Vol. 74 No. 5 May 1997 • Journal of Chemical Education 543

In the Classroom

The Coupling of Related Demonstrations To IllustratePrinciples in Chemical Kinetics and Equilibrium1

Richard A. PacerDepartment of Chemistry, Indiana University–Purdue University, Fort Wayne Campus, Fort Wayne, IN 46805

While sample problems can be worked in lecture to il-lustrate how a rate law may be found from data obtainedexperimentally, student interest is heightened considerablyif the set of data used is generated before the students’ eyesby means of a demonstration. The dependence of the rate ofreaction of magnesium with HCl lends itself well to thispurpose. If the length of a uniform magnesium ribbon andtemperature are kept constant, a rate law

Rate = k[H+]n

can be found by varying HCl molarity and measuring therate of formation of H2 gas. A novel feature of this demon-stration is the use of a baby bottle to collect and measurethe H2 gas. Davenport (1) notes that the baby bottle is thecheapest and most durable volumetric glassware on themarket. The facts that it is graduated in cubic centimeters(as well as in ounces) and that the nipple serves as a pres-sure-sensitive two-way valve make it ideal for a number ofchemistry experiments.

It should be emphasized that what is being describedhere is an approximately 15-minute demonstration. Moreelegant means are available for determining n in the ratelaw for the Mg/HCl reaction. An example is the experimentdescribed by Birk and Walters (2), based on careful pres-sure measurements. But theirs is a student experiment re-quiring some two hours of laboratory time, not a short lec-ture demonstration.

Later in the semester, when students are introducedto ionic equilibria, equal lengths of magnesium ribbon canbe placed in beakers containing equal volumes of equimo-lar HCl, CH3COOH, and H3BO3. The differing rates can nowbe related to different concentrations of H+ provided by thethree acids, due to different degrees of ionization and Kavalues. This very simple demonstration shows up well onan overhead projector. Students can be reminded of the ear-lier rate law demonstration, and the two can be linked to-gether.

Procedure

Demonstration IA 20-cm length of magnesium ribbon is cleaned with

steel wool (if necessary), folded, and placed in the nippleportion of a baby bottle. It must be folded in an irregularmanner (not wound), so that essentially all surface area isavailable for contact with acid. It should be fitted securelyso that it will not drop out when the nipple is inverted. Thebottle itself is filled to the top graduation mark (240 mL inthe bottle I used) with 0.40 M HCl. The nipple is screwedonto the bottle, after which the bottle is inverted and thenipple placed below the surface of the water in a large pailor beaker. Begin timing as soon as the bottle is inverted.After 90 seconds, place your forefinger over the nipple open-ing and remove the bottle from the water bath. After point-ing the nipple end of the bottle away from the audience, theforefinger may be removed.

Measure by inspection the volume of HCl remaining inthe (calibrated) baby bottle. The difference in volumes gives

a measure of the volume of H2 produced, permitting one tocalculate the average rate of reaction.

[CAUTION! Hydrogen pressure will force a stream of HClout of the bottle if the opening is not covered! Be certaintherefore that the bottle is not pointed at anyone as it isbeing removed. Safety goggles are absolutely essential. Al-though only small quantities of H2 are generated, the gasis explosive and calls for a well-ventilated room. If the dis-tance between the instructor’s desk and the first row of stu-dents is small, use of a safety shield is highly desirable.]

The experiment is repeated, using 0.60 M HCl and afresh strip of magnesium ribbon. This time 60 secondsshould be sufficient to give an adequate volume of H2. Fromthe data, the reaction order with respect to HCl concentra-tion may be calculated.

(The 0.40 M and 0.60 M HCl solutions may be preparedby simple volumetric dilution from a common source, suchas 6.0 M or 12 M HCl. Commercial grade HCl is adequate.)If done as a lecture demonstration (with calculationsworked out on chalkboard), about 15 to 20 minutes of classtime will be required.

Demonstration 2

The second demonstration is incredibly simple com-pared to the first. Three small beakers (such as 50-mL size)or Petri dishes are placed on an overhead projector. Intoeach is placed 30 mL of 1 M acid. The acids used are HCl,H3BO3, and CH3CO2H. All solutions should be at room tem-perature. (A 1.0 M solution of boric acid is fairly close tosaturation, but should easily go into solution with mildheating and stirring. The experiment will also work wellwith slightly lower concentrations, such as 0.80 M acids.) Astrip of magnesium (3.2 mm wide, commercial grade,cleaned with steel wool, if necessary) is cut into 1.0-cmlengths. A piece is dropped into each of the three acid solu-tions at essentially the same time, and results are noted onthe overhead.

Discussion

In demonstration 1, one might anticipate an n valuereasonably close to 2 (Birk and Walters [2], for example, re-ported a value of 2.06). Typical n values, however, range be-tween 1.6 and 1.7, most likely because of diffusion rate limi-tations; hence, average rates are somewhat different frominitial instantaneous rates. Nevertheless, useful data aregenerated before the students’ eyes, with which the instruc-tor may use logarithms to evaluate n.

A typical sample calculation is given below.

Rate = k[HCl]n

R2

R1= k(0.60 M)n

k(0.40 M)n = (1.5)n

R2

R1=

45 mL H 2 / min23.3 mL H 2 / min

= 1.93

544 Journal of Chemical Education • Vol. 74 No. 5 May 1997

In the Classroom

(1.5)n = 1.93

n log (1.5) = log (1.93)

n =log (1.93)log (1.5)

=0.286

0.176= 1.63 or 1.6

From the data one may also calculate a rate constant.Using the rate law developed by Birk and Walters (2),

Rate = k (surface area of Mg)a [H+]b

where a = 1 and b = 2, one may use the data given above tocalculate an average rate constant, kAVE, of 1.80 × 10{3 mL H2s{1 mm{2 M{2.

For advanced classes, one may wish to postulate a plau-sible reaction mechanism. The following might be offeredfor discussion:

2H+(aq) + 2e{ → 2H. (adsorbed on Mg surface)

Mg(s) → Mg2+(aq) + 2e{

2H. → H2(g)

In Demonstration 2, students are not told (at least notinitially) what the three acids are, but are asked to drawconclusions based on their observations. In a few minutes,

the magnesium strip in 1 M HCl is completely consumed;the students readily conclude that that beaker must con-tain a strong acid. But the difference between the other twoacids is both striking and puzzling. In 1 M acetic acid, H2 isevolved at a fairly significant rate, propelling the Mg stripabout the beaker. But only an occasional bubble is seenforming on the strip in boric acid. This provides an excel-lent opportunity to involve students in a discussion of themeaning of Ka. Even though H3BO3 and CH3CO2H are bothweak, there is an enormous difference in their relativestrengths. The Ka for acetic acid is 1.75 × 10{5, whereas thatfor boric acid is 5.81 × 10{10. Then, one can tie this demon-stration to the earlier one, which showed the dependence ofthe rate of reaction of magnesium with acid on [H+]n, rein-forcing the principles learned earlier.

Note

1. Presented before the Division of Chemical Education atthe ACS National Meeting in Denver, March 31, 1993 (Paper #333).

Literature Cited

1. Davenport, D. A. J. Chem. Educ. 1969, 46, 878–879.2. Birk, J. P.; Walters, D. L. J. Chem. Educ. 1993, 70, 587–589.