February 2011 Archives

Lab reports are due next week - I had about half of my class turn theirs in today.
They should weigh their product from Exp 3 which should take no more than 5 minutes - if you didn't have them prepare their %yield calculations before they left last week, you might want to email them a reminder to have them prepared before they come to class. In my prelab, I discuss why their %yield is greater than 100% because almost everyone is confused.

Remind them to remove the protective sleeve on the digital thermometers before using them. Also remind them to use the same well in the pink Styrofoam calorimeter for each trial.

I discuss specific heat in my prelab and have them visualize boiling a pot of water - the pan gets hot faster than the water does, which accounts for the difference in Csp for water and aluminum. I also discuss practical uses for calorimetry so they understand that we aren't just trying to torture them with math in this lab.

Part A:
The calculations are well described on p 5-7. Some get confused about finding the mass of the solution using the density of NaCl or what to do since the 1.0M concentration of NaCl doesn't match the 2.0M solutions they started with.

For Q7, some students assume %error is the same as %yield. It's not. Have them see Appendix A. I saw %errors ranging from 6% to 30%.

Part B:
Set up a 600mL beaker with about 15pcs Al in boiling water. The students should bring their tongs, calorimeter, thermometer and partner to the Al and transfer the metal quickly. They can monitor the temperature while walking back to their desk

Have them show you their calculation for the first trial of their calorimeter constant. It should be between 0 and 15.

Part C:
I had no questions about this part, though the %errors here were much greater than that for the enthalpy of neutralization (between 50 and 150%)

WASTE:
Neutralize acid/base and pour it down the drain.

Lab took almost the entire period today with the first students finishing in 2 hours and the last about 5-10 minutes before the end of class. They should be working in partners.

Pick up your TA evaluation forms from Smith 115. These go straight to you so that you can improve your teaching. I get better feedback if I ask them to write at least one good thing and one thing that needs improvement when I hand them out.

Show the students how to make an ice bath to rinse their precipitate with cold DI water. They should start with this so that their water is cold by the time they need it. The lab manual does not mention an ice bath, so they will skip this step if you don't tell them to do it. Also ask them why they should use ice cold instead of boiling water because none of my students understood this.

Part 1:
Many students didn't know that [ ] means concentration, so they were confused by the table on p 3-4. Many only recorded one sig fig from the bottle instead of all three (0.3M instead of 0.300M)

It can save time to have each group measure 50-51 mL of each reactant initially - enough for all three trials and a few drops leftover for Part 3. If they decide not to do this, make sure that they label their grad cylinders with the reactant they contain because if they get switched, they will end up with a reaction occurring in their grad cylinder.

Part 2:
Remind your students to weigh their filter paper for each trial. As soon as they have their first trial filtering, they can move onto their second as long as they label everything. There are sticky labels under the chalkboards. After filtering, they can unfold their filter paper and place it on watch glasses or the cover of their spot plate to dry over the weekend.

MAKE SURE THEY DON'T DISPOSE OF THEIR FILTRATE BEFORE COMPLETING PART 3.

Part 3:
I discussed limiting and excess reactants during my prelab by drawing a beaker containing the reactants with an arrow going to a beaker containing the products. I proposed three different molar feed ratios and asked my students which of the reactants would still be present at the end. They seem to understand Q3-9 better when you use "excess" reagent rather than "limiting" reagent when you lead them to the answer.

I also drew a diagram of the spot plate wells on the board:

T1 T2 T3
Ca(NO3)2 O O O -> Q3-6
NaCO3 O O O -> Q7-9

Before they leave, they should complete a separate page in their notebook with their % yield calculations set up for next week. Exp 5 takes the entire period, so it is vital that they have this ready before they get to class next week. It should take them no more than 5 minutes to weigh their samples and turn everything in. I had them write a conclusion this week in which they discussed potential reasons for their %yield being >100% and <100% so that, whatever their results, they won't have to do this next week.

WASTE: Everything can go down the drain.

Most students today finished within 1.5hrs, and my last student left after 2.5 hrs. It is helpful to divide students into three groups and have each group start with a different part of the lab so that they aren't all waiting in line for the same chemicals. Also watch for students bringing chemical bottles back to their benches - they usually forget to put them back.

PRE-LAB - I first went over the grading for the lab reports from the past two weeks and discussed common mistakes - incorrect sig figs, missing units, missing observations, missing or illegible calculations and again what I expect to see in a good purpose, procedure and conclusion. There is plenty of time in lab today, so I wanted to make sure that my students understand my expectations - they pay more attention now than they did on check-in day since they already have experience with writing reports and have received grades back.

I gave examples of acid-base and precipitation reactions, including the molecular, complete ionic and net ionic equations for these types of reactions. They should have already covered this in class, but several still needed a review. We also discussed possible observations and what they mean (bubbles, color change, texture change, heat, turns cloudy). I also discussed the difference between 'clear' and 'colorless' since most students use these interchangably.

PART C - Almost every student thought that Na2CO3 should be a solid in the reaction and then wouldn't break up into ions. Some students thought this was a ppt rxn because they used too much Na2CO3 and assumed the solid in the bottom was a product (NaCl (s)) rather than leftover reactant. Make sure they stir/shake the reaction mixture.

Q4 - You can make up your own question, or omit - I had my students answer this question before doing Part C. I had them design an experiment to verify whether Na2CO3 is solid or aqueous. I expect them to be able to put a small amount of Na2CO3 into a test tube and add DI water to see whether it dissolves. They could verify the presence of ions using conductivity.

Waste - 2 beakers - the one with Ag waste goes in jug. Acid/base beaker gets neutralized and poured down the drain. Send them to look at the colors on the back of the universal indicator bottle to see the yellow/green a neutral solution should be. You might want to have them check with you when they get to this step - many add too much acid or base and then swing back and forth ending up with 500mL of waste at the end.