Greenhouse Effect

Carbon dioxide in our oceans.
As carbon dioxide is pumped into the atmosphere, primarily from the burning of fossil fuels, its concentration increases at a rate of 1 ppm every year. As well as being part of our atmosphere, carbon dioxide also dissolves in the oceans. Large quantities of carbon dioxide are dissolved in oceans which act as carbon stores. At room temperature, 100 ml of water can dissolve 90 cm3 of carbon dioxide.

 

Once dissolved in water, only 1% of the CO2 reacts to form carbonic acid according to the equation below.

CO2(aq) + H2O(l) <=> H2CO3(aq)

An equilibrium is soon established between the dissolved carbon dioxide and carbonic acid. The set of related reactions that now take place are shown on the right.

Carbonic acid is a very weak acid. A small percentage of molecules will ionize in a two step process, as shown on the right, to form carbonate (CO3-2) ions. An equilibrium is established between dissolved carbon dioxide and the carbonate ion.

The carbonate anion reacts with metal cations in seawater, such as Ca2+, to form solid calcium carbonate (limestone). Most carbonates are insoluble.

As more calcium carbonate is precipitated extensive deposits of limestone are formed. This traps the carbon in huge rock formations deep underground.

H2O(l)+ H2CO3(aq)<=> HCO3-(aq)+ H3O+(aq)

 

H2O(l)+ HCO3-(aq)<=> CO3-2(aq)+ H3O+(aq)

Although the calcium carbonate is insoluble it will readily dissolve in acidic solutions. The carbonate (CO3-2) ion behaves as a Lowry Brønsted base according to the equation on the right.

 

 
CaCO3(s) + 2 H+(aq)<=> Ca2+(aq) + H2CO3(aq)

The carbonic acid dissociates to produce carbon dioxide gas and water according to the equation on the right.

 
H2CO3(aq) => H2O(l)+ CO2(aq)
The reaction of carbonates and acids can be readily demonstrated be reacting vinegar and baking powder, as shown on the right. Click to see a 300 kb video.
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