[nisha1]

Please can you provide me the formulas used for calculating thermodynamic parameters like free energy, enthalpy and entropy,using Langmuir adsorption isotherms. Mostly we study langmuir isotherm for solution of different concentrations at different temperatures. Then we find the slope and intercept from its graphs. I wanted to know that what values we exactly put to calculate thermodynamic parameters?

[Borek]

Please stop posting same questions several times.

http://www.chemicalforums.com/index.php?topic=34611

[Hunt]

You have an expression for K=f(theta) , so you can determine  :delta: G^o = -RTlnK

Then you can use basic thermodynamic equations to find  :delta: S ,  :delta: H , etc.

[Hunt]

For example : dG = Vdp - SdT

So ΔS^o = - ((∂ΔG^o)/(∂T))_p

You just differentiate the expression for G w.r.t T while keeping p and theta constant.

[Hunt]

Give this a quick read if you like.

[nisha1]

Please can you provide me the formulas used for calculating thermodynamic parameters like free energy, enthalpy and entropy,using Langmuir adsorption isotherms. Mostly we study langmuir isotherm for solution of different concentrations at different temperatures. Then we find the slope and intercept from its graphs. I wanted to know that what values we exactly put to calculate thermodynamic parameters?

sir beneath is the site address of a research paper in which they have obtained the value of 'b' from the slope and intercept of Langmuir isotherm and have placed the value of 'b' in the formula of Free energy and enthalpy. please explain the theory behind it.
http://pubs.acs.org/doi/pdf/10.1021/ie051111f

thankyou

[Hunt]

I dont have the time to read it , plus u should do it urself and then ask specific questions if u dont understand.

Concerning the plot , it's quite simple.

Theta = coverage = q / Q

bC ( 1- theta ) = theta

 bC ( 1 - q/Q ) = q / Q

A simple rearrangement gives you :

1 / q = 1/QbC + 1/Q

You plot 1/q versus 1/C and get the slope 1/Qb where 1/Q is the intercept, so u can figure out from here .

All the variables are already defined in the paper.

[nisha1]

I dont have the time to read it , plus u should do it urself and then ask specific questions if u dont understand.

Concerning the plot , it's quite simple.

Theta = coverage = q / Q

bC ( 1- theta ) = theta

 bC ( 1 - q/Q ) = q / Q

A simple rearrangement gives you :

1 / q = 1/QbC + 1/Q

You plot 1/q versus 1/C and get the slope 1/Qb where 1/Q is the intercept, so u can figure out from here .

All the variables are already defined in the paper.

thanks for making it easier. but the question i wanted to ask was that can we use directly 'b' which is the adsorption capacity in the formula of free energy

[Hunt]

Hi,

Yes b is the equilibrium adsorption consant so - RTLnb =   :delta: G^o

Then you can determine the enthalpy by using the vant hoff equation :

d Lnb / dT =  :delta: H^o / RT²

You integrate this equation and ofcourse taking :delta: H^o independent of T.

The result, I think, is included in the paper.

[nisha1]

Hi,

Yes b is the equilibrium adsorption consant so - RTLnb =   :delta: G^o

Then you can determine the enthalpy by using the vant hoff equation :

d Lnb / dT =  :delta: H^o / RT²

You integrate this equation and ofcourse taking :delta: H^o independent of T.

The result, I think, is included in the paper.

Thanks, very grateful to you sir