CP Chemistry Theodore Roosevelt High School Lab #3-2 Calorimetry Lab

Introduction Much like in the previous lab where we melted an ice cube, you can use q = CmΔT to calculate the heat change that occurs when you dissolve an ionic solid. You can then determine the molar heat of solution, ΔHsoln, in kJ/mol by dividing the heat change by the moles of chemical dissolved. Positive molar heats represent endothermic changes, which cause a decrease in the temperature of the surroundings, while negative molar heats have the opposite effect. In this lab, you will compare the calculated heat change of each chemical to its accepted value to determine your percent error. Be sure to work quickly, measure accurately and keep your calorimeter lid secure to ensure the best possible data! Purpose Based on the introduction above, determine the purpose of this lab and include it in your lab write-up. Prediction During the winter, calcium chloride is used to melt ice from streets and sidewalks. Do you think this is an endothermic or an exothermic reaction? Explain. Equipment beaker, 250 mL cardboard lid electronic balance, 0.01 g graduated cylinder, 100 mL

Styrofoam cups, 2 Vernier LabQuest Vernier temperature probe

Materialsammonium chloride [NH4Cl] calcium chloride [CaCl2] Safety Considerations • Safety goggles must be worn at all times. • Sometimes chemicals from previous labs still remain in glassware and on other lab equipment;

wash all lab equipment before and after performing this lab. • Wash your hands thoroughly after completing this lab.

Procedure 1. Turn on the Vernier LabQuest and connect the temperature

probe. Tap on the settings box on the right of the sensor screen and set the collection time to 10 minutes and the frequency to 60 times per minute (once per second). Save the settings by tapping the “Done” button.

2. Assemble your calorimeter per the diagram shown to the right. 3. Using a graduated cylinder, measure out exactly 100.0 mL tap

water and pour it into your Styrofoam cup calorimeter. 4. Using the LabQuest’s temperature probe, measure the starting

CP Chemistry Theodore Roosevelt High School Lab #3-2

temperature of the water. Record this value. 5. Obtain approximately 5 g of calcium chloride. Using weigh paper, measure and record the

mass of the chemical. 6. Quickly place the calcium chloride in the calorimeter, securely replace the lid and begin recording

data on your LabQuest by pressing the play button. 7. Gently swirl the calorimeter to help the calcium chloride to dissolve completely and evenly; be

careful not to spill any water. 8. Once all of the chemical has dissolved, stop recording data on your LabQuest. Using the stylus,

select all of the data on the graph and then tap the ‘Analyze’ menu, then ‘Statistics’ and finally ‘Temperature’. Record the maximum value displayed on the right as the final temperature of the water.

9. Clear the recorded data and perform steps 3-8 again using ammonium chloride instead of calcium chloride, and record the minimum temperature instead of the maximum.

Data – you should create a data table in your lab write-up that looks something like this:

Chemical Mass Starting Temp

Final Temp q* ΔHsoln* % error*

calcium chloride

Calculations (Include these answers in your Conclusion!) 1. Knowing the number of moles of each chemical that were dissolved, and knowing the heat

change caused by each chemical (q), determine the molar heat of solution (ΔHsoln) for each chemical in kJ/mol.

2. The accepted heat of solution (ΔHsoln) of calcium chloride is -82.8 kJ/mol. Using your experimental value for ΔHsoln of calcium chloride, calculate your percent error in this experiment.

3. The accepted heat of solution (ΔHsoln) of ammonium chloride is +14.7 kJ/mol. Using your experimental value for ΔHsoln of ammonium chloride, calculate your percent error in this experiment.

Questions 1. Why is the molar heat of solution (ΔHsoln) of calcium chloride a negative value? Why is the molar

heat of solution (ΔHsoln) of ammonium chloride a positive value? 2. Compare your results to those of another lab group. Were their values for ΔHsoln similar to or

different from yours? Explain why or why not. 3. How do you think you might apply this technique to determining the amount of chemical potential

energy stored in food? Errors Describe two possible errors you may have committed in this lab that may have somehow affected your results. Explain the specific steps you will take to avoid each of these errors in the future. Conclusion Write two or more paragraphs summarizing your results, examining the validity of your prediction, and explaining your calculations.

CP Chemistry Theodore Roosevelt High School Lab #3-2 Calorimetry Lab workspace: