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Kinetics of Chemical Reactions A Low-Cost and Simple Apparatus G. ~ a ~ a g e o r g i oK.u Ouzounis, ~ and J. Xenos Department of Primary School Education, Democritus University of Thrace, Alexandroupolis Kinetics of chemical reactions is one of the most interesting subjects with which the curriculum of chemistry education deals in Greece and other wuntries a s well (1). Kinetics is taught, a t least in Greece, theoretically and not through experiments, although it is a highly difficult subject. Havingin mind this situation, we devised a simple low-cost apparatus, the use of which facilitates the under-
standing of lessons about kinetics of chemical reactions. Our main tareets were (1)to develoo a simole . aooaratus .. that would h s p students of.seconda& educatmn to understand and consolidate terms about klnrtlcs. and (!: to keeo the cost of t h e apparatus within the b;dget of ever; teacher of secondary education. Description of the Apparatus The operation principle of the apparatus looks like that of a photometer. Alight beam coming from a common lamp of 2.5 V goes through the solution under study straight to a photoresistance. A photoresistance can alternate its rates according to the accepted intensity of light. Thus, when this is connected to a milliammeter through a proper
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Figure 1.The apparatus: (1) cylindrical receiver, (2)lamp, (3)photoresistance, (4) potentiometer, (5) and (6) positions for 3 V supply connection, (7) and (8) positions for 1.5 V supply and milliammeter
Figure 2.The apparatus during function: (1) the main apparatus, (2) a 100-mL beaker containing the solution under examination, (3)3 V supply, (4) 1.5 V supply, (5)lab milliammeter.
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Table 1. Correspondence Concentration-Readout for the Reaction-forming AgCl
Concentration (M x 1o-~)
Figure 3. Electrical diagram of the whole arrangement: (1) the solution under examination. (2) lamp, (3) beam of light, (4) photoresislance, (5) milliammeter, (6) 1.5 V supply, (7) 3 V supply, (8) ~otentiometer. circuit, its rate variation can be taken down as the corresponding variations of intensity of the current on the milliammeter. Figure 1shows the apparatus made a t the beginning. To operate it we need one more milliammeter, a 1.5-V battery, a supply of direct current with a n output of 3 V (or combination of two DC batteries in serial connection), a s well as the necessary connecting cables, objects easily found in a science lab. The apparatus consists of a cylindrical receiver (1)glued with silicone on a piece of iron-plate suitable to hold a 100mL beaker. The cylindrical receiver, in this case, is a plastic drainage pipe fastener. Two holes of 0.8 cm each, were made one facing the other. Outside one of them, there has been applied a small lamp of 2.5 V and 0.5 A (2) (using a piece of iron-plate glued firmly with silicone); whereas, outside the other one, the photoresistance 1-100 Kn (3) has been glued (with silicone again) and has been colored
Readout (%ofscale)
black, so that it is not affected by the light outside the apparatus. The inknsity of the lamp is adjusted by a potentiometer 10 i2 (4).(In our case, we have equally used a potentiometer 100 R parallel to a resistance of 100 0). On the apparatus there are also two positions ( 5 ) and (6) where a 3 V supply is connected (from a supply or a battery) and another two positions (7) and (8)where a battery of 1.5 V and a lab milliammeter 0-3 or 0-5 mA (in serial connection) are connected. Figure 2 shows the apparatus during function. Figure 3 shows the electrical diagram of the whole arrangement. When such an arrangement is to be used in a classroom (and not in a lab) we have made a compact alternative construction easily camed (from classroom to classroom)(Fig. 4). The electrical diagram of this apparatus is similar to that of Figure 3, with the difference that a double switch has been added (so that the two circuits to turn onloffa t the same time). Applications How the Teacher Can Use the Apparatus
This apparatus enables the teacher to show, through experiments, the process of a reaction either counting its relative velocitv e v e n moment or Living information on its mechanism. For this purpose we select a reaction that either forms or consumes a hardly soluble substance in suspension or a colored one. In this way the change in.concentration of such a substance can be noted in the apparatus screen, because it affects the amount of light coming to the photoresistance and, therefore, the current intensity eventually counted by the apparatus. We must add the following:
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(1)The absorption of the beam of light by the solution (mntaining the substance under examination) is not a linear function (the Lambert Beerb Law is not in farce here); and (2) the intensity of the beam of light finally falling upon the photoresistance is not a function of the intensity of the current noted an the apparatus screen.
Figure 4. A compact alternative construction: (1) a 100-mL beaker (intothe cylindrical receiver),(2) pofentiometer, (3)double switch, (4) milliammeter. (The values of the apparatus readings are receivev by noting the position of the needle with a permanent marker on the celluloid sheet placed on the screen). 648
Journal of Chemical Education
So, before we examine the kinetics of a reaction, we must make a standard corresponding curve of the solution concentration and of what is noted on the apparatus screen. This will be done by using the known concentrations of the substance under examination in the solution. To study the reaction producing AgCl from &NO3 and HCI we write down the apparatus readings presented in Table 1, for the AgCl concentrations. In this way we plot the curve of Figure 5. To count the instant velocity of the reaction we mix solutions of HCI nnd &NO3 and writc down the apparatus readings a t given time intervals. Because the process of the reaction develops rapidly, the values of the apparatus readings are received by noting the position of the needle
times and thus to come to conclusions about its change and about the relationship between this velocity and the concentration. The instructor also could ask them to plot a curve of velocity-time. Similarly, in order to study the reaction between Ag2CrO4 and HN03, we write down the apparatus readings for different
2
5.o-
1.4 1.2 1 2 0.8 +. 5 0.6 2 0.4 0.2 0
:S : -.\\
Ag,CrOa In orderconcentrations. to count the instant velocity we mix AgzCr04 with HN03 solution and write down the apparatus readings as in --CI 0 the case of AgCI. Based on the received 0 20 40 60 80 100 values we plot the curve of Figure 7. Also we &n plot curves that will inform Readout (% of scale) the students on the mechanism of the reactions. Figure 5. Correspondence concentration-readout of the apparatus, for the reaction forming ~ h study , of reactions between KI and AgCI. Hz02 (in presence of starch indicator) and between Na2SzO3and HC1 gives the following results: When diagrams of wncen0.9 tration-time of the above reactions are compared with that of the reaction forming AgCl, a delay in the process of the reaction is observed (Figs. 8 and 9). Based on this observation the teacher / can explain that there is more than one g 0.5 .step in the process of the reaction and also can tell a possible relative duration for 9 0.4 + c each one. The curve in Figure 8, describ8 0.3 ing the process of forming the iodinec starch complex, shows a delay caused by 0.2 the initial reaction of the I2 formation 0.1 (from KI and HzOz). So the existence of the two steps is obvi0 0 5 10 15 20 25 i0 ous (formation of 1, and formation of Izstarch wmplex) as well as the fact that the Time (sec) first step (the delay of Fig. 8) is much faster than the second. Figure 9 also Figure 6. Process of the reaction forming AgCI. shows that there is more than one step. In this way it is proved that the reaction bewith a permanent marker on the celluloid sheet placed on tween NaS203 and HC1, usually presented in one step in schoolbooks, actually takes place in more steps (2): the screen (Fie. 4). These values nresented in Table 2 correspond with ihe use of the standard curves, to values of HCI+ Na2S203+NaCl + NaHS203 (1) concentrations. So, fmally, we plot the m e of Figure 6. Based on this curve the teacher can report the rate of NaHSz03+ NaHS03 + S (2) change of the AgCl wncentration value and the way of deNaHS03+ HC1+ NaCl + H20 + SO, (3) termination of the reaction velocity at any time. The teacher can ask the students to calculate the velocity Na2Sz03+ 2HC1+ 2NaCI + HzO +SOz+ S (4) of the reaction from the curve tangent of Figure 6 at given The curve in Figure 9 describes the process of the reaction (2) forming sulfur. Table 2. Process of the Reaction-formingAgCl
;::k/,. Pr.-/
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8
lime (3)
Readout (% of scale)
Concentration (M ~t
0-7
Experiments 1. Plotting a Standard Curve forA0C1 . -
In a 100-mL volumetric flask containing approximately 50 mL of distilled water, we add 0.2 mL of HCI 1M solution, 0.2 mL &NO3 1M solution and dilute this solution to 100mL. So a AgC10.002 M suspension is formed. In a 100mL beaker we add 60 mL of distilled water and we place it in position 1of the apparatus (Fig. 1).We adjust the intensity of the light beam, so that in the scale of ammeter screen the maximum readout appears (100% of the scale). We throw out the distilled water and after stirring it, we add 60 mL ofthe AgC10.002 M suspension. We write down the apparatus reading. The same work also is repeated for solutions prepared from the proper dilutions of the AgCl 0.002 M solution. The receivedvalues are presented on Table 1. Volume 71 Number 8 August 1994
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I
0
10
20
30
40
Time (sec)
Figure 7. Process of the reaction consuming Ag2Cr04.
Time (min) Figure 9. Process of the reaction forming sulfur.
and diluting to 30 mL. Also, we prepare a HN03 solution diluting 0.4 mL of HN03 0.1 M solution to 30 mL distilled water. In a 100-mLbeaker we add 60 mL of distilled water and adjust the maximum readout of the ammeter. We throw out the distilled water and in a 100-mL beaker placed in position 1of the apparatus (Fig. 1) we pour in immediately both Ag2Cr04and HN03 solutions starting wunting time simultaneously. Every 2 s we write down the readout as in case 2. 5. Plotting a Standard Curve for lw'ine-Starch Complex
Time (sec)
Figure 8. Process of the reaction forming I2starch complex.
We prepare an iodine-starch wmplex 0.002 M suspension from a reaction between 4 mL of H2022%WIVsolution, 2 mL of KI 3% wlv solution and 1mL of starch indicator and diluting to 100 mL. Working as in case 1we write down the readout of the apparatus. 6. Studying the Reaction-Forming Iodine-Starch Complex
2. Studying the Reaction Forming AgCl
We prepare a HCI solution diluting 0.1 mL HCI 1M to 30 mL distilled water and a AgN03 solution diluting 0.1 mL AgN03 1M to 30 mL of distilled water. In a 100-mLbeaker we add 60 mL of distilled water, and we adjust the maximum readout of the scale of ammeter as in case 1. We throw out the distilled water and in the 100-mL beaker placed in position 1of the apparatus (Fig. 1) we pour in immediately both HC1 and AgN03 solutions starting counting time simultaneously. Every 2 s we write down the positions on a celluloid sheet placed on the screen of the ammeter. Table 2 presents the received values. 3. Plotting a Standard Curve forAgzCr04
We prepare a solution mixing2 mL of HzOz2% w/v solution and 1rnL of starch indicator and diluting to 30 mL and another solution diluting 1mL of KI 30% w/v solution to 30 mL of distilled water. We adjust the maximum readout of the ammeter as in case 2. In the 100-mLbeaker placed in position 1ofthe apparatus (Fig. 1) we pour inimmediately both H2O2and KI solutions while we start counting time simultaneously. Every 2 s we write down the readout of the apparatus. 7. Plotting a Standard Curve for Suspending Sulk~r
We prepare a sulfur 0.02 M suspension from reaction between 8 mL of Na2S2O34% w/v solution and 2.2 mL of HCl 2 M solution and dilution in 100 mL. Working as in case 1 we write down the readout of the apparatus.
We prepare a 5 x lo4 Ag2Cr04suspension from a reaction between 0.2 mL K2Cr045% wlv solution and 0.2 mL &NO3 1M solution and diluting to 100 mL. As in case 1,we write down the ammeter readings for the above AgzCr04suspension and for the suspensions prepared by proper dilution of the first.
diluting 2 mL ofHCl2 M to 31)mLofdistilled water. Workw as in &e 2 we write down the readout of the apparatus.
4. Studying the Consuming Reaction AgrCr04
Literature Cited
We prepare a AgzCr04 suspension mixing 0.1 mL of K2Cr045%wlv solution with 0.1 mL ofAgN03 1M solution
1. Thedorapoulau, P.;PspszLsi, K Ckimiko C h m i k a Cln. Educ 1381,53,12C&122. 2. G m l i n HondbvchderAnorganisehe Chemk, Verlag Chemie:Wenhem.1960,Vol.9 B2, p 876.
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Journal of Chemical Education
8. Studying the Reaction-Forming Sulfur Suspension
We prepare a Na2S203solution diluting 10 mL of Na2&03 4% w vmlutionto30mLofdistilled waterand a HCI solution
