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[Need Help] Finding Gibbs Energy of a reaction

Hello!

Can anyone help me if i want to find the Gibbs free energy of the following reaction at different temperatures? The H2O in the reactant is the superheated steam at 3barG and 380oC.

CaCl₂.2H₂O(l) + 2H₂O(g) → Ca(OH)_{2 (s)}+ 2HCl(g) +2 H₂O(g)

:smile:

Reply 1

JockerLW

Original post by dalila_amnani

[delta]G_r(T,P)= [delta]H_r(T) - T*[delta]S_r(T) + R*T*ln((P_partial(HCl)/P_total)^2)

You must find [delta]H_r(T) and [delta]S_r(T) All data are in a handbook about thermodynamic values of substances.

But you must remember that [delta]H_r(T) = [delta]H_r(298) +{integral}([delta]Cp(298)*dT) and
[delta]S_r(T) = [delta]S_r(298) +{integral}([delta]Cp(298)*dT/T)

It is very easy.

Reply 2

dalila_amnani

how about the H₂O(g) in the product? do we need to consider the pressure also?

Reply 3

JockerLW

Original post by dalila_amnani

how about the H₂O(g) in the product? do we need to consider the pressure also?

No. The pressures of gaseous H₂O are reduced in logarithm. And in general you calculate to the equation:

CaCl₂*2H₂O → Ca(OH)_{2 (s)}+ 2HCl(g)

2 molecules of H₂O (g) is as in reagents and in products. You can reduce their mentally.

And I think, that CaCl₂*2H₂O not (l) but (s))))) this is a solid substance))

(edited 10 years ago)

Reply 4

Big-Daddy

Or if you're working with a real life issue I can assume you have access to data-books or the Internet for data. Then find the enthalpies of formation of each of your products and reactants, find the standard enthalpy change of the reaction; molar entropies for each of your products and reactants and find the standard entropy change; calculate from ΔG(stn)=ΔH(stn)-TΔS(stn) the standard Gibbs' free energy; and then you will have to use ΔG=ΔG(stn)+R*T*ln(Q) where Q=P(HCl)^2=P(total)^2 * x(HCl)^2 where the likelihood that HCl is not the only gas contributing to your total pressure is treated. All this assumes that enthalpy change and entropy change are independent of temperature; that can be fixed by using heat capacities to modify them for the desired temperature.

(edited 10 years ago)

Reply 5

JockerLW

Original post by Big-Daddy

Write out the problem properly for God's sake.

Or if you're working with a real life issue I can assume you have access to data-books or the Internet for data. Then find the enthalpies of formation of each of your products and reactants, find the standard enthalpy change of the reaction; molar entropies for each of your products and reactants and find the standard entropy change; calculate from ΔG(stn)=ΔH(stn)-TΔS(stn) the standard Gibbs' free energy; and then you will have to use ΔG=ΔG(stn)+R*T*ln(Q) where Q=P(HCl)^2=P(total)^2 * x(HCl)^2 where the likelihood that HCl is not the only gas contributing to your total pressure is treated. All this assumes that enthalpy change and entropy change are independent of temperature; that can be fixed by using heat capacities to modify them for the desired temperature.