[guferr]

I'm try to figure out the equilibrium constant for the reaction CH₃COOH + NaHCO₃ ⇌ CH₃COONa + H₂CO₃ (in aqueous solution).

And I'm confused about exactly what processes i need to take in account and what I have to consider here.

For instance, I've seen people considering that most of the  CH₃COOH is not going to be dissociated (while most of the NaHCO₃ is) and considered that most of the CH₃COONa would be kept dissociated as CH₃COO- + Na+ and thus taking the Na + out of the equation and transforming that into:

CH₃COOH + HCO₃- ⇌ CH₃COO− + H₂CO₃

I've myself done that before seeing that other person doing it, and in this way I can use the dissociation constants of H₂CO₃ and  CH₃COOH to find the Keq of this reaction.

However, I then noted that CH₃COONa isn't going to be necessarily mostly dissociated.

It's going to be completely dissolved in water, as it's very soluble, but this doesn't mean it's going to be fully dissociated.

So, was that approach valid?

Cause if CH₃COONa is kept in that form then you can't take Na+ out of the equation like that.

Also, should CH₃COOH react directly with HCO₃-, or will the reaction only happen between the already dissociated ions (Na+,HCO₃-,H+ and CH₃COO-) in the aqueous solution?

I'm confused about what processes I need to take in account to find the Keq of the first reaction I wrote here (which is the one I'm interested in).

[Orcio_87]

CH₃COOH + HCO₃- ⇌ CH₃COO⁻ + H₂CO₃

Maybe you should look at this as two separate reactions:

CH₃COOH ---> CH₃COO^- + H⁺

and

H⁺ + HCO₃^- ---> H₂CO₃

Also, should CH₃COOH react directly with HCO₃-, or will the reaction only happen between the already dissociated ions (Na+,HCO₃-,H+ and CH₃COO-) in the aqueous solution?

Do you dont worry about the H₂CO₃ ---> H₂O + CO₂ reaction ?

[Borek]

However, I then noted that CH₃COONa isn't going to be necessarily mostly dissociated.

You can safely ignore that. Yes, in more concentrated solutions sodium acetate will be not 100% dissociated, but there will be also plenty of other, more important problems that are very difficult to quantify - and they will be source of a much larger errors.

Dissociation constants we use are technically correct only for infinitely diluted solutions, in every other solution effects related to electrostatic interactions between all ions in the solution start to change these constants. We do try to deal with the effect (google Debye–Hückel theory) but our best attempts still fail for more concentrated solutions (or rather: we have models, but they require plenty of experimentally determined coefficients, which make them impractical to use). Then, even non reacting, oppositely charged ions tend to cluster and create ion pairs (or higher conglomerates), which is technically similar to creating undissociated compounds. We ignore all of these, even if they are typically producing much larger errors then the ones you are afraid of.

[guferr]

CH₃COOH + HCO₃- ⇌ CH₃COO⁻ + H₂CO₃

Maybe you should look at this as two separate reactions:

CH₃COOH ---> CH₃COO^- + H⁺

and

H⁺ + HCO₃^- ---> H₂CO₃

Also, should CH₃COOH react directly with HCO₃-, or will the reaction only happen between the already dissociated ions (Na+,HCO₃-,H+ and CH₃COO-) in the aqueous solution?

Do you dont worry about the H₂CO₃ ---> H₂O + CO₂ reaction ?

I can consider the H₂CO₃ ---> H₂O + CO₂ afterwards.

But I already did the steps you mentioned for the CH₃COOH + HCO₃- ⇌ CH₃COO⁻ + H₂CO₃ reaction, I already got an Keq for that one.

I just don't know if it's the correct approach.

[guferr]

However, I then noted that CH₃COONa isn't going to be necessarily mostly dissociated.

You can safely ignore that. Yes, in more concentrated solutions sodium acetate will be not 100% dissociated, but there will be also plenty of other, more important problems that are very difficult to quantify - and they will be source of a much larger errors.

Dissociation constants we use are technically correct only for infinitely diluted solutions, in every other solution effects related to electrostatic interactions between all ions in the solution start to change these constants. We do try to deal with the effect (google Debye–Hückel theory) but our best attempts still fail for more concentrated solutions (or rather: we have models, but they require plenty of experimentally determined coefficients, which make them impractical to use). Then, even non reacting, oppositely charged ions tend to cluster and create ion pairs (or higher conglomerates), which is technically similar to creating undissociated compounds. We ignore all of these, even if they are typically producing much larger errors then the ones you are afraid of.

I'm not worried about it not being dissociated due to concentration, but rather because it might keep that form even in a dilute solution.

Just like CH₃COOH itself does. It's fully dissolved but only a tiny fraction of it will be dissociated.

Same could happen for CH₃COONa, even why it fully dissolves in ethanol and other solvents where it doesn't dissociate.

I just wasn't sure if I could consider it would fully dissociate just because it's fully dissolved

[Babcock_Hall]

Have you tried adding or subtracting equations?