A. Carbon dioxide (CO2) is an abundant greenhouse gas that is present in high at

Question

A. Carbon dioxide (CO2) is an abundant greenhouse gas that is present in high atmospheric concentrations as a result of increasing air pollution. The two processes for the natural removal of CO2 from the atmosphere are photosynthesis and dissolution into the oceans. Complete the following chemical equation for the dissolution of CO2 in water. Remember to include the physical states of the products.

CO2 (aq) + H2O <--> _____________________

B. What happens to the pH of the ocean as more CO2 dissolves?

Explanation / Answer

At room temperature, the solubility of carbon dioxide is about 90 cm3 of CO2 per 100 mL
of water. In aqueous solution, carbon dioxide exists in many forms. First, it simply dissolves.
CO2(g) ---> CO2(aq)
Then, an equilibrium is established between the dissolved CO2 and H2CO3, carbonic acid.
CO2(aq) + H2O(l) <====> H2CO3(aq)
Only about 1% of the dissolved CO2 exists as H2CO3. Carbonic acid is a weak acid which dissociates in
two steps.
H2CO3    <====> H+ + HCO3 1- Ka1 = 4.2 × 10-7
HCO3 -1 <====> H+ + CO32-   Ka2 = 4.8 × 10-11

When water (H2O) and CO2 mix, they combine to form carbonic acid (H2CO3). Carbonic acid is weak compared to some of the well-known acids that break down solids, such as hydrochloric acid (the main ingredient in gastric acid, which digests food in your stomach) and sulfuric acid (the main ingredient in car batteries, which can burn your skin with just a drop). The weaker carbonic acid may not act as quickly, but it works the same way as all acids: it releases hydrogen ions (H+), which bond with other molecules in the area.

Seawater that has more hydrogen ions is more acidic by definition, and it also has a lower pH. In fact, the definitions of acidification terms—acidity, H+, pH —are interlinked: acidity describes how many H+ ions are in a solution; an acid is a substance that releases H+ ions; and pH is the scale used to measure the concentration of H+ ions.

The lower the pH, the more acidic the solution. The pH scale goes from extremely basic at 14 (lye has a pH of 13) to extremely acidic at 1 (lemon juice has a pH of 2), with a pH of 7 being neutral (neither acidic or basic). The ocean itself is not actually acidic in the sense of having a pH less than 7, and it won’t become acidic even with all the CO2 that is dissolving into the ocean. But the changes in the direction of increasing acidity are still dramatic.

So far, ocean pH has dropped from 8.2 to 8.1 since the industrial revolution, and is expected by fall another 0.3 to 0.4 pH units by the end of the century. A drop in pH of 0.1 might not seem like a lot, but the pH scale, like the Richter scale for measuring earthquakes, is logarithmic. For example, pH 4 is ten times more acidic than pH 5 and 100 times (10 times 10) more acidic than pH 6. If we continue to add carbon dioxide at current rates, seawater pH may drop another 120 percent by the end of this century, to 7.8 or 7.7, creating an ocean more acidic than any seen for the past 20 million years or more.

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