Cases where water is included in the equilibrium expression?

[commoncold]

I was always sure that water was excluded from the equilibrium expression. However, I came across a question in the Princeton Review hyperlearning science workbook that kind of confused me.

The passage stated that" In an experiment, 4 mixtures containing different ratios of pure acetaldehyde and ethylene glycol were allowed to react in toulene (using 1.0 mL of sulfuric acid as a source of a catalytic amount of H+ to initiate the reaction) and come to equilibrium."

The question asked, "What is the equilibrium expression for the acetal reaction"? and they listed this as the correct answer: [acetal] [H2O]/ [CH3CHO]*[HOCH2CH2OH] stating that water is not the solvent here; this is the toulene solution.

In addition, if the toulene solution is included in the expression, why is it not considered a reactant?

Could someone clarify what is going on? Many thanks!

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[Mornhavon]

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[sciencebooks]

In this case, water is not in the equilibrium expression because it is not the solvent here, toluene is.\

*Sorry, didn't realize you'd included that.

 

[idontcaureuknow]

In this case, water is not in the equilibrium expression because it is not the solvent here, toluene is.

Whether or not water is the solvent isn't an issue, the question is if it's part of the reactants or products, and what the K value is being referenced.

Think acid base chemistry (yes, I'm choosing a common "confusing" example). For a generic acid dissociation in water we have:

HA + H2O <=> A- + H30+

The equilibrium expression is:

K = [H30+][A-]/[HA][H2O]

Since water is the solvent, the concentration is roughly known in terms of molarity, (IIRC 55.4M or something). So what is commonly done in aqueous acid base chemistry is the following:

K[H2O] = [H30+][A-]/[HA]

To simplify, we look up a Ka value, which is the acid dissociation equilibrium constant in water, in other words:

Ka = K[H2O] = [H30+][A-]/[HA]

 

[sciencebooks]

Whether or not water is the solvent isn't an issue, the question is if it's part of the reactants or products, and what the K value is being referenced.

Think acid base chemistry (yes, I'm choosing a common "confusing" example). For a generic acid dissociation in water we have:

HA + H2O <=> A- + H30+

The equilibrium expression is:

K = [H30+][A-]/[HA][H2O]

Since water is the solvent, the concentration is roughly known in terms of molarity, (IIRC 55.4M or something). So what is commonly done in aqueous acid base chemistry is the following:

K[H2O] = [H30+][A-]/[HA]

To simplify, we look up a Ka value, which is the acid dissociation equilibrium constant in water, in other words:

Ka = K[H2O] = [H30+][A-]/[HA]

I've seen it overly simplified like that and I'll admit I probably just memorized a line without completely understanding it.