How acidic is the rain if atmospheric CO2 concentration doubles?
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Students use basic aqueous geochemistry knowledge to investigate controls of the atmospheric carbon dioxide level on pH values of the wet precipitation at standard conditions (25 oC, 1 atmospheric pressure).
Upper level biogeochemistry course.
Skills and concepts that students must have mastered
Basic knowledge of chemical equilibria among dissolved inorganic carbon species in aqueous systems and charge balance equations.
How the activity is situated in the course
Give students some understanding how natural precipitation pH varies as a function of atmospheric CO2, and I put this into context of discussion on sources of acidity of acid rains in northeast US. Could be used as a stand-alone exercise.
Content/concepts goals for this activity
Chemical composition of rain, and general controls on natural water chemistry
Higher order thinking skills goals for this activity
This exercise involves looking for equilibrium constants for important inorganic carbon reactions.
Other skills goals for this activity
Working in groups
Description and Teaching Materials
Using 1 bar total atmospheric pressure, 25oC, and pCO2=10^-3.5 bar as an initial condition, students will write down chemical reactions that dissolve CO2 and H2CO3, which further dissociates into protons and bicarbonate. Then students will look for data for equilibrium constants for these reactions and calculate the pH of the final solution.
Then do the same calculation again when atmospheric CO2 concentration doubles and compare pH values in two scenarios.
Teaching Notes and Tips
This back of the envelope calculation helps us to discuss also the impacts of rising CO2 levels on climate change and ocean acidification.
I will bring a volunteer group to the white board and ask them to explain how they have done the calculation. Then students can compare notes and know where they might have done wrong.
References and Resources
I use two books for reference and resources:
The geochemistry of natural waters by James I. Drever
Aqueous Environmental Geochemistry by Donald Langmuir.