We are just chugging along in chemistry this year. On to Unit 7! First let’s recap Unit 6:
- Chemical reactions can be identified by a change in color, temperature or odor or the formation of a precipitate or a gas
- Particles can rearrange during a chemical reaction but mass must be conserved (total number of particles does not change)
- Chemical reactions occur in predictable patterns
- It takes energy to break bonds and energy is released when bonds are formed
- Exothermic reactions release heat when the chemical energy of the system is decreased. Endothermic reactions absorb heat when the chemical energy of the system is increased.
We have finally arrived at the mole! I know this ordering of units is a little strange but I have found that students do much better with stoichiometry if they are coming right off the mole unit.
Packing Peanut Challenge
The beginning of my mole unit is based on the concept of relative mass. I start by presenting students with this large bag of packing peanuts, a balance, and a small sample of packing peanuts and say “figure out how many packing peanuts are in here, you can’t open the bag.”
It takes students a few minutes to formulate a plan but eventually they realize they need to use the mass of their sample of packing peanuts to set up a proportion. This establishes the idea that we can count by massing. This technique is really useful when you have a large amount of something or when you need to count things that are very small (in the case of atoms, both!).
Relative Mass Activity
The packing peanuts challenge leads nicely into a more in-depth relative mass activity. I adapted this relative mass activity from the Modeling Instruction materials because I didn’t have any hardware but I did have paperclips, metal shot and pennies. In the activity, students are given vials with the same number of the aforementioned objects in each vial.
Students complete a series of calculations converting between mass and number of items. The activity ends with students calculating the relative masses of the items and comparing those relative masses to numbers on the periodic table. At this point, I make the connection that the atomic masses on the periodic table are all relative (first to hydrogen, now to carbon). Since scientists could not measure the mass of a single atom, a common sample size of particles was needed to compare masses of different elements: this is the mole. Right now, it does not matter how many particles are in a mole. All we need to know is the atomic mass on the periodic table is the mass of one mole of an element. Hence, we call this the molar mass.
I extend this discussion with a bean challenge. I give each group a vial of 50 white beans, a vial of 50 red beans, a vial with an unknown quantity of bean compounds (2 white beans and 1 red bean) and an empty vial. Students are given the challenge to determine how many bean compounds are in the mystery vial. This task requires students to find the mass of 2 white beans and 1 red bean (like finding the molar mass of a compound) and then set up a ratio to determine the number of bean compounds in the vial (like calculating the number of moles in a sample when given the mass). Students are generally able to then quickly make the connection between calculating the mass of a bean compound and calculating the molar mass of a chemical compound.
After completing these two activities, students can very easily move to practicing mass/mole conversion calculations.
Once students have relative mass down, we can figure out exactly how big a mole is.
Size of Mole
I start the discussion about the size of a mole by asking students to measure out a mole of water. This takes a little bit on thinking initially but eventually students remember from Unit 1 that the density of water is 1 g/mL so 1 mole of water would be equal to about 18 mL of water.
I then ask students “how many particles of water do you think make up that 18 mL by order of magnitude?” Students usually guess around the order of magnitude of one trillion to 1oo trillion. They are always very surprised to learn that they grossly underestimated. I follow up this discussion with some fun, size of a mole calculations to put that giant number in perspective. Did you know that a mole of basketballs would fit in a ball bag roughly the size of the Earth?
Students are then able to complete mole/particles conversion calculations and two-step conversion calculations. While students complete these calculations, I also have them working on the multi-day nail lab.
I use the nail lab to introduce the concept of empirical formula. Students observe the reaction of an iron nail with copper (II) chloride, only they do not know which ion of copper was used. Students figure out how much copper was produced and how much chlorine was used, and then calculate the mole ratio and find the empirical formula. This lab takes 3 days (set-up, collect the precipitate, dry and measure the precipitate). Since each step does not take a whole class period, I do this in conjunction with mole/particle conversion calculations. I have also used the synthesis of magnesium oxide lab for determining an empirical formula which can be done in one class period (not counting the discussion).
Empirical and Molecular Formulas
After the nail lab, I jump right into calculating empirical and molecular formulas. For next year, I think I will make a more distinct transition from empirical to molecular formulas as this year my students had some trouble delineating the two. I use hydrogen peroxide and glucose as my poster child examples for the difference between empirical and molecular formulas.
To practice with empirical and molecular formulas, I have students play a round of whiteboard speed dating (see Kelly O’Shea’s blog) with a crime scene problem. The FBI has analyzed a white powder and they need to know if it is Tylenol (like the suspect claims) or cocaine. Students analyze the data and decided what to report back to the FBI.
Additionally, I have students work on “The Strange Case of Mole Airlines.” This activity was originally published in the Journal of Chemical Education and can be easily found with a quick Google search. This activity provides a wealth of practice with empirical formulas and also gives students the chance to form some conspiracy theories! Next year, I hope to set up a whole crime scene for students analyze!
Unit 7 Practicum
As will all units, I wrap up Unit 7 with a practicum. I had students calculate the formula of a hydrate. Students came up with the general lab procedure as a class (evaporate off the water and calculate the change in mass) and completed the experiment and calculations within their groups.
The practicum puts a wrap on Unit 7! Let’s sum up what we added to the model so far…
- Molar masses on the periodic table are relative to 12 g of Carbon-12 or 1 mole of carbon
- There are 6.02 x 10^23 particles in a mole
- Empirical formulas represent the simplest ratio in which elements combine and can be calculated using mole ratios
- Molecular formulas represent the actual number of atoms of each element that occur in the smallest unit of a molecule. This may be the same as the empirical formula.
That unit sets us up well for what I call the top of chemistry mountain, stoichiometry!