Experiment In this experiment a high vacuum sublimation was performed to determine the vapor pressure and finally the enthalpy of sublimation of Vanillin and o-Vanillin. AKnudsen cell with sample was weighed 10 times maybe just say weighed by difference, they don’t usually like it when you spell out the steps like that no way it was like 0. 5 grams, check the lab again and at least 0. 5 g of Vanillin or o-Vanillin was added to the cell and it was reweighed 10 times.
This experiment depended heavily on the method of weigh by difference when determining the mass of sample (Vanillin or o-Vanillin) loss in sublimation. A water bath (70 °C for vanillin or 40 °C for o-Vanillin) is prepared for the sample. The Knudsen cell was inserted into the sample tube and the closed end of the sample tube was placed into the prepared water bath. The tube with the O-ring was connected to the sublimation apparatus. The diffusion pump was connected to the water supply and turned on to 90 V. The cold trap of the apparatus was filled with Liquid nitrogen every hour of the experiment.
Maybe talk about the type of vacuum pump used since there was so much about it in the lab manual After the sample had reached and sustained the desired temperature for about 5 minutes the sublimation region was roughed.. Just say that you roughed, they don’t usually like this step by step stuff with the valves, but I do like that talk about the pressures that we reached The time between the opening of valve 1 and 3 was the uncertainty in total time. The sublimation apparatus was place under high vacuum to initiate the sublimation process.
The mass loss was determined by weighing the difference in the mass of the Knudsen cell + sample before and after sublimation. The area of the Knudsen cell orifice was determined by using the area of a circle equation A=?? (d/2)2 . The dimensionless correction factor (? ) was determined using the equation ? =1-0. 5(l/d)+0. 2(l/d)2 . Using the measured values of mass loss (m), total time (t), and temperature (T), along with calculated thickness parameter (? ) and area (A) we determined the vapor pressure of our samples using the following equation:
Vapor Pressure (P) = ((m)? (?? A? t))? SQRT((2??? R? T)/(M)) Where R = 8. 314 J/kmol ? The calculated vapor pressure for each trial was used along with the measured average temperature from each trial in the temperature bound integrated Clausius-Clapeyron equation to determine the ? H°Sub . Enthalpy of Sublimation ? H°Sub = – Ln(P2/P1)(T2-T1)? R Using the Integrated Clausius-Clapeyron equation and the vapor pressure equation discussed the raw data was used to determine the following analyzed results for vapor pressure and ? H°Sub for Vanillin and o-Vanillin..
Probably should add standard deviations, even if they are those weird numbers we were still talking about in lab Thursday. | Vanillin T1| Vanillin T 2| o-Vanillin T 1| o-Vanillin T2| m (kg)| 8. 59E-06±6. 37E-06| 6. 816E-05±2. 458E-07| 2. 916E-05±1. 350E-07| 6. 125E-05±9. 156E-07| T (K)| 329. 02±0. 30| 343. 54±0. 16| 311. 40±0. 08| 302. 73±0. 06| t (sec)| 5732±24| 6870±13| 6291±25| 6291±40| P (Pa)| 1. 0107±0. 2931| 6. 8358±0. 1378| 3. 0406±0. 0622| 6. 2972±0. 1372| ? H°Sub (J/mol)| -230. 728 ±239. 311| | 52. 4913 ±43. 7797| | ? | 0. 991928| | | | A (m^2)| 5. 024E-07 ± 1. 0053E-08| | | |