L3 - Carbonate systems and equilibrium Flashcards
What makes rainwater naturally acidic?
The dissolution of atmospheric CO2 forms carbonic acid, making rainwater naturally acidic
What role does carbonic acid play in natural waters?
Carbonic acid regulates the pH and alkalinity of natural waters, determining the reactivity of many chemical compounds and solids.
What is the major long-term sink for dissolved carbon?
Calcium carbonate (CaCO3) is the major sink for dissolved carbon in the global carbon balance
How does the carbonate system in oceans affect Earth’s temperature?
It controls the partial pressure of CO2 in the atmosphere, helping to regulate the planet’s temperature.
What are the six chemical species in the CO2–H2O system?
- CO2 (g) – Carbon dioxide gas
- H2CO3* – Carbonic acid (H2CO3* = CO2(aq) + H2CO3)
- HCO3− – Bicarbonate
- CO3 2− – Carbonate
- H+ – Proton
- OH− – Hydroxide
What are the four key reactions in the CO2–H2O system and their equilibrium constants?
- CO2(g) + H2O ↔ H2CO3*; Henry’s law constant KH= [H2CO3∗] / PCO2
- *H2CO3 ↔ H+ + HCO3−**; K1 =
[HCO3−][H+]/ [H2CO3∗] - HCO3− ↔ H+ + CO3 2−; 𝐾2 = [CO3 2−][H+] / [HCO3−]
- H2O ↔ H+ + OH−;
𝐾w = [H+][OH−]
What is the mass balance equation for total dissolved inorganic carbon (CT)?
CT=[H2CO3∗]+[HCO3−]+[CO3
2−]
What is the charge balance equation in the CO2–H2O system?
[H+]=[HCO3−]+2[CO3
2−]+[OH−]
What happens when CO2 dissolves in water?
CO2 exists in equilibrium with carbonic acid:
CO2(aq)+H2O↔H2CO3
What is the equilibrium constant for CO2(aq) ↔ H2CO3 at 25°C?
K = [H2CO3] / [CO2]= 1.70 x 10^-3
What ratio of CO2 to H2CO3 exists in dissolved water?
The ratio is approximately 600:1, with most dissolved CO2 remaining as CO2(aq)
What is the first dissociation reaction of carbonic acid and its constant?
H2CO3↔HCO3−+H+, K1 =4.60×10 ^−7 at 25°C
What is the second dissociation reaction of bicarbonate and its constant?
HCO3−↔CO32−+H+,
𝐾2=4.68×10 ^−11 at 25°C
How does the carbonate system’s equilibrium change with pH?
- pH < 6.34 (pK1): H2CO3 predominates.
- 6.34 < pH < 10.33 (pK2): HCO3− predominates.
- pH > 10.33: CO3 2− predominates.
What are the equivalence points in the carbonate system?
- At pH = 6.34,
[HCO3−]=[H2CO3]. - At pH = 10.33, [CO3 2−]=[HCO3−].
What are the two polymorphs of calcium carbonate?
- Calcite – Rhombohedral, density = 2.7 g/cm³.
- Aragonite – Orthorhombic, density = 2.9 g/cm³
What are the solubility products (Ksp) for calcite and aragonite?
- Calcite: Ksp = 3.31×10 ^−9
.
Aragonite: Ksp = 3.71×10 ^−9
.
How do pH and pCO2 affect calcite solubility?
Solubility increases with CO2 (lowering pH).
Solubility decreases as temperature increases
How does pressure affect calcite solubility?
Solubility increases with pressure, especially at ocean depths.
Why is inorganic calcium carbonate precipitation rare in oceans?
Despite being oversaturated with CaCO3, the IAP/Ksp ratio is typically only 3–5 (well below the required 20–25).
What drives biological precipitation of calcium carbonate in oceans?
Biomineralization by organisms like foraminifera (calcite) and pteropods (aragonite).
How does increasing CO2 affect ocean calcification?
Ocean acidification reduces calcification rates, leading to declines of 3–60% for a doubling of atmospheric CO2.
What is the significance of the carbonate system in ocean chemistry?
It buffers pH and atmospheric CO2, playing a crucial role in global carbon cycling
What is a Bjerrum plot used for?
to show the relative concentrations of H2CO3, HCO3−, CO3 2−, H+, and OH− in the CO2–H2O system under constant total concentration conditions.
How does calcium carbonate act as a buffer in the ocean?
It reacts with H+ ions to neutralize ocean acidification caused by increased atmospheric CO2
