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# CHEM 146 Lab 3 Worksheet | 2

CHEM 146
Lab 3 Worksheet |
2

Lab 3 Worksheet

Instructions. Carefully review the material on pages 3 – 8 of the
CHEM 146 Lab 3 Manual (especially the fully worked examples). In this online activity, you are processing and analyzing data identical to what you would obtain in lab.

1. Experimentally Determining KC at 3.8 oC . A student loads 3.0 mL of an ethanol solution of
BDa dye into a cuvette (b = 1.00 cm). Then places the loaded cuvette into a spectrophotometer equipped to maintain the cuvette and its contents at 3.8 oC. She obtains the following equilibrium absorption spectrum at 3.8 oC:

Wavelength: 494 nm

Absorbance: 0.633

Wavelength: 628 nm

Absorbance: 0.438

Determine
KC for this system at 3.8 oC. To receive credit, you must show all work. Remember, first symbolically (with mathematical symbols,
not actual values) state the equation(s). Then, present the
calculation in which actual values are plugged in and the results shown. Be certain sig figs are reasonable
.
You are expected to follow this format in all subsequent lab work without being prompted.

2. Van’t Hoff Analysis. In
Problem 1 above, you obtained
KC at a specific temperature for
BDa. In a similar manner
KC can be determined for
BDa and
BDb over a range of discrete temperatures. Assume, after some effort, you obtain the following
KC vs.
T results for molecular dyes
BDa and
BDb:

 KC at Various Temperatures T for BDa KC at Various Temperatures T for BDb T ( ̊C) KC T ( ̊C) KC 2.18 1.5 2.26 6.7 1.89 3.4 0.974 9.7 1.61 5.4 0.454 12.9 1.36 7.7 0.185 16.5 1.14 10.1 0.0747 20.1 0.957 12.1 0.0347 23.8 0.814 13.9 0.0168

Using EXCEL determine
H and
S for
each dye

. Paste-in the plots below with the
linear trend-line equation and

R2
Remember, in this course all graphs must be appropriately scaled, titled and labeled. Please remember to do this without further reminders. Include all calculations, that is, show all work to clearly indicate how you determined the enthalpy and entropy values for
BDa and
BDb. (Remember to first symbolically state the equations).

3. Gibbs Free Energy. Given the
H and
S results above,
calculate
G at 1.0 ̊C
for
both
dyes
.
Repeat the calculations for
both dyes, but at 5.0 ̊C
. Show all work. It is highly recommended to carry out the sub-calculations with 4 – 5 significant figures, and only round to the correct SF at the end – this avoids “rounding errors.” Assume
H and
S remain constant over the 1 – 5 ̊C
range.

4. Analysis I. Given the results above, what do the calculated
G results indicate about the directionality (product vs. reactant) of each system (BDa and BDb) at 5.0 ̊C versus 1.0 ̊C?

5. Analysis II. As you know, certain raw vegetables when subjected to freezing temperatures loose desirable qualities such as flavor and firmness. Suppose you are tasked with designing an inexpensive thermochromic thermometer that can be attached to vegetable shipping pallets and indicate if freezing temperatures exist. In this application the thermometer must sharply undergo a chromic shift (give a rapid, distinct color change) around 2 – 4 ̊C. Given the results from
Questions 2 – 4 above, and assuming the ε and
λmax values of both dyes are essentially identical, which dye,
BDa or
BDb, would be the best choice for such a thermochromic thermometer? Clearly justify your claim using the relevant data. To receive credit, you must respond with well written, very legible, complete sentences that correctly employs chemical terminology and concepts.

146 Lab 3 Worksheet
v1.4

7-14-22
UA-CBC Technical Press, University of Arizona

## image2.jpeg

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