chemistry, first year university
posted by julie .
Solid NH4N3 decomposes explosively according to the following equation:
NH4N3(s) ⇌ 2N2(g) + 2H2(g)
A small amount of ammonium azide was placed in a sealed container and the air was removed. The sample was then detonated. At equilibrium the total pressure in the container was found to be 421,959.129 Pa. Calculate Kp.

Assuming the temperature was at STP (0°C).
Consider a 1L volume.
Total pressure
= 421959.129Pa
= 421.96 kPa
= 4.1654 atm at 0°C
Partial pressure of each product
= 2.08272 atm
This implies
P_{N2}=P_{H2}
=2.08272 atm
Kp=P_{N2}²P_{H2}²
= 2.0827^4
= 18.8158475075608
= 1.882*10^{1} approx.
Note:
solids do not enter into the definition of equilibrium constants.
In any case, in an explosion, we assume all (solid) reactants have been consumed.