Less than zeroAdvanced Chemistry (STEM) Lab

Unit 2: Thermodynamics

Overview: Your objective in this lab is to cool an aqueous solution to below zero Celsius. All groups will start by running the same reaction with the same apparatus. Each group will then make a second run utilizing design improvements. It is in this "improved" run that you may be able to lower the temperature to less than zero.

Part I:

1)Obtain a 150 ml beaker, a thermometer, and a grad cylinder.

2)Use the grad cylinder to obtain 25 ml of 2 M HCl. Measure the temperature of the acid.

3)Weigh out 4 grams of solid baking soda (sodium bicarbonate)

4)Put the baking soda in the beaker.

5)Carefully add the acid to the beaker (much fizzing will occur). Stir with the thermometer and see how low the temp goes.

Calculations:
The reaction that occurs in the beaker is:

NaHCO3 (s) + HCl (aq)  NaCl (aq) + H2O (l) + CO2 (g)H = +28 kJ/mol

Remember:qsurr = m x (T) x Cs [Cs = 1.0 cal/goC, 4.184 J/goC]

a)In your beaker, you have 25.0 ml of solution. This is roughly equivalent to 25.0 g H2O. How many calories wereremoved from this quantity of water to cause your temperature decrease? How many joules does this correspond to?

b)Use the H value given above to calculate the number of joules that would be absorbed by the reaction assuming you start with 4.00 grams baking soda.

c)Compare your experimental heat lost (a) to the calculated heat absorbed (b). Consider and write down what adjustments you might make to get these numbers closer together.

Part II: Going below zero!

Your objective now is to make the cooling reaction work better than it did in Part I. You are allowed to make any modifications you like, within reason. The following questions are designed to make you consider possible avenues for modification. These are NOT the only things that you could change. I would like you to think of all the improvements you would like to make and then do one experiment that is your "best shot". Put the results of your experiment (temperature-wise) on the board. If you mess up in some way, you can repeat your experiment, but otherwise think that you only have one try at reaching the goal of less than zero.

Questions to consider in determining your “best shot.”

1)Does the container affect the results of the experiment?

2)Is there an advantage to scaling up or scaling down the reaction? (you are allowed to use as little as 1 gram or as much as 15 grams baking soda and HCl concentrations from 0.5 M to 6.0 M)

3)Is the ratio of baking soda to acid that you used in the first reaction the "right" ratio? Would there be an advantage to using excess baking soda or excess acid? Note that the generic baking soda you are using is NOT a reagent grade ultrapure chemical – that may make a difference in your thinking.

Lab Report Write-up: Please submit your results and summary of the lab on your lab notebook pages with the following contents:

Pre-Lab: This has the usual components, plus you may want to include some pre-part 1 ideas for improving your results.

Data Table: Consolidate all of your data for Parts 1 and 2 in a single table (no calculated values here please). After this, record your observations.

Questions: Work out with steps, calculations for questions a), b) and c).

Explanation: Give your reasons for your “best shot” choices (reaction container, mass of baking soda, volume and concentration of acid), justifying them with sentences of explanation and calculations. Explain the reason for your not getting below zero, if you did not.

STEM Society activity: In order to have this activity qualify for a STEM Society portfolio item, you must get a reaction to go below zero. Then submit the following components in your reflection:

Data Table and Questions as requested above in the Lab Report Write-up section.

Explanation: Verify your successful recipe with the following calculations:

  1. The mole quantities of your baking soda and HCl.
  2. The expected heat lost by the water/acid solution.
  3. The actual heat lost by the water/acid solution.
  4. The expected T of the water.

Summarize your experiment in a paragraph in which you explain your progression of learning as you tested various recipes and made adjustments. Lastly, conclude with what you think are the most significant factors that allowed for your success.