Calculating Enthalpy Changes in Aqueous Reactions: A Quick Guide"

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Discussion Overview

The discussion revolves around calculating enthalpy changes in aqueous reactions, specifically addressing the use of enthalpy values for ions versus compounds in a chemical reaction involving iron and sodium hypochlorite. Participants explore the implications of using ionic enthalpy values and the complexities involved in enthalpy calculations for aqueous solutions.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant questions whether to use the enthalpy values for ions when dealing with aqueous reactants and products, citing the lack of available enthalpy values for certain compounds like NaOCl and NaCl.
  • Another participant raises a concern about needing the correct enthalpy values for specific ions, suggesting that the oxidation states may complicate the calculations.
  • A different participant argues that the enthalpy of formation for aqueous compounds involves substeps, including solvation, which must be considered in the calculations.
  • One participant corrects another's understanding of oxidation states in NaOCl and discusses the relationship between free energy change and enthalpy change, emphasizing the need to account for solvation and lattice energies.
  • A participant expresses a desire to organize data related to these reactions, indicating an ongoing effort to clarify the information involved.

Areas of Agreement / Disagreement

Participants exhibit disagreement regarding the appropriate approach to calculating enthalpy changes, particularly about whether to use ionic enthalpy values or those of the compounds. The discussion remains unresolved with multiple competing views presented.

Contextual Notes

Participants highlight the complexity of enthalpy calculations due to the need for various energy values, such as solvation and lattice energies, which are not straightforwardly available or defined in the context of the discussion.

mrjeffy321
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In a reaction, if I have a product/reactant that is aqueous, do I use the values of enthalpy for their ions?

for example:
2Fe (s) + 3NaOCl(aq) --> Fe2O3 (s) + 3NaCl(aq)
which could also be:
2Fe (s) + 3Na-1(aq) + 3O+2(aq) + 3Cl-1(aq) --> Fe2O3 (s) + 3Na+1(aq) + 3Cl-1(aq)

since I can't find a value for the enthalpy of NaOCl, then this would work out nicely to use the ions, and I don't see an aqueous enthalpy value for NaCl, so this is why I am thinking that I should use the ion's enthalpy, am I right.
 
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but then again, if this is true, then I would need to have the values of enthalpy for a sodium ion that is negative 1, rather than positive, and what an oxygen ion?
 
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I don't believe so. The enthalpy of formation of the following aqueous compounds is the net enthalpy of formation from the corresponding elements from their most stable states; there are substeps involved, one of these substeps I believe pertains to solvation...formation of ions, then solvation.


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mrjeffy,

I see you're still rusting, with bleach this time - for your thermit weld, I guess ?

Okay, you've gotten the oxidation states in NaOCl wrong - it should be Na(+1)O(-2)Cl(+1). It's the Chlorine ion that has a weird state. I'm sure the the reduction potential for :
Cl(+1) + 2e- ---> Cl (-1) is documented.

From this, you can calculate the Free Energy change, and from that, the enthalpy change.

But it's not so simple, because you will also have to use the solvation energy for O(-2) as well as the lattice energy for Fe2O3 OR the ionization and lattice energies for Fe - depending on what data you use.
 
yes, I am still rusting, but mostly just trying to organize all this data I have and get rid of all the scrap paper I have laying around, so I can have all the info I could possibly ever want on these reactions without the clutter.

ok, all see what I can do with this method of doing it.
 

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