|Lesson 6a: solid NaNO3, and gaseous NH3 and HNO3 at 60% relative humidity|
|Note: the above should be entered on the
"comprehensive" calculations page of Model III
The equilibrium between solid NH4NO3(s) and the two gases is described by:
NH4NO3(s) = NH3(g) + HNO3(g)
Kp = pNH3 × pHNO3 = 4.356E-17 atm2
The output in the other browser window shows that the actual partial pressures – calculated from the number of moles of each gas present in the system volume of 1 m3 – are 2.4E-9 atm (NH3) and 4.8E-9 atm (HNO3), yielding a product of 1.15E-17 atm2. Because this is less than the value of Kp above, no solid NH4NO3(s) can form. In contrast, the constituent ions of NaNO3(s) are not volatile and the ambient relative humidity is just below the deliquescence relative humidity of NaNO3(s). Consequently, this salt exists as a solid at equilibrium.
If no partitioning of NH3 and HNO3 was allowed into the gas phase, then a aqueous phase containing both salts would exist. (You can try this by repeating the calculation above but with trace gas partitioning switched off.) In this case the equilibrium partial pressure of HNO3 above the liquid (4.067E-5 atm) corresponds to 1.663E-4 moles of the acid in 1 m3 of gas phase – orders of magnitude greater than is present in the system. Consequently the HNO3 would be expected to evaporate along with the NH3 whose equilibrium partial pressure above the liquid would increase as it became less acidic. The end result would therefore be expected to be a particle containing only NaNO3, which is what we find.
Proceed to Lesson 6b to learn how the system behaves at a high enough relative humidity for deliquescence to take place, or return to the main page for this lesson.