chemical principles revisited
edited by MURIEL BOYD BISHOP Ciernson University Ciernson, SC 29631
The Effect of Pressure on the Equilibrium of the N204-NO2 System, and its Classroom Demonstration Zhiming Yang Guizhou Normal University, Guiyang, Guizhou 550001,People's Republic of China Some textbooks (1-5)cite the following chemical system as a n example to introduce the effect of change of pressure or volume on gaseous equlibrium: Nzo4(g) 2 2 Noz(g) brown colorless At constant temperature, an increase in pressure (decrease in volume) shifts the system a t equilibrium in the direction that produces more Nz04,and a decrease in pressure (increase in volume) shifts it in the opposite direction, giving more NOz. Unfortunately, in some textbooks (3-5) the equilibrium shift is inappropriately connected with the color change in the system. In these books it is claimed
Initial
Change
Final
Figure 1. Calculated concentrations versus time for N,04 (dashed line) and NO2 (solid line) when the volume of an equilibrium mixture is halved. (Temperatureassumed constant.) 94
Journal of Chemical Education
that, when pressure on the system is increased (its volume decreased), the net reaction shiRs to the leR until equilibrium is re-established, and the color of the system becomes lighter (more Nz04). Conversely it is claimed that, when the pressure on the system is decreased (its volume increased), the color darkens (more NOz) because the equilibrium has shifted from left to right. Such claims are in error, however. Even though the equilibrium shifts in the anticipated direction, we should not expect the color to change in the manner described. The wlor changes depend on the concentration of NOz (or its partial pressure), assuming that comparisons are made using tubes of equal thickness. The change in concentration of the NO2 depends on both the change in volume and also the accompanying shift in equilibrium. For example, when the system is compressed (at constant temperature) the concentrations of both Nz04 and NOz increase. which would favor the color of the system becoming darker Meanwhile, the concentration of NO? decreases a< a result uf the shiRin~of the rquilibrium towards the side with more Nz04, which favors k e color becoming lighter. These two processes, then, have opposite effects upon the color change. It has been proved both theoretically and by experiment (6) t h a t t h e change i n volume affects t h e change in color more significantly that does the shift in equilibrium. We take the case of increasing pressure to demonstrate this fact in detail. Figure 1 shows the effect of increasing pressure on the equilibrium for the N2O4-N02 system. Ci is the concentration (or partial pressure) of the NOz in the initial equilibrium. C, expresses the concentration of NOz when the volume of the system is decreased but no shiR takes place, and Cf is the calculated concentration of NO2 a t the new equilibrium condition. These three concentrations are related as follows: C, > Cr> Ci. Since C, > Cf, the equilibrium obviously shifts to the leR, as expected. But since Cr > Ci, the color of the gas mixture should become darker. Experimental Demonstration According to Figure 1, if we could compare the colors of the imagined system (compressed but not re-equilibrated) with the final system, we should fmd that the color becomes lighter (C, > Cr) due to the shift in equilibrium. Is it possible to choose a suitable reference system for following this part of the color change? Since the reaction between Nz04and NOz is so fast, (relaxation time on the order of lo4 s), this system cannot be used as a probe for this color change. Therefore, we have chosen bromine as a suitable system because the color of bromine is similar to that for NO2 and because there is no equilibrium process to contend with. Starting with a bromine concentration that
Initial
(1)When the total pressure exerted by these gases is 1atm, liquifaction will occur at 21.15 'C or lower to give a heterogeneous system. It is necessary that the sum of partial pressures of Nz04and NO2 be less than 1.2 atm at 25 'C if liquifaction is to be avoided.
Final
(2) At mom temperature, air (containingN2, 02, etc.) present in the svstem should not take mart in the chemical reaetion. Hence ~t 1s unnecessary that pure dinitragen tetmxidr nnd nitrogen dioxide be used in thesystem. Air that is present u ill not influence the conclusion. ~
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Summary
Figure 2. Schematic diagram showing the results of compressing (at constant temperature Br2 (a and b) and N20hN02 (a' and b'). The number of dots looselv indicates what haooens of mol, ~ ,to the ~ number ~ ec, es responsloe I& color (Br2 and hO,) in each case. (Tney are not however meant lo Imply that the mo ar concentrations of Br,ana NO2 neeaed to gwe tne same nlt al color are eqLa1.j ~
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gives the same color as a sample of Nz04-NO2 and com~ r e s s i n ethe bromine. we see the color we should exDect &om N&NOZ in theabsence of re-equilibration (Fig.'2, a and b.) Compressing similarly the N204-NO2 sample (Fig. 2, a' and b') yields a final color that is lighter than the final bromine color, demonstrating that the chemical eauilibthe rium has in fact shifted from right to left. final color of the N204-NOz system is darker than the initial color of either system, illustrating that Cr> Ci. Two facts related to the Nz04-Nz system should be kept in mind in running this demonstration.
c ow ever,
In the reaction Nz04(g, colorless) 2 2 NO2 (g, brown), a n increase in pressure causes the equilibrium to shift to the left, but the mlor becomes darker, not lighter. A decrease in pressure causes a shift to the right, but the color becomes lighter instead of darker. The competing processes affecting the color can be demonstrated using bromine gas as a comparison system. Acknowledgment I thank J. P. Lowe of the Pennsylvania State University for his assistance with English. Literature Cited 1. MeQuarry, D.A.;RackP.A.Ce-lchemist'y, 2nd ed.,Freemsn:New York. 1987:p 451. 2. Knz, J.C.;Purcell,K F.Chem~t'y~ndChhhiml&oefii~;Sssddd:PhililddIphia,
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