units of Kp.. so I know that Kc doesn\'t have any units (the equilibrium concent

ID: 837194 • Letter: U

Question

units of Kp..

so I know that Kc doesn't have any units (the equilibrium concentration) but what about Kp?

I know that Kp = Kc (RT)^delt n

but, since R = .08206 L atm / K mol,

shouldn't it have a unit of L atm / mol?

ex: if Kc = .0508 at temperature 439 K then Kp at 439 K = .0508 [(.08206 L atm / K mol) x ( 439 K)]^delta n

which means that Kp = 1.83^delta n L atm / mol

so, does this mean that Kp has the unit of L atm / mol?

somehow i feel like Kp doesn't have any unit but the above informations suggests to me that there might be units for Kp...

someone please clarify this for me. thanks

Good question!

For a reaction aA(g) + bB(g) = cC(g) + dD(g),

Kp = (PCc x PDd) / (PAa x PBb) where P terms denote the partial pressures.

So, apparently Kp is unitless.

But this is true, only when c + d = a + b

For example, for the reaction 2SO2(g) + O2(g) = 2SO3(g)

Kp = PSO32 / PSO22 x PO2

This will have a unit of P2 /P2 x P = P-1 e.g. atm-1 or Pa-1

On the other hand, for the reaction CH4(g) + H2O(g) = 3H2(g) + CO(g)

Kp = (PH23 x PCO) / (PCH4 x PH2O)

So the unit of Kp will be P3 x P / P x P = P2 e.g. atm2 or Pa2

Therefore, although Kc is unitess, the unit of Kp depends on the specific reaction.

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