HSAB: Hard Soft Acid Base | Ferdinand's Curiousity

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Discussion Overview

The discussion revolves around the Hard Soft Acid Base (HSAB) theory and its application to predicting the direction of a chemical reaction involving HI, NaF, HF, and NaI. Participants explore the nature of acids and bases in the context of this reaction and consider the conditions under which the reaction may proceed in either direction.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Ferdinand introduces the reaction and categorizes the species involved according to HSAB theory, questioning the direction of the reaction and the formation of NaI.
  • One participant suggests calculating the equilibrium constant, indicating that the reaction may favor the left side based on their initial assessment.
  • Another participant asks for clarification on the state of the reaction (in water or in a different phase), which is confirmed to be in water by several participants.
  • A participant calculates the Gibbs free energy change for the reaction and concludes that the equilibrium constant suggests a preference for the left side, indicating higher concentrations of reactants.
  • Another participant notes that the equilibrium direction can depend on the starting materials, suggesting that adding HI to the NaF solution could shift the equilibrium to the right.

Areas of Agreement / Disagreement

Participants express differing views on the direction of the reaction, with some suggesting it goes to the left and others proposing that it could shift to the right under certain conditions. There is no consensus on the overall direction of the reaction.

Contextual Notes

Participants rely on specific Gibbs free energy values and assumptions about the reaction conditions, such as being in aqueous solution, which may affect the conclusions drawn. The discussion does not resolve the implications of borderline acid-base classifications.

Who May Find This Useful

Individuals interested in chemical equilibria, acid-base theory, and the application of HSAB principles in predicting reaction outcomes may find this discussion relevant.

fairburn96
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Hi guys, my name is ferdinand. I'm curious about HSAB, especially to predict where the reaction goes.
Exp: HI + NaF >>> HF + NaI
H is hard acid
I IS soft base
Na is hard acid
F is hard base, and

HF Is hard.hard
NaI Is hard soft.
The reaction will goes to right or left? And why?
And since hard acid like to bind hard base, why NaI formed?
Thank you...
Terima kasih.And how about the borderline? Is it count as hard or soft?
 
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Have you calculated the equilibrium constant? My quick look says the reaction goes to the left, as you suspect.
 
In water? Or a solid and a gas?
 
Borek said:
In water? Or a solid and a gas?
Good question. I assumed water. OP?
 
insightful said:
I assumed water.

Then it should go to the right, HF is a weak acid.
 
Borek said:
Then it should go to the right, HF is a weak acid.
I used DeltaGf of (all aqueous at 25C):
-129 kcal/gmol for NaF
-71 for NaI
-12 for HI
-66 for HF

and got DeltaGrxn = +4kcal/gmol and using DeltaG = -RTlnKeq, got Keq = 1.2E-3 indicating left side reactants at higher concentrations than right side products.
 
Last edited:
Well, it depends on what your starting material is. Yes, in general the equilibrium is to the left, but if you add HI to the NaF solution, fluoride gets protonated and the shift is to the right.
 

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