KSP vs Ka

[rwabdu]

My question is about how to understand when to leave out pure substances from Ka or Ksp equations

Ok so for ksp we include the ion when it is not dissolved example:

NaCL --> Na + Cl

Ksp= [Na][Cl]/NaCl

and same for Ka:

HA --> A + H

Ka= [A][H]/[AH]

but isn't AH and NaCl a pure substance there for shouldn't it be left out of the rate equation, (for example Kw doesn't include [H2O] in the denominator)?

Thank you!

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[PiBond]

We generally leave out pure solids and liquids in equilibrium expressions because we consider them to be unchanging. Therefore,

NaCl ---> Na + Cl

Ksp = [Na][Cl]

the NaCl is left out as it is a pure solid

For the Ka expression, we understand that the Ka represents an acid dissociation constant in H20

HA + H20 ---> A- + H30+

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

Water is left out because it is a pure liquid and the solvent of this reaction, thus it is unchanging.

you referenced rate equations in your question, but these are equilibrium expressions and don't pertain to rate.

 

[rwabdu]

thank you. the key was realizing that pure substances are unchanging. i didnt mean to say rate i understand the rate equations.

Thanks so much!