Carbon dioxide dissolves in water according to the equations:
CO₂(g) + H₂O(l) ⇌ H₂CO₃(aq)
H₂CO₃(aq) + H₂O(l) ⇌ HCO₃^–(aq) + H₃O⁺(aq)
Carbon dioxide levels in the atmosphere have increased about 20% over the last century. Given that Earth's oceans are exposed to atmospheric carbon dioxide, what effect might the increased CO₂ be having on the pH of the world's oceans? What effect might this change be having on the limestone structures (primarily CaCO₃) of coral reefs and marine shells?

Verified step by step guidance

1. Understand the dissolution of CO2 in water: When CO2 gas dissolves in water, it reacts with water to form carbonic acid (H2CO3). This is a reversible reaction represented by the equation CO2(g) + H2O(l) ⇌ H2CO3(aq).
2. Recognize the dissociation of carbonic acid: Carbonic acid (H2CO3) can further dissociate in water to form bicarbonate (HCO3-) and hydronium ions (H3O+), as shown by the equation H2CO3(aq) + H2O(l) ⇌ HCO3-(aq) + H3O+(aq). The formation of H3O+ ions results in a decrease in pH, making the solution more acidic.
3. Relate increased CO2 to ocean acidity: With a 20% increase in atmospheric CO2, more CO2 dissolves in the ocean, leading to higher concentrations of carbonic acid and subsequently more H3O+ ions. This increase in hydronium ions lowers the pH of ocean water, making it more acidic.
4. Examine the effect on limestone structures: The increased acidity of ocean water can affect limestone structures such as coral reefs and marine shells, which are primarily composed of calcium carbonate (CaCO3). Acidic conditions increase the dissolution of CaCO3, potentially leading to the weakening or degradation of these structures.
5. Consider the broader environmental impact: The weakening of coral reefs and marine shells due to increased ocean acidity can have significant ecological consequences, affecting marine biodiversity and the overall health of marine ecosystems.

Key Concepts

Carbonic Acid Formation

When carbon dioxide (CO2) dissolves in water, it reacts to form carbonic acid (H2CO3). This process is crucial because carbonic acid can dissociate into bicarbonate (HCO3-) and hydronium ions (H3O+), which contribute to the acidity of the solution. The increased presence of CO2 in the atmosphere leads to more carbonic acid in oceans, affecting their pH levels.

Ocean Acidification

Ocean acidification refers to the decrease in pH levels of ocean water due to the absorption of excess atmospheric CO2. As CO2 levels rise, more carbonic acid is formed, leading to a lower pH. This change in acidity can have detrimental effects on marine life, particularly organisms that rely on calcium carbonate (CaCO3) for their shells and skeletons, such as corals and shellfish.

Impact on Calcium Carbonate Structures

Calcium carbonate structures, like coral reefs and marine shells, are sensitive to changes in pH. As ocean acidity increases, the availability of carbonate ions (CO3^2-) decreases, making it more difficult for marine organisms to form and maintain their calcium carbonate structures. This can lead to weakened shells and reduced growth rates, threatening marine biodiversity and ecosystems.