Water in Reactions

[Lilly Catarozoli 1B]

When do we include water in K calculations? Lavelle said something about when it is not a solvent but I am still confused. Thanks!

[KatarinaReid_3H]

No pure substances (liquids and solids) go into the K expression. Therefore, if water is liquid and everything else in the equation is aqueous then it won't go into the K expression because water in liquid form is the solvent and will be present in a huge quantity compared to the other species in solution. The only case that I have seen where water was included in the K expression was when it was in gas form with the other species also in gas form because then water was not the solvent.

[Juliet Cushing_2H]

Include water in its gas form because it yields a partial pressure. When we look at reactions in solutions we almost always see water as a solvent (I've never seen a problem where another species is the solvent). If water is the solvent then omit it from Keq equations [A]/[B] because moles Water / Liters Water won't produce a concentration (thats just a pure liquid). I guess theoretically, if you have mols of water / Liters of a different solvent chemical, then water might have a concentration? But im 99.999% sure we won't see that in chem 14b. So essentially, include water in K when is is a gas with a partial pressure.

[Siwa Hwang 3G]

when the water is in aqueous or gaseous form

[Eileen Quach Dis 2A]

You only include water in reactions when it's not a liquid. Liquids or solids aren't included in k because they're in so much excess that any change to their concentration doesn't change their overall concentration by much so their concentrations would cancel out in the numerator and the denominator of k.

[Vanshika Bhushan 1A]

Water is omitted from the equilibrium expression only if it is a solvent in that reaction because it is a pure liquid. We can't increase the concentration of a pure liquid or pure solid.

[VincentLe_3A]

We only include water in equilibrium constant K calculations when water is not in excess as a solvent. Essentially, this means only include water in K calculations when it is in its gaseous phase with a partial pressure, where it does not act as a solvent in the system.

[Kathy_Li_1H]

Hi! I believe you can use water when it is in an aqueous or gaseous form since the equilibrium constant can only be calculated with concentrations of gases or aqueous solutions.