of Chem I. My goal for summer work is to start the process of getting them to think on a higher level.
I give them a very few pencil-and-paper questions, but each one combines more than one topic, such as a stoichiometry and gas laws. (In Chem I, they usually learn topics separately but rarely combine them.) Most of the problems are excerpts from AP exam questions that they should be able to answer from what they learned in Chem I. I usually end with a ridiculously difficult extra credit question, such as the following. (My purpose in giving it to them is to get a sense of how much of a multi-level problem they can keep track of. By making it an extra credit question, I also get to find out which students are the ones who will actually attempt a complex question like this and which ones will simply give up.)
350 mL of a 2.0M aqueous solution of permanganic acid is reacted with 2.0 L of 2.0 N hydrochloric acid, producing manganese (II) chloride and chlorine gas. The hydrochloric acid was made from chlorine-33, which has a half-life of 37.24 minutes and undergoes beta minus decay. The reaction is conducted in a sealed 25 L vessel at an initial pressure of 99.5 kPa. The vessel is kept at a constant temperature of 25°C throughout the reaction. What is the pressure inside the vessel after 75 minutes?
In order to solve this problem, you will need to take the following into account:
- This is a redox reaction, which you will need to balance. Note that because both reactants are acids, you may assume that the reaction is happening under acidic conditions.
- This is also a stoichiometry problem. Because the quantities are specified for both reactants, you will need to use the balanced equation to determine which reactant is limiting, and to determine how many moles of chlorine gas are produced.
- Because chlorine-33 undergoes radioactive decay, the final number of moles of gas will also depend on the decay process. Assuming that chlorine (which is diatomic) decays into a monatomic element, each chlorine molecule will produce two molecules of the decay product. If the decay product is a gas, this will increase the total number of gas molecules; if the decay product is a solid or liquid, it will decrease the number of gas molecules. (Note: You do not need to take into account what happens if when only one atom of a diatomic molecule decays. You may assume that the decay process produces a chloride radical, which rapidly combines with another chloride radical to produce chlorine gas.
- Because the reaction produces a gas and takes place in a sealed container, once you know how many moles of gas are in the container, you can use the volume of the container and the gas laws to determine the final pressure. You may assume that all gases in this problem behave ideally.
I also give them two Alka-Seltzer tablets and instruct them to perform and write up an experiment in which they determine the mass of CO2 produced. I also ask them to calculate the percent yield based on the label’s claim that each tablet contains 1916 g of bicarbonate.
Most of them struggle with the assignment, and only a few of them get the experiment to work, but it sets the stage nicely for the kind of thinking I’m looking for.
Originally posted to the ap-chem discussion list.
