Lab Report 5 Heat of dissociation of N2O4 PDF

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I passed the class with a B+. Lab reports worth about 80% of the class grade. I even got a D+ on the final, but still passed with a B+ because I got good grades on the lab reports....

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Expt #5: Heat of Dissociation of N2O4 Yiyeon Kim Partners: Mark Kuhlman, Gabriela Lopez Section 22 TA: Fan Yang

1. Introduction: A. Purpose - The objective of this experiment is to ultimately analyze the strength of bonds via the dissociation reaction of dinitrogen tetroxide into nitrogen dioxide. The apparent molecular weight of the equilibrium mixtures of these two chemicals will be measured to arrive at the dissociation and equilibrium constants. B. Procedure - A reaction flask with a stopcock was used for this experiment. First, the flask was evacuated with a mechanical pump, releasing the air inside and warmed up if needed to clear any moisture. It took a few minutes to sufficiently evacuate the flask. The flask was then cooled and weighed. The flask was then filled with dry air and reweighed. The temperature and pressure was then recorded. After, the flask was allowed to equilibrate and was reweighed, it was evacuated again for a few minutes. The empty flask measurements agreed in weight and were progressed to the next steps. Dinitrogen tetroxide was then inserted into the flask. Some pressure was released by releasing some of the gas inside flasks. The flask was clamped in a large beaker of water with a thermometer and a stirring rod. The bath was warmed to 30°C and maintained. The pressure was allowed to equilibrate for a thirty seconds and the bath temperature was recorded. The bulb was then removed from the water, dried and cooled to room temperature in order to be weighed. This procedure was repeated at 40°C, 50°C, and 60°C. C. Discussion - ΔH is the enthalpy or totally heat content of a system. It is also the energy needed to break a bond. ΔS is the entropy or disorder in a system. It is the energy a system needs to convert for mechanical work. ΔG is the Gibbs free energy or 1 ΔS is the maximum amount of reversible work for a system. lnK p (T ) =− ΔH R * T + R derived equation that relates Kp, ΔS, and ΔH. The natural logarithm of the equilibrium constant Kp is a function of the temperature with some constant dependence on the enthalpy and entropy. The slope in the equation comes out to be the enthalpy whereas the y-intercept of the entropy. 2. Data and Calculations: A. Show one sample calculation and tabulate the rest. Calculate Kp, G, H, and S at each temperature. m T )( 294.3 )( 92 ) X t = ( mair )( T0 )( 92 ) − 1 = ( 0.3123 0.8066 304.15 29 − 1 = 0.2721 29 t

2x ) = 0.954 ( 2 *0.2721 ) = 0.4081 atm P N O2 = P 0 ( 1+x * 1+0.2721

P N 2O 4 = P 0 ( 1−x ) = 0.954 * ( 1−0.2721 1+0.2721 ) = 0.5458 atm 1+x K p (T ) =

P NO

2

2

P N 2O 4 * P

*

=

0.40812 0.5458

= 0.3051

ΔG =− RT lnK p (T ) = − 8 .314 * 2 94.3 * l n (0.3051) = 3 001.56J/K * mol

Δ H = − R * s lope =− 8.314 *− 6677 = 55512.58J Δ S = R * y − i ntercept = 8 .314 * 2 0.8 = 1 72.93J/K

Temperature(K)

Kp

ΔG (J/K*mol)

ΔH (J)

ΔS (J/K)

304.15

0.3051

3001.56

55512.58

172.93

313.85

0.6069

1302.69

55512.58

172.93

323.45

1.0740

-191.98

55512.58

172.93

332.34

2.0226

-1946.95

55512.58

172.93

1 l nK p (T ) =− 6677 308K + 20.8 =− 0.8786

ΔG =− RT lnK p (T ) = − 8.314 * 308K *− 0.8786 = 2249.84 K ΔS =

ΔG−ΔH −T

J * mol

= 2249.84−55512.58 = 172.93 KJ −308

Temperature(K)

ΔG (J/K*mol)

ΔH (J)

ΔS (J/K)

308

2249.84

55512.58

172.93

318

520.46

55512.58

172.93

328

-1208.86

55512.58

172.93

B. Include plot of Kp vs. T and 1n Kp vs. 1/T.

3. Discussion: A. Literature Values and Results Comparison - The literature values obtained for Δ  H, ΔS, and ΔG from reference (3) were 57.20 kJ, 175.72J/K, and 4.81 kJ/(K*mol) at 298.15K, respectively. ΔH% = | 57200−55512.58 | = 2.95% 57200 ΔS% = | 175.72−172.93 | = 1.58% 175.72 ΔG% = | 4810−3953.14 | = 17.8% 4810 B. Error Discussion - Some uncontrollable factors of errors that may have affected the experimental values to differ from literature values may have been from the fact that the gases being observed are not ideal. Thus, heat is loss, there is not a perfect relationship between the thermodynamic properties, and the contents of the flask may not be completely pure. Even though the flask was evacuated of air several times, some air still resided inside as well as water which would affect the results. Overall, the experimental values came very close to the literature values with only around 20% error. C. Underlying Assumption - Some assumptions that were made in this lab were that the gases were ideal. With that assumption, the moles of the gas were able to be calculated from the ideal gas law which then were used in finding Kp. The partial pressures of each dinitrogen tetroxide and nitrogen dioxide were found this way. Although this could give a relatively accurate result and calculation, the gases in the systems observed were not ideal. Another assumption is that the enthalpy is completely independent of temperature change but in reality, it does vary with change in temperature. Thus, even though the linear fit showed a linear relationship in Kp vs. 1/T, making enthalpy the same value for all of T, enthalpy did actually vary. D. General Comments - Looking at the percent error, they’re relatively close to the literature values, so the experiment was done successfully.

4. Questions:

1. We should never open the stopcock unless the temperature of the bulb is higher than it was at the time you previously opened the stopcock, because raising the temperature of the flask also increases the pressure inside the flask while lowering the temperature of the flask decreases the pressure inside the flask. When the flask is opened to the atmosphere during each step, it equilibrates and the inside becomes the same pressure as atmospheric pressure. Then, if the flask is heated, having atmospheric pressure, the inside will pressurize and release when opened. Conversely, if it is cooled, the inside will have a lower pressure and air will come in when the flask is opened. 2. We should make sure we let air back into the bulb before leaving, because not keeping a vacuum inside the flask is a safety precaution. Air and atmospheric pressure is kept in the flask after being done with it so that, over time, there would not be a chance of the flask collapsing in on itself. 5. References: 1. Y. Yin, “Supplementary Laboratory Manual.” 2. TA Fan Yang. 3. http://chemistry-reference.com/reaction.asp?rxnnum=448...