So far we have expressed equilibrium constant of the reactions in terms of molar concentration of the reactants and products, and used symbol, Kc for it.
For reactions involving gases, however, it is usually more convenient to express the equilibrium constant in terms of partial pressure.
The ideal gas equation is written as, pV = nRT
Here, p is the pressure in Pa, n is the number of moles of the gas, V is the volume in m3 and T is the temperature in Kelvin
Therefore, n/V is concentration expressed in mol/m3
If concentration c, is in mol/L or mol/dm3, and p is in bar then p = cRT
We can also write p = [gas]RT
Here, R= 0.0831 bar litre/mol K
At constant temperature, the pressure of the gas is proportional to its concentration i.e.,
For reaction in equilibrium H2(g) + I2(g) ⇔ 2HI(g)
We can write either
where ∆n = (number of moles of gaseous products) – (number of moles of gaseous reactants) in the balanced chemical equation. It is necessary that while calculating the value of Kp, pressure should be expressed in bar because standard state for pressure is 1 bar.
We know from Unit 1 that:
1pascal, Pa=1Nm-2, and 1bar = 105 Pa