Thermo Lab Write Up 6944m

Thermodynamics - Enthalpy of Reaction and Hess’s Law Lab Partner: Drew Simonson Objective: Hess’s law Data Tables: Part 1 Initial Temperature C 50.0 mL H20- room temperature __________ Heated

__________

Mixing Data: Time (sec)

Temperature(C*)

Time (sec)

20

120

40

140

60 80

160 180

100

Tmix C_______

qcal J ____________

Tave C _______ Ccal J/C ___________ Part 2 Reaction #1: HCl + NaOH  NaCl +H2O

Mixing Data:

Temp (C*)

Time (sec)

Temperature(C*)

Time (sec)

20

120

40

140

60

160

80

180

Temp (C*)

100

Tmix C_______

qcal J ____________

Tave C _______ Ccal J/C ___________ Reaction #2 NH4Cl +NaOH  NH3 + NaCl + H2O Initial temp: 50 ml 2.0 M NH4Cl ___________ 50 ml 2.0 M Hcl

___________

Mixing Data:

Time (sec)

Temperature(C*)

Time (sec)

20

120

40

140

60

160

80

180

100

Temp (C*)

Tmix C_______

qcal J ____________

Tave C _______ Ccal J/C ___________

Post Lab Calculations and Analysis: Part 1: 1. On graph 2. See data tables 3.see data tables 4.see data tables Part 2: See data tables for all Part 3: 1.

2.

3.

Post-Lab Questions: 1. What is meant by calorimetry? Calorimetry is the science of measuring the heat of chemical reactions or physical changes. Calorimetry involves the use of a calorimeter. 2. How does graphical analysis improve the accuracy of the data? Graphical analysis allows us to extrapolate the recorded data with great accuracy using the least squares regression line. This allows us to obtain the temperature at time zero when the two solutions are mixed. Graphing the data also allows us to identify any outliers that result due to fluctuating temperatures as the solution equalizes. 3. The equation for calculating the heat evolved in each reaction is : qrxn = ..[(grams of solution specic heat of solution Tsol) + (Ccal Tsol)] What is the meaning of the negative sign in front of the brackets? The negative sign indicates that heat is released from the system as a result of the reaction. 4. Do the lab results Hess's Law? Yes, the lab results do Hess's Law. The enthalpies of reactions 1 and 2 can be manipulated to sum to the enthalpy of reaction 3, with a 19.8% margin of error. For example, the enthalpy of reaction is -71.04 kJ/mol and it can be added to (+)5.66 kJ/mol to have the theoretical enthalpy value for reaction 3 at -65.38 kJ/mol. Experimentally, the enthalpy of reaction 3 is -54.57 kJ/mol. Given the errors that could have occured as well as the unreliability of the equipment, the results do coincide with Hess's Law.

5. How could the procedure be modified to achieve greater accuracy? Modifications to the procedure would yield better results. For example, instead of a least squares regression line of best fit, a curve should have been used. This would have suited the data much better, as well as for any variation in the temperatures, and would have yielded a more accurate initial time zero temperature. Also, instead of the use of a styrofoam calorimeter was very inaccurate towards the final results. If a more accurate calorimeter was used, greater accuracy could have been obtained. 6. Calculate Hrxn for the three equations in the experiment using a reference table. Do these values Hess's Law?