In various chemical processes such as reactions and the dissolving of salts, heat is either absorbed or given off. We call these events either an endothermic (heat in) or exothermic (heat out) process. It is usual to detect these heat events by measuring the temperature change associated with the process. In this problem, you will dissolve several salts in water, measure the resulting temperature change, and then make deductions about the nature of the process.

  1. To start this activity, click this link for Endothermic vs. Exothermic from the list of assignments. The lab will load in a new tab. Click back to this tab to read further instructions and complete the questions below.
  2. There will be a bottle of sodium chloride (NaCl) on the lab bench. A weigh paper will be on the balance with approximately 2 g of NaCl on the paper.
  3. The calorimeter will be on the lab bench and filled with 100 mL water. Make certain the stirrer is On (you should be able to see the shaft rotating). In the Thermometer section of the Live Data tray menu click Save Data to begin recording data. Allow 20-30 seconds to obtain a baseline temperature of the water.
  4. Drag the weigh paper with the sample to the calorimeter until it snaps into place and then pour the sample into the calorimeter. Observe the change in temperature until it reaches a minimum or maximum and then record data for an additional 20-30 seconds. (You can click on the clock on the wall labeled Accelerate to accelerate the time in the laboratory.) Click Stop Saving. Open the Lab Book menu in the tray. Record the temperature before adding the NaCl and the highest or lowest temperature after adding the NaCl below.

TBefore= ºC

TAfter= ºC

  1. Click the red disposal bucket or the Clear Lab button to clear the lab. Open the Presets menu in the tray and select the preset experiment called Heat of Solution-NaNO3 and repeat the experiment with NaNO3. Record the initial and final temperatures below.

TBefore= ºC

TAfter= ºC

  1. Click the red disposal bucket or the Clear Lab button to clear the lab. Open the Presets menu in the tray and select the preset experiment called Heat of Solution-NaAc and repeat the experiment with NaCH3COO (NaAc).  Record the initial and final temperatures below.

TBefore= ºC

TAfter= ºC

7.         Calculate ΔT (ΔT = TAfter – TBefore) for each mixture and record it below.

ΔT NaCl =  ºC

ΔT NaNO3 = ºC

ΔT NaCH3COO = ºC

  1. An exothermic process gives off heat (warms up). An endothermic process absorbs heat (cools off).

Which solutions are endothermic and which are exothermic? What is the sign of the change in enthalpy ΔH in each case?

NaCl

  • slightly endothermic
  • slightly exothermic
  • + ΔH
  • – ΔH

NaNO3

  • exothermic
  • endothermic
  • + ΔH
  • – ΔH

NaCH3COO

  • exothermic
  • endothermic
  • – ΔH
  • + ΔH

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