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### Topic: Gas Equilibrium Constant  (Read 2228 times)

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#### mystreet123

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##### Gas Equilibrium Constant
« on: June 01, 2016, 06:46:53 AM »
If a reaction involves both gases and aqueous solution, should we use Kp or Kc for equilibrium constant?

How to include concentration of aqueous solution in gas equilibrium constant?

• Rhyming Chemist
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##### Re: Gas Equilibrium Constant
« Reply #1 on: June 01, 2016, 07:04:51 AM »
You've asked an interesting question. I'd go with Kc but I'm going on a hunch.

The page below advises that solids and liquids should be omitted from equilibrium constant expressions.

http://chemwiki.ucdavis.edu/Core/Physical_Chemistry/Equilibria/Chemical_Equilibria/The_Equilibrium_Constant

While this page features examples of equilibria exclusively involving gases yet taking the Kc constant.

http://chemwiki.ucdavis.edu/Core/Physical_Chemistry/Equilibria/Chemical_Equilibria/The_Equilibrium_Constant/Calculating_An_Equilibrium_Concentration_From_An_Equilibrium_Constant/Writing_Equilibrium_Constant_Expressions_involving_solids_and_liquids

I couldn't find any explanations of heterogeneous equilibria involving both aqueous and gaseous species, suggesting it's either insanely complicated, or not appreciably different from heterogeneous equilibria which only involve one or the other. In other words, leave out solids and liquids and go with Kc. I suspect it's the insanely complicated option and may involve activities. Is there any particular reason you want to know?

#### mystreet123

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##### Re: Gas Equilibrium Constant
« Reply #2 on: June 01, 2016, 07:19:07 AM »
You've asked an interesting question. I'd go with Kc but I'm going on a hunch.

The page below advises that solids and liquids should be omitted from equilibrium constant expressions.

http://chemwiki.ucdavis.edu/Core/Physical_Chemistry/Equilibria/Chemical_Equilibria/The_Equilibrium_Constant

While this page features examples of equilibria exclusively involving gases yet taking the Kc constant.

http://chemwiki.ucdavis.edu/Core/Physical_Chemistry/Equilibria/Chemical_Equilibria/The_Equilibrium_Constant/Calculating_An_Equilibrium_Concentration_From_An_Equilibrium_Constant/Writing_Equilibrium_Constant_Expressions_involving_solids_and_liquids

I couldn't find any explanations of heterogeneous equilibria involving both aqueous and gaseous species, suggesting it's either insanely complicated, or not appreciably different from heterogeneous equilibria which only involve one or the other. In other words, leave out solids and liquids and go with Kc. I suspect it's the insanely complicated option and may involve activities. Is there any particular reason you want to know?

I'm taking A level but I guess this is not included in specification then if it is complicated?

#### mjc123

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##### Re: Gas Equilibrium Constant
« Reply #3 on: June 01, 2016, 07:25:31 AM »
There's nothing to stop you using a hybrid equilibrium constant including aqueous concentrations and gas pressures, especially since the usual standard state for solutions is 1M concentration and for gases 1 atm pressure.
For example, the Nernst equation (which is related to equilibrium constants) for the H+/H2 electrode would be
E = E° + RT/F*ln([H+]/pH21/2)