Activity - Equilibrium Expressions

[i_love_science]

When I use the concept of activity to express the equilibrium constant expression, for either equilibrium pressures or concentrations, the units cancel and the value of K has no units (which is how K is customarily reported). But because of the difference in reference states (the reference pressure is 1 atm (or 1 bar) and the reference concentration is 1 mol/L), K_c is not equal to K_p.

Why is there an exception to this principle when the sum of the powers in the numerator and the denominator of the equilibrium expression are the same? Aren't the reference states still different? In this case K_c is equal to K_p. Thank you!

 

[mjc123]

Consider changing the reference pressure, say from 1 bar to 0.5 bar. Each reagent pressure (expressed as a multiple of the reference pressure) will increase by a factor of 2. The expression for K_p will therefore change by a factor 2^Δn. If Δn = 0, the value of K is independent of the reference pressure.

 

[i_love_science]

Thanks, I understand now.