N204

[ChemSiddiqui]

Ok I did an experiment to find the dissociation enthalpy of N204 and measured the equillibrium vapour pressure relative to the atmosphere. Plotted the graph of result (plot was ln(K) against 1/T*10^3/K-1) and got a straight line with the negative slope which equals -delta H/R(gas constant). I am getting 53 KJ mol-1 as my enthalpy of dissociation. So i was wondering why is it that becuase from my calculation from entalpies of formation of N2O4 and NO2 from the equation ;

 

N2O4 <-------> 2N02

 

I get 57.3 KJmol-1. My experimental value isn't that off the theoratical value but i am wondering why it is 4 kJmol-1 short. One reson i can think of is that NO2 used isn't pure because when I cooled it using liquid nitrogen it kinda turned pale blue rather than white. But could there be an error is my measurement?

 

what you folks think!

 

Comments or suggestion appreciated.

[CaptainPanic]

I think that it's a miracle if you get the exact literature value. You're within 10%!

 

Lesson #1 about measurements: there is always an error in the measurement. The question is how large is the error?

 

You can reduce the error in the measurement by:

1. Calibration of the measurement devices (thermometers, pressure meters)

2. Doing multiple tests and taking the average (perhaps even after kicking out that experiment where you screwed something up).

 

If you measure a temperature, you must take into account heat losses to the environment. If you do proper insulation (I have no clue about your setup), and the value is closer to the theoretical value, then you know you can blame the setup.

[ChemSiddiqui]

I think that it's a miracle if you get the exact literature value. You're within 10%!

 

Lesson #1 about measurements: there is always an error in the measurement. The question is how large is the error?

 

You can reduce the error in the measurement by:

1. Calibration of the measurement devices (thermometers, pressure meters)

2. Doing multiple tests and taking the average (perhaps even after kicking out that experiment where you screwed something up).

 

If you measure a temperature, you must take into account heat losses to the environment. If you do proper insulation (I have no clue about your setup), and the value is closer to the theoretical value, then you know you can blame the setup.

 

I guess you are right. the value is not that off the literature value so I get i shall celebrate i did the experiment well. I like physical chemisrtry experiment and do well in them but dont ask for inorganic or organic...they are a nightmare. thanks

[UC]

One reson i can think of is that NO2 used isn't pure because when I cooled it using liquid nitrogen it kinda turned pale blue rather than white. But could there be an error is my measurement?

 

Pale blue? That would indicate the presence of NO in your sample of NO2, forming N2O3 upon cooling. It is fairly dark blue when pure, so the concentration is probably quite low. I imagine that measurement error of some sort is to blame instead.