Equilibria and Environment

Learning Goals

You will become a bit more appreciative of the complex environment in which we live.

Synopsis

Although it is very important to remember that we live in a non-equilibrium system (Section 19.12), we can apply some of our understanding of equilibrium reactions to our entire ecosystem. Our biosphere contains a large number of carbon-containing compounds in various states of existence. The molecule holding the entire carbon-based system together is CO₂. Carbon dioxide is naturally formed in the biological processes of respiration and naturally consumed by photosynthetic organisms. The oceans hold about 60 times as much carbon dioxide as the atmosphere, so the ocean is a giant "sink" for carbon dioxide.

Atmospheric water also reacts with carbon dioxide:

CO₂(g) + H₂O(l) H₂CO₃(aq) with Ka = 4.2 x 10^-7

The dissolved carbon dioxide makes the rain acidic (pH ~ 5.6). The oceans not only contain dissolved and reacted carbon dioxide, they also contain vast amounts of CaCO₃ from which comes the carbonate ion:

CaCO₃(s) Ca²⁺(aq) + CO₃^2-(aq) with Ksp = 3.8 x 10^-9 This then leads to

CO₃2-(aq) + H₂O(l) HCO₃^-(aq) + OH^-(aq) with Kb = 2.1 x 10^-4

The production of the hydroxyl ion plus the carbonic acid content leads to a sea water pH of ~ 8.2. The reactions above also form a buffer solution that buffers added acids and bases.

The added acids from underground volcanic activity and other sources are countered by:

HCO₃^- (aq) + H₃O⁺(aq) H₂CO₃(aq) + H₂O(l)

and

2 H₃O⁺(aq) + CO^3-(aq) 3H₂O(l) + CO₂(g)

The slow ocean mixing also removes acid by reactions with calcium carbonate:

H₃O⁺(aq) + CaCO₃-(s) HCO₃^-(aq) + Ca²⁺(aq) + H₂O(l)

The added bases are countered by:

OH^-(aq) + HCO₃^-(aq) H₂O(l) + CO₃^2-(aq)

From these discussions, you can gain a little appreciation for the complexity of our biosphere. Again, it is important to remember that we actually live in a nonequilibrium environment.

Discussion Questions for Bulletin Board Activities

Do a web search for the "greenhouse effect" and pick one of the discussions to share on the bulletin board of WebCT.