L3 - Carbonate systems and equilibrium

Question 1

What makes rainwater naturally acidic?

Answer 1

The dissolution of atmospheric CO2 forms carbonic acid, making rainwater naturally acidic

Question 2

What role does carbonic acid play in natural waters?

Answer 2

Carbonic acid regulates the pH and alkalinity of natural waters, determining the reactivity of many chemical compounds and solids.

Question 3

What is the major long-term sink for dissolved carbon?

Answer 3

Calcium carbonate (CaCO3) is the major sink for dissolved carbon in the global carbon balance

Question 4

How does the carbonate system in oceans affect Earth’s temperature?

Answer 4

It controls the partial pressure of CO2 in the atmosphere, helping to regulate the planet’s temperature.

Question 5

What are the six chemical species in the CO2–H2O system?

Answer 5

1. CO2 (g) – Carbon dioxide gas
2. H2CO3* – Carbonic acid (H2CO3* = CO2(aq) + H2CO3)
3. HCO3− – Bicarbonate
4. CO3 2− – Carbonate
5. H+ – Proton
6. OH− – Hydroxide

Question 6

What are the four key reactions in the CO2–H2O system and their equilibrium constants?

Answer 6

1. CO2(g) + H2O ↔ H2CO3*; Henry’s law constant KH= [H2CO3∗] / PCO2
2. *H2CO3 ↔ H+ + HCO3−**; K1 =
   [HCO3−][H+]/ [H2CO3∗]
3. HCO3− ↔ H+ + CO3 2−; 𝐾2 = [CO3 2−][H+] / [HCO3−]
4. H2O ↔ H+ + OH−;
   𝐾w = [H+][OH−]

Question 7

What is the mass balance equation for total dissolved inorganic carbon (CT)?

Answer 7

CT=[H2CO3∗]+[HCO3−]+[CO3
2−]

Question 8

What is the charge balance equation in the CO2–H2O system?

Answer 8

[H+]=[HCO3−]+2[CO3
2−]+[OH−]

Question 9

What happens when CO2 dissolves in water?

Answer 9

CO2 exists in equilibrium with carbonic acid:
CO2(aq)+H2O↔H2CO3

Question 10

What is the equilibrium constant for CO2(aq) ↔ H2CO3 at 25°C?

Answer 10

K = [H2CO3] / [CO2]= 1.70 x 10^-3

Question 11

What ratio of CO2 to H2CO3 exists in dissolved water?

Answer 11

The ratio is approximately 600:1, with most dissolved CO2 remaining as CO2(aq)

Question 12

What is the first dissociation reaction of carbonic acid and its constant?

Answer 12

H2CO3↔HCO3−+H+, K1 =4.60×10 ^−7 at 25°C

Question 13

What is the second dissociation reaction of bicarbonate and its constant?

Answer 13

HCO3−↔CO32−+H+,
𝐾2=4.68×10 ^−11 at 25°C

Question 14

How does the carbonate system’s equilibrium change with pH?

Answer 14

• pH < 6.34 (pK1): H2CO3 predominates.
• 6.34 < pH < 10.33 (pK2): HCO3− predominates.
• pH > 10.33: CO3 2− predominates.

Question 15

What are the equivalence points in the carbonate system?

Answer 15

• At pH = 6.34,
  [HCO3−]=[H2CO3].
• At pH = 10.33, [CO3 2−]=[HCO3−].

Question 16

What are the two polymorphs of calcium carbonate?

Answer 16

1. Calcite – Rhombohedral, density = 2.7 g/cm³.
2. Aragonite – Orthorhombic, density = 2.9 g/cm³

Question 17

What are the solubility products (Ksp) for calcite and aragonite?

Answer 17

• Calcite: Ksp = 3.31×10 ^−9
  .
  Aragonite: Ksp = 3.71×10 ^−9
  .

Question 18

How do pH and pCO2 affect calcite solubility?

Answer 18

Solubility increases with CO2 (lowering pH).

Solubility decreases as temperature increases

Question 19

How does pressure affect calcite solubility?

Answer 19

Solubility increases with pressure, especially at ocean depths.

Question 20

Why is inorganic calcium carbonate precipitation rare in oceans?

Answer 20

Despite being oversaturated with CaCO3, the IAP/Ksp ratio is typically only 3–5 (well below the required 20–25).

Question 21

What drives biological precipitation of calcium carbonate in oceans?

Answer 21

Biomineralization by organisms like foraminifera (calcite) and pteropods (aragonite).

Question 22

How does increasing CO2 affect ocean calcification?

Answer 22

Ocean acidification reduces calcification rates, leading to declines of 3–60% for a doubling of atmospheric CO2.

Question 23

What is the significance of the carbonate system in ocean chemistry?

Answer 23

It buffers pH and atmospheric CO2, playing a crucial role in global carbon cycling

Question 24

What is a Bjerrum plot used for?

Answer 24

to show the relative concentrations of H2CO3, HCO3−, CO3 2−, H+, and OH− in the CO2–H2O system under constant total concentration conditions.

Question 25

How does calcium carbonate act as a buffer in the ocean?

Answer 25

It reacts with H+ ions to neutralize ocean acidification caused by increased atmospheric CO2