Lab #1: Am I Blue?
Investigating the chemistry and energetics of a hydrated salt
Overview: In this lab, you will run two reactions involving a hydrated salt. Hydrated salts contain water
molecules within their crystals. In this case, you will be working with a colorful copper sulfate salt.
In the lab, you should be using your stoichiometry skills as well as applying your understanding of
chemical energetics.
Part A: Dehydrating the salt
1. Obtain a clean, dry evaporating dish. Mass this dish and record its mass.
2. Mass out between 4 and 6 grams of the hydrated copper sulfate salt into the evaporating dish. Record the mass
that you have added to the dish, and the total mass of dish + salt.
3. Heat the salt in the evaporating dish over a Bunsen burner flame (use a ring and a wire gauze to support the dish).
Your goal is to drive off ALL the water that is originally present in the copper sulfate crystals. Use a glass stirring
rod to help the process along (be careful—the bowl will be hot!). You will need to use good judgment to
determine when the salt has been completely dehydrated. Record your observations and your method for judging
when the reaction is done.
4. Allow the dish to cool and then re-mass the dish with the dehydrated salt. In your lab book, show (with clearly
labeled numbers) a METHOD for calculating the MASS OF WATER and the MASS OF THE DEHYDRATED SALT. Be
Neat! If it is not legible it is wrong!
5. Calculate the mass of water as a PERCENTAGE of the original hydrated salt’s mass.
6. Using your numbers from question #4, calculate the MOLES of water driven off in your reaction and the MOLES of
CuSO4 that remain in the bowl.
7. The original formula of the hydrated copper sulfate salt is one of formulas shown below. Choose the
formula that you think is correct based on your EXPERIMENTAL data. Explain your reasoning. Note:
you should base your answer either on the percentage of water you calculated in q #5 or on the mole
ratio you calculated in q #6.
CuSO4 • 3 H2O
CuSO4 • 4 H2O
CuSO4 • 5 H2O
CuSO4 • 6 H2O
8. Here is a list of possible sources of error, classify each into one of the categories listed below. Explain
your reasoning (a generalization would be most powerful!)
A. Didn’t cook the salt long enough
B. Lost some salt on the stirring rod
C. Original bowl was slightly damp (prior to adding the salt)
a. Water molecule calculation too low (i.e. the group calculated 3.5 water molecules when the truth is
4 water molecules)
b. Water molecule calculation too high (i.e. the group calculated 5.5 water molecules when the truth is
5 water molecules)
9. Which of the following energy profiles accurately describes the energy change associated with the
dehydration reaction? Explain your reasoning thoroughly. Note: please discuss both flow of energy
and change in bonding states in your answer. Remember: It takes energy to break bonds.
hydrated
dehydrated
Chemical
potential
energy
Chemical
potential
energy
hydrated
dehydrated
Part B: Rehydrating the Salt
In this part of the lab, you will try to reverse the process you did in Part A (i.e. go from the white copper
sulfate salt back to the blue hydrated copper sulfate salt).
10. Before doing any experimentation, make a prediction about whether the rehydration process will be exothermic
or endothermic. Provide a brief argument for your prediction.
11. Measure 20 mL of distilled water in a small graduated cylinder. Use a digital thermometer to record the
temperature of the water.
12. Transfer your CuSO4 to a pre-massed STYROFOAM CUP and add the 20 mL of distilled water. Stir gently with
your thermometer and record the FINAL temperature and the OVERALL CHANGE in temperature of the water.
13. Sketch an ENERGY PROFILE that describes the relative change in POTENTIAL energy that occurs during the
rehydration reaction and discuss whether your this reaction is EXOTHERMIC or ENDOTHERMIC. Note: after
completing step #16, place a DH value on this energy profile. Note: DH = Heat (q) in kJ / moles
14. Calculate the number of joules of heat produced in the rehydration reaction using the equation shown below:
Hint: this is a CALORIMETRY equation. Examples of “SPECIFIC HEAT” are pasted to your magic bean machine.
Energy (q) (joules) = specific heat of water (C) • (mass of solution) • (DT)
15. Your answer from question 14 has determined the number of JOULES of heat produced from a small sample of
copper sulfate. Use this information (and some proportional reasoning) to determine the DH for the reaction in
terms of KILOJOULES PER MOLE of copper sulfate. Note: you have calculated JOULES in step 15 and MOLES of
copper sulfate in step 6. Put these together proportionally to figure out the kilojoules of heat produced from 1 mole
of copper sulfate.
16. Which form of copper sulfate is more STABLE: the white salt or the blue salt? Explain briefly.
17. Suppose you wanted to make a portable hand warmer (like the chemical heat packs you might use when skiing).
Which form of copper sulfate would you use in your hand warmer—the white salt or the blue salt? Explain. Note:
make sure to discuss POTENTIAL energy in your answer!
18. When you are ready to clean up, pour your blue salt into the big CuSO4 RECYCLING BEAKER.