Table 1.
Free energy per mole for NH3-generating reactions under Venus cloud conditions
| Reaction | Free energy of reaction (kJ/mol) | Free energy required per mole of surplus NH3 (kJ/mol) | Water consumed per surplus NH3 | |
| 1 | 4N2(aq) + 11H2O(l) → 2NH4+OH−(aq) + 3NH4+NO3−(aq) | 1,730 to 2,024 | 865 to 1,012 | 6.5 |
| 2 | N2(aq) + 8H2O(l) → 2NH4+OH−(aq) + 3H2O2(aq) | 1,203 to 1,471 | 602 to 736 | 4 |
| 3 | 2N2(aq) + 10H2O(l) → 4NH4+OH−(aq) + 3O2(aq) | 1,000 to 1,306 | 262 to 343 | 2.5 |
| 4 | 4N2(aq) + 17H2O(l) + 3HCl(aq) → 5NH4+OH−(aq) + 3NH4+ClO4−(aq) | 1,364 to 1,634 | 273 to 323 | 3.4 |
| 5 | N2(aq) + 6H2O(l) + 3SO2(aq) → (NH4+)2SO42-(aq) + 2H2SO4(aq) | 1,193 to 1,313 | N/A | N/A |
Free energies of NH3-producing reactions are calculated from refs. 83–85. Ranges are minimum to maximum over a range of pH = −3 to pH = +4 and temperature from 2 °C to 115 °C. Concentrations of SO2 and H2O are as described in ref. 34. O2 fractional abundance is assumed to be 10−6. Table columns are as follows. First column: reaction number. Second column: possible chemical reaction that produces NH3. Third column: free energy of reaction assuming that NH3 is accumulated to 2 molar concentration. For the fourth and fifth columns, values were calculated in terms of “surplus NH3,” which is the amount of NH3 synthesized as NH4OH. Fourth column: free energy per mole of “surplus NH3” produced. Fifth column: number of water molecules consumed per “surplus” NH3. Reaction 3 (bold type), which produces molecular oxygen as an oxidized byproduct, is the most efficient, in both its use of energy and its use to water. We note that reaction 4 could produce hypochlorite, chlorite, or chlorate as an oxidized product, but, as perchlorate is relatively stable and is the weakest oxidizing agent, we have shown this reaction for illustration only. Reaction 5 generates more acid than it consumes, and so cannot be a source of the base which neutralizes H2SO3. We also note that reaction 1 and reaction 4 (reactions making nitrate and perchlorate, respectively) clouds also alternatively explain the presence of O2. Nitrate and perchlorate would “rain out” and decompose to N2 and O2 or HCl, Cl2, and O2, respectively, below the clouds. In situ measurements of NOx and ClO4 abundance in the clouds could rule out these reactions as a potential source of indirect formation of O2.