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Chemistry Forums for Students => Physical Chemistry Forum => Topic started by: arcinus on December 11, 2017, 11:34:53 AM

Title: Calculating pKa changes from different Tempuratures
Post by: arcinus on December 11, 2017, 11:34:53 AM

I am currently trying to calculate the change in pKa values for N,N Diethylhydroxylamine with alternating temperature dependency.

I have the standard temperature/pressure pKa value of 5.67

I was trying to use the following equation:

ln (K2/K1)=-ΔHΘ/R×(1/T1)×(1/T2)

However I'm not 100% sure how to go about obtaining the standard enthalpy change for this reaction:
(C₂H₅)₂NOH  ::equil:: H⁺ + (C₂H₅)₂NO^-

Title: Re: Calculating pKa changes from different Tempuratures
Post by: arcinus on January 02, 2018, 06:37:18 AM

I have gotten an answer for this after a lot of toil and effort, if anyone needs help with calculating temperature dependencies for weak acids and bases feels free to contact me.

Title: Re: Calculating pKa changes from different Tempuratures
Post by: Borek on January 02, 2018, 07:42:33 AM

Can't you outline your approach here?

Title: Re: Calculating pKa changes from different Tempuratures
Post by: Babcock_Hall on January 02, 2018, 08:40:32 AM

For common aqueous buffering substances, there are published values of ΔpK_a/ΔT.  However, I once found a buffer for which there were two different values given (I don't recall the details), and your compound is not one of the most commonly encountered, and I am not sure that it would appear in such a table.  Can you clarify what you mean by "alternating?"

Title: Re: Calculating pKa changes from different Tempuratures
Post by: arcinus on January 03, 2018, 05:44:52 AM

I went off of a maximum assumption for calculating the ΔHΘ of a strong acid base reaction, which would be -57.1 Jmol-1K-1, and found a pKa under standard conditions of 5.67 for Diethylhydroxylamine (DEHA). Using the Van't Hoff equation, where T1 and K1 would be our standard conditions, T2 would be the temperature we wish to know the pH value of.

I then made an excel with headers as follows :
T2 (°C) _ T2 (°K) _ 1/T2 _ (1/T1-(1/T2) _ K2/K1 _ K2 _ ln(K2) _ pH

pH was then calculated via pOH (steps below) and use the formula to cover a range of temperatures from 10-150.

Then I made a table of a range of concentrations of DEHA under standard conditions.

After having a method for doing temperature ranges and concentration ranges I combined the two.

Knowing the Ka value at specific temperatures,

[OH-]^2 = [DEHA] * Ka
sqrt[OH-]^2=[OH]
pOH=-log10([OH])
pH = 14-pOH

Ended up with a massive table, going to be running an experiment with a boiler and some probes to test my theoretical values in the coming week.

Probably not the greatest explanation of how I got there. Basically effort and caffeine...

I'll check with my boss and see if I can post my excel or if it is company property.