Cinética Del Carbonato De Calcio En Cáscara De Huevo

In the Laboratory
edited by

Second-Year and AP Chemistry

John Fischer
Ashwaubenon High School Green Bay, WI 54303-5093

Using a Datalogger To Determine First-Order Kinetics and Calcium Carbonate in Eggshells

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Martin M. F. Choi* and Pui Shan Wong Department of Chemistry, Hong Kong Baptist University, Kowloon Tong, Hong Kong, SAR, China; *mfchoi@hkbu.edu.hk

The Earth’s crustconsists of about 3.4% (by mass) calcium. Calcium exists primarily as calcium carbonate, calcite, chalk, and marble on Earth or in the ocean (1–4). Acid rain has become a serious environmental problem in most industrial countries as it can destroy buildings, monuments, and statues with high calcium carbonate content. Acid rain is caused by emissions of sulfur dioxide from coal- and oil-burning powerplants and of nitrogen oxides from automobiles. In the atmosphere, these emissions are chemically converted into sulfuric acid and nitric acid that accumulate in cloud droplets and fall as precipitation, for example rain or snow. Acid rain has a corrosive effect on limestone and marble as calcium carbonate reacts readily with acids (5):
+ CaCO3(s) + 2H (aq)

Ca2+(aq) + CO2(g) + H2O(l)

(1)The study of acids on calcium carbonate is frequently taught in basic chemistry courses. Equation 1 can be applied to determine inorganic carbon in rocks and soils when carbonate is converted into carbon dioxide. Unfortunately, these experiments often require expensive equipment, such as a Fourier transform infrared

spectrophotometer (6, 7), which is not commonly available in secondaryschools. Reaction kinetics are often taught in secondary school or introductory college levels (8). Experiments demonstrating first-order kinetics have been discussed in the literature (9–12). Equation 1 can be used to teach firstorder reaction kinetics if acid is present in excess and the rate of liberation of carbon dioxide is monitored. The application of a datalogger (a computer interfaced to one ormore sensors) in biology, chemistry, and physics laboratories in Hong Kong secondary schools is becoming popular. The major advantage of a datalogger is that students can simultaneously monitor, in real time, various parameters of an environment, such as temperature, electric current, electric potential, pressure, photons, pH, and oxygen. Roser and McCluskey have successfully demonstrated the useof a pressure sensor to study the stoichiometry of the reaction of hydrogen carbonate or carbonate with hydrochloric acid (13). This article describes the application of a Pasco CI-6532 pressure sensor used in conjunction with a datalogger (Figure 1) to monitor the liberation of carbon dioxide in eq 1. The proposed method is simple, safe, and convenient to use. It not only serves for thedetermination of the chemical kinetics of eq 1 but also can be extended to obtain quantitatively the concentration of calcium carbonate in eggshells. Principles Equation 1 is first order with respect to calcium carbonate and acid, respectively. If acid is present in large excess, only a small quantity of acid will be used up in the reaction. The concentration of acid is practically constant, and thereaction rate will depend on the concentration of calcium carbonate alone. The reaction, thus, is pseudo-first-order with respect to the concentration of calcium carbonate,

rate =

d [ CaCO3 ]t dt

= −k [ CaCO3 ]t

(2)

where [CaCO3]t is the concentration of calcium carbonate at time t, and k is the rate constant. The integrated from of this equation is,

kt = ln [CaCO3 ]0 − ln [CaCO3 ]t(3)

Figure 1. The experimental setup for the determination of calcium carbonate in eggshells.

where [CaCO3]0 is the initial concentration of calcium carbonate, which is proportional to the final pressure (P f ) of CO2 produced. The concentration of [CaCO3]t at time t is equal to [CaCO3]0 minus the concentration of CaCO3 reacted. Since the quantity of carbon dioxide liberated is re•...