B Appendix B: Equilibrium Constants at 25°C
The equilibria and their constant values are borrowed from (Chameides and Perdue 1997) and (Stumm and Morgan 1996).
B.1 Equilibrium reactions in the H2O-H2-O2 system
| Equilibrium reactions | Log K |
|---|---|
| H+ + OH- ⇆ H2O | -14.00 |
| H+ + e- ⇆ H2(g) | 0.00 |
| H+ + e- + ¼O2(g) ⇆ ½H2O | 20.78 |
H+: proton; OH-: hydroxide; H2(g): dihydrogen gas; O2(g): dioxygen gas; H2O: water
B.2 Equilibrium reactions in the CO2-H2O system
| Equilibrium reactions | Log K |
|---|---|
| CO2(g) ⇆ CO2(aq) | -1.47 |
| CO2(g) + H2O ⇆ H2CO3° | -1.46 |
| H2CO3° ⇆ H+ + HCO3- | -6.36 |
| HCO3- ⇆ H+ + CO32- | -10.33 |
| CO2(g) + H2O ⇆ H+ + HCO3- | -7.82 |
| CO2(g) + H2O ⇆ 2H+ + CO32- | -18.15 |
| CaCO3(calcite) + 2H+ ⇆ Ca2+ + CO2(g) + H2O | 9.74 |
CO2: carbon dioxide; H2CO3°: this compound does not exist as such in nature… It corresponds to the combined CO2(aq) and the true carbonic acid H2CO3, such that [H2CO3°] = [CO2(aq)] + [H2CO3]. This expression H2CO3° in the equilibrium is preferentially used in chemistry because the amount of true carbonic acid in solution is about 1000 smaller than CO2(aq) (see further details in Chapter 11); HCO3-: bicarbonate (the prefix “bi-” is rather misleading and does not correspond to the number of negative charges, which would appear more logical nowadays, and corresponds to an outdated naming system); CO32-: carbonate; CaCO3: calcite mineral; Ca2+: calcium cation.
B.3 Equilibrium reactions for nitrogen species
| Equilibrium reactions | Log K |
|---|---|
| NO3- + 2H+ + e- ⇆ NO2(g) + H2O | 13.03 |
| ½NO3- + H+ + e- ⇆ ½NO2- + ½H2O | 14.32 |
| ⅓NO3- + 4/3H+ + e- ⇆ ⅓NO(g) + ⅔H2O | 16.14 |
| ¼NO3- + 5/4H+ + e- ⇆ N2O(aq) + ⅛H2O | 18.88 |
| ⅕NO3- + 6/5H+ + e- ⇆ ⅒N2(g) + 3/5H2O | 21.05 |
| ⅛NO3- + 5/4H+ + e- ⇆ ⅛NH4+ + ⅜H2O | 14.90 |
| N2(g) ⇆ N2(aq) | -3.21 |
| NO(g) ⇆ NO(aq) | -2.73 |
| N2O(g) ⇆ N2O(aq) | 0.54 |
| NH3(g) ⇆ NH3(aq) | 1.76 |
| NH3(aq) + H+ ⇆ NH4+ | 9.28 |
N2: dinitrogen; NO3-: nitrate; NO2(g): nitrogen dioxide; NO2-: nitrite; NO: nitric oxide; N2O: nitrous oxide; NH3: ammonia; NH4+: ammonium
B.4 Equilibrium reactions for phosphorus species
| Equilibrium reactions | Log K |
|---|---|
| H3PO4 ⇆ H2PO4- + H+ | -2.12 |
| H2PO4- ⇆ HPO42- + H+ | -7.21 |
| HPO42- ⇆ PO43- + H+ | -12.67 |
| H3PO3 ⇆ H2PO3- + H+ | -2.00 |
| H2PO3- ⇆ HPO32- + H+ | -6.59 |
H3PO4: phosphoric acid; H2PO4-: dihydrogen phosphate; HPO42-: hydrogen phosphate ; PO43-: phosphate or orthophosphate; H3PO3: phosphonic acid (also phosphorous acid); H2PO3-: dihydrogen phosphite; HPO32-: phosphonate (or phosphite)
B.5 Equilibrium reactions for sulfur species
| Equilibrium reactions | Log K |
|---|---|
| H2SO4(aq) ⇆ H+ + HSO4- | 1.98 |
| HSO4- ⇆ H+ + SO42- | -1.98 |
| H2SO3(aq) ⇆ H+ + HSO3- | 1.91 |
| HSO3- ⇆ H+ + SO32- | -7.18 |
| H2S(g) ⇆ H2S(aq) | -0.99 |
| H2S(aq) ⇆ H+ + HS- | -7.02 |
| HS-~ ⇆ H+ + S2- | -12.90 |
| ⅛SO42- + 5/4H+ + e- ⇆ ⅛H2S(g) + ½H2O | 5.25 |
| ⅛SO42- + 9/8H+ + e- ⇆ ⅛HS- + ½H2O | 4.25 |
| ⅙SO42- + 4/3H+ + e- ⇆ ⅙S(s) + ⅔H2O | 6.03 |
| ½S(s) + H+ + e- ⇆ ½H2S(g) | 2.89 |
| α-FeS(trolite) ⇆ Fe2+ + S2- | -16.21 |
| FeS2(pyrite) ⇆ Fe2+ + S22- | -26.93 |
appendix still under construction