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Ocean acidification and pāua shells

Linked equilibria, buffer ratio, saturation state, gas solubility and entropy

Shellfish such as pāua and kina build shells of aragonite (CaCOX3\ce{CaCO3}). Atmospheric COX2\ce{CO2} dissolves in seawater and is in equilibrium with carbonic acid, hydrogencarbonate and carbonate ions. Use these apparent values for seawater at 15 °C: pKaK_a(HCOX3X−\ce{HCO3-}) = 9.00; KsK_s(aragonite) = 6.5×10−76.5\times10^{-7}; [CaX2+]=0.0103[\ce{Ca^2+}] = 0.0103 mol L−1^{-1}; [HCOX3X−]=1.8×10−3[\ce{HCO3-}] = 1.8\times10^{-3} mol L−1^{-1} (assume constant). Surface seawater pH has fallen from about 8.20 to 8.10 and could reach 7.80 by 2100.
Facts you may need
  • Le Chatelier
  • buffer ratio from pH and pKa
  • Q vs Ks
  • entropy and temperature

(a)

Write equations for the steps by which atmospheric COX2\ce{CO2} produces HX3OX+\ce{H3O+}, HCOX3X−\ce{HCO3-} and COX3X2−\ce{CO3^2-} in seawater, and explain why increasing atmospheric COX2\ce{CO2} lowers the pH.

(b)

The last equation in (a) suggests extra COX2\ce{CO2} should produce MORE carbonate, yet measurements show [COX3X2−][\ce{CO3^2-}] falls. Resolve this apparent contradiction with an equation.

(c)

Calculate [COX3X2−][\ce{CO3^2-}] at pH 8.10 and at pH 7.80, assuming [HCOX3X−][\ce{HCO3-}] stays at 1.8×10−31.8\times10^{-3} mol L−1^{-1}.

(d)

Calculate Q for aragonite at both pH values and compare with KsK_s. Discuss what the trend means for shellfish, including what would happen if pH reached 7.60.

(e)

Discuss why seawater's pH has changed only slightly despite the large amounts of COX2\ce{CO2} absorbed, and why this protection is weakening.

(f)

Cold Southern Ocean water absorbs more COX2\ce{CO2} than warm tropical water. Explain in terms of the entropy changes for COX2(g)→COX2(aq)\ce{CO2(g) -> CO2(aq)}, which is exothermic.
Ocean acidification and pāua shells — Scholarship Chemistry | Xametric