Chemical Forums

Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: ultrashogun on March 16, 2007, 05:06:44 AM

Title: Question about Henry constant
Post by: ultrashogun on March 16, 2007, 05:06:44 AM

Nobody has ever mentioned this to me, but I was just playing around with some calculations and my question is, is the Henry constant in Henrys law the same thing as the equilibrium constant for the process of solution?

CO2(g) <---> CO2(aq)

The law tells me that [CO2(aq)] at equilibrium is h * Ppart, so if I substitute in the expression for K I get K = (Ppart * h)/Ppart = h .

My other question is, if I wanted to find the Partial pressure of CO2 in water, taking into consideration the reaction of the gas with water

CO2(aq) + H20 <> H2CO3 with K,

Would the expression of K for the total reaction be Kt = h * K ? 

Title: Re: Question about Henry constant
Post by: DevaDevil on March 16, 2007, 12:11:32 PM

Henry's law: e^p = e^kc; taking ln: p = k_H*c; in this case k_H = pCO₂ (g) / [CO₂ (aq)]

CO₂ (g) <> CO₂ (aq)  K₁ = [CO₂(aq)] / pCO₂(g) = 1/k_H

CO₂ (aq) + H₂O (l) <> H₂CO₃ (aq) (let's assume H₂CO₃ as a single solvated molecule for simpleness, of course it would dissociate into (bi)carbonate)  K₂ = [H₂CO₃ (aq)] / [CO₂ (aq)] = k_H * [H₂CO₃] / pCO₂ (g)

which means that for the overall reaction: CO₂ (g) + H₂O <> H₂CO₃
K_t = [H₂CO₃ (aq)] / pCO₂ (g) = K₁ * K₂

in principle the same as your approach. Just be careful with Henry's constants - they are given in alot of different formats. (for example both [ L atm / mol], and [M / atm ] - work out if you need the relation p = k * c or k^* = p * c from the dimensions )