Equilibrium expressions

[NA19]

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In an experiment, four mixtures containing different ratios of pure acetaldehyde and ethylene glycol were allowed to react in toluene and come to equilibrium. Once at equilibrium, the amount of acetal and water yield was recorded.

What is the equilibrium expression?

Ans: [acetal][H2O]/[ethylene glycol][acetaldehyde]

I thought that you usually don't use water in the eq'm expression since it's a 'pure substance'. Thoughts?

 

[ashtonjam]

The reason water is excluded from most equilibrium expressions is because it's usually in vast excess. This is especially true in aqueous environments like during acid-base chemistry. The [H2O] term is practically constant and gets divided out since it hardly changes.

In this case, water is probably included because it's actually a product that isn't in crazy excess. You're doing this experiment with organic solvents. Water is only made in small quantities when the acetal is formed, so its concentration matters a lot more.

 

[gettheleadout]

The exclusion of water from the expression is done in cases where the reaction takes place in aqueous solution, and the amount of water is relatively constant. In this case a small amount of solute water is produced in organic solution as a result of the reaction, so it must be included.

Edit: Yeah, what ashtonjam said.