Chemistry Everyday for Everyone
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Tested Demonstrations
Ed Vitz Kutztown University Kutztown, PA 19530
Chemical Domino Demonstration submitted by:
M. Dale Alexander Department of Chemistry and Biochemistry, New Mexico State University, Las Cruces, NM 88003
checked by:
Daniel T. Haworth and Thomas J. Xue Department of Chemistry, Marquette University, Milwaukee, WI 53201
This demonstration consists of a number of different chemical reactions occurring in sequence in a Rube Goldberg– type apparatus. These reactions include the reduction of water by an active metal, the oxidation of a moderately active metal by an acid, reduction of metallic ions by a metal of greater activity, acid–base neutralization reactions in solution monitored with indicators, a gas-phase acid–base neutralization reaction, decomposition of a compound, precipitation of an insoluble salt, substitution reactions of coordination complexes, and pyrotechnic oxidation–reduction reactions including a hypergolic oxidation–reduction reaction, an intramolecular oxidation– reduction reaction, and the combustion of a flammable gas. The design of the apparatus requires only that the demonstrator initiate the first reaction; the others follow in sequence as a result of the “domino effect”. The domino effect is provided by siphon tubes, which allow solutions to be transferred from one vessel to another, and by generating gases to provide the pressure to push solutions into reaction vessels.
20 mL of water is added here to initiate demonstration
125 mL Erlenmeyer flask
magnetic stirrer Calcium metal
(1)
25 mL 0.02 M BaCl2
125 mL flask
from section 1
50 mL Flask Na2CO3 (0.04 mol) with bromocresol green phenolphthalein in 500 mL of water
to section 2
magnetic stirrer
filter paper 50 mL 6 M H2SO4
125 mL flask to section 3
10 mL 0.1 M CuSO4
1.5 % H2O2 500 mL
1 L flask
Figure 1. Section 1 of domino demonstration apparatus.
490
Ca(s) + H2O(l) → H2(g) + Ca(OH)2(s)
copper metal
10 mL 12 M NH3
THF 100 mL
The Demonstration The reactions are carried out in the apparatus as shown in three sections in Figures 1 to 3. The sequence of reactions is initiated by the reaction of calcium metal (under tetrahydrofuran, THF) with water (see Fig. 1), producing hydrogen gas and calcium hydroxide, which forms as a white precipitate (eq 1). The reaction is carried out in THF to dilute the water and decrease the reaction rate to a suitable level. The pressure increase resulting from the production of hydrogen gas in a
60 ml 0.25 M Hg(NO3)2
6M H2SO4 20 mL
The Chemical Domino Demonstration is both educational and entertaining. It provides an excellent means for a review of chemical concepts at the conclusion of a general chemistry course. This particular version of the Chemical Domino Demonstration was presented at the 13th Biennial Conference on Chemical Education at Bucknell University (1). An earlier version was presented at the 10th Biennial Conference on Chemical Education at Purdue University (2).
10 mL 12 M HCl
1 L flask zinc metal (mossy)
25 mL 1 M NaOH 10 mL 0.1 M HCl and phenolphthalein
25 mL 6 M NH3 1 L flask
Figure 2. Section 2 of domino demonstration apparatus.
Journal of Chemical Education • Vol. 75 No. 4 April 1998 • JChemEd.chem.wisc.edu
Chemistry Everyday for Everyone
closed system forces sulfuric acid solution (6 M) out of a test tube into a coarse-fritted funnel, from which the acid is slowly delivered to a gas-generating flask containing sodium carbonate and the acid–base indicators phenolphthalein and bromocresol green. Neutralization of the sodium carbonate occurs in two steps, first to form sodium hydrogen carbonate and subsequently, carbon dioxide and water (eqs 2, 3). H2 SO4(aq) + 2Na2CO3(aq) → 2NaHCO3 (aq) + Na2 SO4(aq) (2) H2SO4(aq) + NaHCO3(aq) → H2O(l) + CO2(g) + NaHSO4(aq)(3)
The phenolphthalein color change (red to colorless) accompanies the first reaction (eq 2) and the bromocresol green color change (blue through green to yellow) accompanies the second reaction (eq 3). The overall color change observed for the neutralization is from purple to blue through green to yellow. Carbon dioxide effervescence is evident as the solution turns yellow in color. Carbon dioxide gas produced in the second step of the neutralization forces mercury(II) nitrate solution (0.25 M) out of a flask into a siphon tube containing copper turnings. Mercury metal plates out on the surface of the copper as copper is oxidized to copper(II) (eq 4). Cu(s) + Hg(NO3) 2(aq) → Cu(NO3)2(aq) + Hg(l) (4) When the solution fills to the top of the siphon arm the copper(II) nitrate solution siphons over into the next siphon tube containing aqueous ammonia (12 M), and the dark blue copper(II) ammine complex is formed (eq 5). (Formation of an insoluble white mercury(II) ammine nitrato complex also occurs in this siphon tube from mercury(II) not removed in the previous siphon tube.) Cu2+(aq) + 4NH3(aq) → [Cu(NH3)4]2+(aq)
(5)
When the mixture in the siphon tube fills the siphon arm it siphons over into the next part of the apparatus, where a portion of the suspension is retained. The remainder is transferred to a coarse-fritted funnel, from which the copper(II) complex is slowly delivered to the next gas-generating flask containing a 1.5% hydrogen peroxide solution. The insoluble mercury(II) complex is collected on the filter. In this gas-generating flask the copper(II) ammine complex catalyzes the decomposition of the hydrogen peroxide into oxygen gas and water (eq 6).
balloon
from section 2
50 mL sodium glycinate 250 mL flask
100 mL 0.1 M Ni(NO3)2
50 mL 1 M ethylenediamine
H2
cellulose nitrate yarn 60 cm glycerin
500 mL flask
KClO3 and sucrose KMnO4
Figure 3. Section 3 of domino demonstration apparatus.
2H 2O2(aq) → O 2(g) + 2H2O(l)
(6)
The pressure increase due to the formation of oxygen gas forces sulfuric acid (6 M) in the next flask into the next siphon tube, which contains a solution of barium chloride (0.02 M). Formation of a white precipitate of barium sulfate occurs (eq 7). Ba2+(aq) + SO42-(aq) → BaSO4(s)
(7)
The mixture subsequently siphons into a funnel equipped with filter paper which allows the barium sulfate to be retained. The filtered acid mixture (unreacted sulfuric acid and hydrochloric acid produced by the reaction) mixes with a solution of copper(II) sulfate and then siphons into the next gas generating flask, which is equipped with the threehole system rather than the traditional thistle-tube system. The three-hole system, with the middle tube open to the atmosphere, allows the acid to be transferred without introducing hydrostatic pressure. As the acid mixture is being transferred, reaction with zinc metal commences (eq 8). Copper(II) ion in the acid mixture facilitates this reaction. Zn(s) + 2HCl(aq) → H2(g) + ZnCl2(aq)
(8)
Even though hydrogen gas is produced by the reaction, there is no pressure increase until the solution in the flask reaches the bottom of the middle tube. Once the liquid in the gas-generating flask reaches the bottom of the middle tube and seals it off, the pressure does increase, which in turn causes the sodium hydroxide solution (0.10 M) in the next flask to be transferred to a test tube also equipped with a three-hole rubber stopper system. The test tube initially contains a solution of hydrochloric acid (0.10 M) and the phenolphthalein indicator, which produces a red color on neutralization of the hydrochloric acid solution (eq 9). HCl(aq) + NaOH(aq) → H 2O(l) + NaCl(aq)
(9)
When the solution reaches the bottom of the center tube the pressure begins to increase, causing an aqueous ammonia (6 M) solution to be forced out of the next flask into a Florence flask containing a small amount of concentrated hydrochloric acid. Ammonium chloride smoke is formed as the ammonia gas emanating from the aqueous ammonia reacts with hydrogen chloride gas present in the flask (eq 10). NH3(g) + HCl(g) → NH 4Cl(s)
(10)
The NH4Cl smoke is discharged through the middle tube in the Florence flask until the solution reaches the bottom of the tube. Once the tube is sealed off, the pressure begins to increase, causing a nickel(II) nitrate solution in the next flask (Fig. 3) to be transferred into a siphon tube containing sodium glycinate solution (1 M). Reaction of green hexaaquanickel(II) ion with glycinate ion (gly᎑) produces a sky-blue tris(glycinato)nickelato(II) complex (eq 11). [Ni(H2O)6]2+(aq) + 3gly᎑(aq) → [Ni(gly)3]᎑ (aq) + 6H2O(l) (11)
This solution subsequently siphons into the next siphon tube, which contains a solution of ethylenediamine (en) (1 M). Since the formation constants of ethylenediamine complexes are larger then the formation constants of the corresponding glycinate nickel(II) complexes, ethylenediamine displaces
JChemEd.chem.wisc.edu • Vol. 75 No. 4 April 1998 • Journal of Chemical Education
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