Goal of the Acid Rain Project

 

The Acid Rain Project is a broad-based science project which can be done by the students in General Chemistry laboratories. The project seeks new knowledge about a topic of current interest, acid rain. It provides a large number of points of connection with the lecture material and integrates many concepts. The Acid Rain Project was begun with NECUSE support in Spring 1991.

Acid Rain Project Manual now available from McGraw-Hill. Manual contains all experiments and a discussion of acid rain and air pollution chemistry. ISBN 0-07-014358-7

STUDENT EXPERIMENTS

1. Acidity and pH - A Discovery Experiment

2. Collection and Analysis of Rain

3. Determination of Sulfate in Rain by Turbidimetry

4. Determination of Sulfate and Nitrate in Rain by Ion Chromatography

General Chemistry Project Activities

1. Two week introduction to the project - students discover various properties of strong and weak acids and learn to make accurate measurements of pH and total acidity. The procedures for obtaining rain samples is explained and a schedule of responsibility for sampling is made. Each student learns to calibrate a pH meter and use the digital titrators to determine total acidity and the pH of strong and weak acids. Hach digital titrators are used for ease, portability, and speed.

2. Students are responsible for monitoring the campus weather station and collecting samples after each rainfall. They take the sample to the laboratory, measure the pH, and titrate the total acidity.

3. Late in the Spring term students determine the sulfate and nitrate concentration in each rain sample.

a. Sulfate is determined by a turbidimetric procedure using BaSO4.

b. Nitrate is determined spectrophotometrically

c. Both nitrate and sulfate are determined by anion exchange high performance liquid chromatography.

4. All data and calibrations are pooled and distributed to all students. Data from other institutions can also be distributed.

5. Each student prepares a written report suitable for submission to the local newspaper detailing the findings of the study. The best reports submitted to local paper for publication.

 

Connections with the Lecture

1. Unit conversions and dimensional analysis. The students calculate the area of the campus and the total volume of rain which falls on campus per inch of rainfall.

2. Stoichiometry. Students calculate the amount of limestone required to neutralize the acid in a given rainfall. They need to assume some mixture of magnesium and calcium carbonates in the limestone.

3. Gas laws, partial pressure, and solubility of gases. Students can discuss the natural acidity of rain due to carbon dioxide in the atmosphere. This can also lead to discussions of the effect of the accelerating greenhouse effect on rain pH.

4. Oxidation and reduction chemistry. Where do the sulfuric and nitric acids come from? What are the reactions involved?

5. Strong acids and weak acids. Why do sulfate and nitrate make the water more acidic than carbonic acid? The initial pH and titration experiment produce a severe "cognitive dissonance" in the students. The resolution of this provides a deep understanding of the properties of strong and weak acids.

6. Acid-Base equilibria. Students can calculate the equilibrium pH of rain due to carbon dioxide in the air. They can do this for the present level of about 350 ppm and for a pre industrial value of 280 ppm. Why does the carbonate contribution to the total acidity vary with seasons? They can also determine the pH of rain if the carbon dioxide doubles in the next 30-60 years. There are a number of more advanced questions which can be developed in this area using spreadsheets to handle the arithmetic. (Eg: Compare the predicted and measured pH for different samples after analysis. Model possible future pollution patterns and predict the effects on the pH of rain.)

7. Social issues related to science and technology. This project will virtually insure that some attention is paid to the causes of acid rain and the possible solutions.

Conclusions

The Acid Rain Project has proved to be quite successful both in raising student's interest and as a vehicle for introducing acid-base chemistry. The students master a number of important laboratory techniques while working on a problem of real scientific interest.