0.12 Thermochemistry  (Page 2/3)

The enthalpy of a reaction might be difficult to obtain directly by experiment but can be determined by measuring (or looking up in tables) the enthalpy changes of reactions which contain the reactants and products in a process governed by Hess's Law of Heat Summation. In Part II of this experiment you are asked to find the enthalpy of hydration of ${\text{CaCl}}_2(s)$ the "target reaction" in this manner.

Target reaction:  ${\text{CaCl}}_2+{\mathrm{6H}}_{2}O={\text{CaCl}}_2\cdot {\mathrm{6H}}_{2}O$

It cannot be measured directly because of the slow kinetics of the reaction in the solid state. However, the heats of dissolution of ${\text{CaCl}}_2$ and ${\text{CaCl}}_2\cdot {\mathrm{6H}}_{2}O$ can be determined and, using Hess's Law, the enthalpy of hydration can be calculated.

Theoretical value of the enthalpy of hydration of ${\text{CaCl}}_2$ anhydrous is -81.33kJ/mol. Theoretical value of the enthalpy of hydration of ${\text{CaCl}}_2\cdot {\mathrm{6H}}_{2}O$ is 15.79 kJ/mol.

Experimental procedure

Materials required

• two Styrofoam cups (serve as the calorimeter)
• styrofoam cover (top to the calorimeter)
• conventional or digital thermometer
• stirring motor and stir bar
• two large Ziploc bag
• two plastic spoons

Setup of the microlab thermistor

• Open the MicroLab Program by clicking on the Shortcut to MicroLab.exe tab on the desktop.
• On the“Choose an Experiment Type”Tab, enter a name for the experiment, and then double click on the MicroLab Experiment icon
• Click“Add Sensor”, Choose sensor = Temperature (thermistor)
• Choose an input, click on the red box that corresponds to the port that your thermistor is connected to.
• Label = Thermistor, sensor units = $°C$ , click next
• Click“Perform New Calibration”
• Click“Add Calibration Point”place the thermistor and thermometer in an ice water bath. Wait until the temperature is constant and then read the temperature on the thermometer (to the nearest 0.1 $°C$ ) and enter it into the“Actual Value”box in MicroLab and hit“ok”.
• Again, Click“Add Calibration Point”place the thermistor and thermometer in warm water bath. Wait until the temperature is constant and then read the temperature on the thermometer (to the nearest 0.1 $°C$ ) and enter it into the“Actual Value”box in MicroLab and hit“ok”.
• Under Curve Fit Choices , click on“First order (linear)”and then“Accept and Save this Calibration”, when prompted to enter units, set as deg C. Save as your name-experiment-date.
• Click“Add Sensor”, Choose sensor = Time
• Choose an input, click on the red box that corresponds any of the Timers.
• Label = Time 1, click next, click Finish.
• Left click on thermistor and drag to the Y-axis over“data source two”, also click and drag to column B on the spreadsheet and also click and drag to the digital display window.
• Left click on Time and drag to the X-axis over“data source one”, also click and drag to column A on the spreadsheet and also click and drag to the digital display window.
• When you are ready to obtain data, click start. When you are finished collecting data, click stop. To run another trial, click repeat experiment.

Part i. reaction of strong acid with strong base

• Weigh the calorimeter (styrofoam cups with lid) WITH a stir bar. Accurately measure 50 mL of 1 M HCl into your calorimeter. Turn on the stirring motor to the medium speed. Fit the thermistor though the cardboard lid to a length such that its tip goes deep into the solution but misses the stir bar. Cover the calorimeter.
• Start the Acquisition program. You should see successive, constant temperature readings of near room temperature. Accurately measure 50 mL of 1 M NaOH and quickly add it to the acid solution in the Styrofoam cups. Cover the calorimeter.
• Continue monitoring the temperature change until thermal equilibrium is reached (the temperature stops changing or starts decreasing).
• Stop the Acquisition program. SAVE YOUR FILE. Remove the cardboard lid, reweigh the calorimeter and record the information on your lab report.
• Repeat the same procedure (Steps 1-4) two more times. Save all your files. Give them different, distinguishable names.