Lesson 2a: a solid salt aerosol at low relative humidity


A system containing ammonium (NH4+) and nitrate (NO3) at low relative humidity is studied. The vapour pressure product of the gases HNO3 and NH3 above solid ammonium nitrate is discussed.

Entering the Data

First, equilibrate the system to a relative humidity of 50%:

1st Calculation
  1. Enter the values and select the options under the following headings:

    Ambient Conditions
    Relative humidity = 0.50 (i.e., 50%).

    Ionic Composition in Moles
    Ammonium = 1.0, Nitrate = 1.0.

    Solid Phases
    There are no entries under this heading.

  2. Click on the "Run" button at the end of the page to do the calculation.
Note:  the above should be entered on the "simple" calculations page of Model III (http://www.aim.env.uea.ac.uk/aim/model3/model3a.php).

Interpreting the Results

We see first that there is no aqueous phase for this relative humidity. In the "Solids" section the "Moles" column reads 1.0, and the "Species" column shows that NH4NO3 (ammonium nitrate) solid is present.

There are no gases present because the input page of the E-AIM model that was used does not permit partitioning into the gas phase to occur. However, the output does show that the equilibrium pressure product of the gases HNO3 and NH3 above the solid is 4.356E-17 atm2. This is the value for the equilibrium constant for the following reaction:

NH4NO3(s) = HNO3(g) + NH3(g)

for which the equilibrium constant Kp is given by:

Kp(NH4NO3(s)) = pHNO3 × pNH3

The activity of solid NH4NO3(s) does not appear in the denominator of the above equation as the value for a pure solid phase is unity by definition.

Notice that the equilibrium constant specifies only the product of the two partial pressures and does not constrain the individual values (except that they must both be greater than zero). The value of Kp is a function only of temperature, and the equilibrium partial pressure product of the two gases will consequently be the same in any system that contains the solid, whatever the value of the relative humidity and whatever other solids or liquid phases are present.

Proceed to Lesson 2b to learn how the behaviour of the system changes at a higher relative humidity, or return to the main page for this lesson.