[ch101em]

Hi

K_a for acetic acid at 25 degrees Celsius is 1.754*10^-5.  At 50.0 degrees Celsius, K_a is 1.6333*10^-5.  What are the changes in enthalpy and entropy for the ionization of acetic acid?

I know I may now have given more information to solve the problem, but I was wondering how to solve it.

Thanks

[Erichsen]

I dont know the exact number in change, but when Ka gets smaller the equilibrum goes to the left (less acetate and more acetic acid).

When u mix acid and water u get an exoterm reaction, that means the equilibrum is exoterm from left to right, and therefore endoterm from right to left. When u raise the temperature the equilibrum will try to lower the temperature by going to the left (endoterm) This means your enthalpy will get smaller.
Im not sure about the entropy though, but i think it will get smaller too because acetic acid is more organised (less disorder) than H₃O⁺ + acetate.

Hope this helps

[Yggdrasil]

Assuming the changes in entropy and enthalpy change negligibly between 25 and 50 deg C, you don't need any more information to solve this problem.  My one big hint is to think of ΔG and how you can relate K to ΔG, then how ΔH and ΔS relate to ΔG.

[ch101em]

Thanks, but I still don't get one thing.

I know the gibbs free energy formula of G=H-T*S, but how would I be able to use the acid ionization constants of the acetates to help me in this formula.

Oh, the answers are -2.29 kJ/mol, and -98.7 J/(mol*K) if they can help, because I still am trying to figure out how the acid ionization constants will help with this problem.

[Yggdrasil]

The equilibrium constant and the change in Gibbs Free Energy are related by the following equation:

K = e^(-ΔG/RT)
or
ΔG = -RT ln(K)

[ch101em]

OK.

And one more thing, how is the acid ionization constant related to the Equilibrium constant?  It looks similar I know that, but yeah.

[Yggdrasil]

Ka is an equilibrium constant.  Write out the expression for the acid ionization constant and then you can see the reaction for which Ka is the equilibrium constant.