Exp 9 Part 2 - Polymers

The first students left an hour early, the slowest stayed the entire period. There were hardly any questions for me

The students are introduced to IR spectroscopy in this lab - relate it back to the Food Dye lab and UV-vis spectroscopy to help them understand the concept. I do not usually get many questions about Part D, except for some students still confused with Lewis structures or polymer repeat units in Q19. Refer them back to Q2 from Part 1 that you passed back.

Part C:
Make a set-up to show the students because the drawing in the manual is a bit confusing for them. Your buret clamp should be hanging from the faucet over the sink. There are rulers on the back flap of their lab manuals (which they can cut off to make it easier) - one partner adds water while the other one holds up the ruler to make measurements. They can weigh their bottles on the triple beam balances instead of the analytical balances. The PVA films do not always come away from the boat without tearing, so students with compromised samples can borrow data from a neighbor if they need it for their graph. Graphs are done by hand today, instead of on excel. It is easier for them to see the trend if they graph all lines on the same set of axes.

There are only 4 pairs of scissors per section, so have half your class start with the spectroscopy section.

Experiment 9 Part 1 - Polymers

Almost half my class stayed until the end of the period. Divide your class into three sections and have them each start with a different part of the lab to keep lines down.

Part A - PETE
The phthalic anhydride should be weighed on the analytical balances while wearing gloves. The cover should be closed as soon as a student is finished weighing. Make sure the students use the disposable culture tubes and weigh boats on the back benches instead of test tubes from their drawers. When heating polymers, use Bunsen burners angled into the hood making sure not to heat the clamps. There are matches under the chalkboards and extra hoses and glass hose connecters at the stockroom window to make the burners reach to the fume hoods. Many students needed help figuring out how to adjust the air/gas ratio on their burners. When the liquid turns a light amber color, they should stop heating and pour into a weigh boat within 10-30 sec (not the 30 sec - 1min mentioned in manual). If they pour their polymers at different times, make sure they observe the viscosities well enough to compare. All waste goes in the trash cans.

Part A - PVA films
Most students are confused about using the triple beam balances. Have them 'tare' their weigh boats (about 5g), and then add 15g for a total of 20g of weight. They then slowly pour in liquid until the arrow points to the line at the zero point. Watch out for students pouring excess liquid back into the bottles. They should dye each sample a different color and record this in their notebook so that when they cut their polymer away from their labeled boats next week, they will be able to distinguish them. They should remove as many bubbles as possible, or at least push them to the side so that they have a large section with no bubbles for testing next week. The boats should then be placed flat in their drawers to dry.

Part A - PEO demo
Do this 1 or 2 times per lab period. We had one TA stand on a stool and demo for all three sections at a time. To cleanup, pour the polymer into the waste container, add water, stir and pour into waste container again. Then use copious amounts of soap, water and brushing in the sink to remove the rest of the polymer.

Part B - Density
There is no PVC in the jars. Make sure the students do not shake or tip them too much. If you've never seen these before, go look at the colors and make sure you can identify all of the pieces because I always get a lot of questions about the difference between the blue and purple pieces.

I received lots of questions about Q5 because I did not discuss the definition of "cross-linking" in my prelab, which I will definitely do next time.

Experiment 8 - Lewis Structures and Molecular Shape

There are no goggles this week, and no make-up labs. The first students left with about 40 minutes left and there were several students barely finishing the questions at the end.

The students should be fairly familiar with writing Lewis structures and formal charge, so you don't need to spend lots of time explaining this. You'll lose their attention fast. Do explain the wedge/dash 3D bond drawings as they have not been introduced to this before, and the effect of 3D structure on polarity. They also will need a description of the types of isomers (especially cis and trans) and lots of help with line drawings. Most every student asks for help with the cis and trans isomers in Q14.

Have your students turn in the top/blue copy of their notebook this week because lone pair dots can be difficult to see on the carbon copy. Tell your students to make them obvious!

Experiment 7 Part II - Food Dye Spectroscopy

Lab moves quickly today, with the last student leaving about 45 minutes before the end of the period.

Part A:
The manual says that TAs will assign dyes this week - you may do this, or let your students choose their own as long as they choose something w/ λmax >50nm away from their previous dye. Some of them don't understand that the 'shoulder' on a peak is really a second, overlapping peak. I had some questions about why the dyes in the mixture have different absorbances even though their concentrations are the same (it's because of ε). Have them take a complete spectrum of their new dye or look at the spectrum of a group from last week before predicting the combined spectrum. Also remind them again that they need to take a reference/blank with DI water.

Part B:
The unknowns from the stockroom are PAAS easter egg dye tablets or kool-aid powder. Have your students use a SMALL amount of solid unknown...they should break or scrape pieces off of the tablet rather than dissolving the entire thing. Even then, they usually end up with too high of a concentration. Most students' instinct is to take all 200mL of concentrated dye and add another 400mL of water to dilute instead of taking a 1mL aliquot and adding 2mL water to that.

Make sure your students record their procedure for both Parts B and C. This is a good lab to differentiate between quantitative and qualitative techniques...here they want to identify the dye, but they don't need to quantify anything. They could measure the mass of dye used and calculate a concentration in units of g/mL, but it is not necessary for identification. It's up to you how detailed you want their procedure, so make sure you specify if you will take pts off for missing masses.

Part C:
This is the fun part. Each student should do their OWN unknown. M&Ms and skittles work great if they use about 5 pcs of the same color and leave them in 5 or 10mL of water for only a few seconds (don't dissolve the white part or the chocolate). The stockroom will provide hotplates (if you ask) for dissolving solids, or concentrating dilute liquid samples. Leave the troubleshooting to the students rather than giving them a quick answer. They have plenty of time to think for themselves this week. If they brought an unknown that won't work (cranberry juice, brown soda etc), have them try it anyway and then explain to you why it didn't work. Also, stress that there is no food or drink consumption allowed in lab!

Typically one student uses the computer and the other student mixes chemicals. Since this experiment is so close to what they did last week, I use this lab as an opportunity to have partners switch roles. They don't like it at first, but it seems to help their comfort level and confidence in future experiments.

Experiment 7 Part I - Food Dye Spectroscopy

Lab took the entire period today with the first students leaving 45 min early.

Student Checkout: laptop, 25mL volumetric flask
TA Pick-up: Ocean Optics Spectrometers, cords, cuvettes

Part A:
The spectrometers should already be set up on the back benches. Just double check that the chalk is the correct height and angle so that the light is reflected up to you. The students can do Part A at any time during the period. They turn the wavelength dial on the instrument and record the corresponding colors. Make sure they do not go below 425nm because that is UV range and can damage their eyes.

Part B:
Assign dyes to each group to ensure that each dye is used at least once (this is important for next week). Dyes are in the plastic carboys. Remind students to record the name and concentration of their dye.

Follow Set-Up Appendix G ("Collecting a Complete Spectrum"). Note the importance of keeping cuvettes clean and orienting them consistently. The cuvettes only need to be filled about 3/4 full.

To copy & paste their data into Excel, have them highlight the entire column by clicking on the "wavelength" or "absorbance" tab at the top of the column, instead of trying to click & drag. They only need the data in the visible region (360-750nm), so they can scale the x-axis on their Excel graph accordingly (right-click on the x-axis, select "format axis" and change the fixed min and max values).

Make sure they HIGHTLIGHT THE GRAPH BEFORE PRINTING or they will have about 70 pages of numbers. They should connect to the network before printing.

Part C:
Have them think about the volumes themselves rather than suggesting numbers. 5, 10, 15 and 20 mL of stock solution work well, except for Yellow #5 where the lowest dilution might be too pale to get a good absorbance.

Make sure they use their burets (get rid of bubbles in the tip, record volumes to the nearest 0.01 mL) and that they dispense the stock solution DIRECTLY into their volumetric flasks. I have seen groups dispensing from the buret to a beaker and then pouring it into their volumetric flask.

They can collect their data in one of two ways:
1) Collect a full spectrum as they did in Part B, then scroll through their data to find the absorbance at their maximum wavelength (as found in Part B). They can record each of these absorbance values in their table then make their own graph in Excel. (I think this is easier for most students)
2) Follow Appendix G "Collecting Absorbance vs. Concentration Data". They'll just have to insert their own concentration values, and a graph will be generated for them.

When typing their concentrations into Excel, several students tried "4.5 x 10^-5". Excel doesn't recognize this as a number and gives them an axis with points 1,2,3,4 rather than their data points. They have to type "4.5E-5" or "=4.5*10^-5" for Excel to recognize the numbers.

Give them the hint that only one of their two graphs in Part C will be linear - have them graph both before answering Q12 and Q13. They can use the R2 value to determine which is more linear.

A lot of students have trouble determining the units and understanding Beer's Law - remind them that slope is "rise over run", so the units of slope will be the units of "rise" divided by the units of "run".

Remind them that they have to bring their own unknown food dye for next week. Things that work well are skittles, M&Ms (only leave them in water for a few seconds), jello, mtn dew, jolly ranchers, gatorade and gummy worms. Things that don't work well are brown sodas, vitamin water, teas, and cranberry or orange juice (color is not from a dye).

Also, you might want to remind your students that they need to be dressing appropriately for lab now that the weather is warming up.

Experiment 6 - Atomic Spectrum of Hydrogen

Time was an issue for the majority of the students today. It helps to split your section in half, and to set up the yardsticks for them, writing the length of the tape on the tape. One half starts with Part I and the other half can choose either II or III. After an hour, make the groups switch. Give several announcements of time remaining to keep them on task.

Goggles are not required for this lab, except for Part III using the hydrogen discharge tubes. There are no makeup labs allowed except for students making up this week's experiment.

Part I:
Tape the LED to the 9V battery so that it stands up easier. When taking a measurement, they should cover the LEDs not being measured to make it easier to identify which spot they are looking for.

To connect the voltage clips to the computer they need both the gray Go!Link Adapter and the black USB Adapter. When they detect the voltage, make sure that they attach the clips to the metal rather than to the plastic.

Once they have a trendline equation, they must change the formatting such that their slope has 3 sig figs. To do this, right click on the equation, click on "format trendline label", and click on "scientific" to display 2 decimals. Even though it says to do this in their manual, and I discuss it in prelab, several groups still omit this step and have slopes of "2E-06". Have your students login to the network to print.

Part II:
There is a typo on Page 6-11 under #6 on the 2nd line - it should direct them towards Table 6-2, not 6-1.

Some students get confused with typing in the equations in excel, particularly the cell references. Show them to click on the cell they want in the equation rather than typing in the corresponding letter and number of the cell - this way they understand better which numbers they are subtracting and how they relate to the equation.

Even with the example of the electron transition diagram for Q7, some students still get confused. They do not understand how to change their axis to match the scale in the manual. Have them divide their values by 10^-18 to get the same scale.

Part III:
The spectroscopes must be rotated to the correct angle for the lines in the spectrum to be visible. Check it out before the students come in because they will surely ask you for help. It helps to turn off the lights in that area of the lab. Your students must wear goggles for this portion.

Q10 was difficult for some students. Guide them to the same calculation they used for prelab Q2c and the example on p6-9. Even with this hint, some of them don't know that 1/∞=0

Experiment 5 - Chemical Calorimetry

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%)

Neutralize acid/base and pour it down the drain.

Experiment 3 Synthesis of Calcium Carbonate

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.


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.

Experiment 2 Part 1 - Chemical Reactions

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.

Experiment 1 Part 3 - Titrations using Conductivity

Lab today was easier than last week since the students are more familiar with the equipment and the layout of the lab. Most of my students were finished in about two hours, and one student stayed until the end.

Check-out and assembly are the same as last week. Remind the students to plug their computers into the power or the batteries will die during lab. They will also need to log in to the network to print their graphs.

Pre-Lab Discussion:
I discussed the definition of a precipitation reaction and drew out the complete and net ionic equations from their prelab questions. Many of them did not know how to write ionic equations, and this helped them think about the concentration of ions in solution and conductivity. From there, I discussed the uses of titrations, the equivalence point and how to properly set up and read a buret - keep buret vertical, use a funnel to fill, fill the tip before beginning, and keep the buret tip and cup close together to eliminate splashing. I also mentioned that burets can be read to two decimal places, and they measure the amount dispensed rather than contained, but I still had about half the groups write "1,2,3,4..." and read the buret incorrectly. It helps to double-check a measurement for each group just to make sure they are reading it properly.

Other tips:
Students can stir their reaction with the probe rather than using a stirring rod. There is no need to rinse the probe between measurements here.

Ask your students why their solution is turning cloudy. Most of them will be confused and scared at first, but eventually they figure out that it's due to the solid AgCl that is forming. They should be recording their observations after every addition.

Computer use was much more fluid this week. Several students tried to add a linear or curved trendline, which is unnecessary for this experiment.

Almost every group asked me about Q16 - how to calculate the original AgNO3 concentration. It helped to draw out a beaker with ions floating in it, write the chemical equation and talk about moles of ions and converting from moles of one reactant to moles of the other reactant. For some reason, many of them wanted to use the 0.028g of NaCl and the molar mass to convert to moles of AgNO3 instead of the volume at the low point on their graph.

All silver-containing waste goes in the "Expt.1 Waste" jug at the end of the benches.