Equilibrium constant, Kc

[MAGGOT]

Hi everyone,
In the Kc formula, do you not include any solid/liquid reactants in the formula or you do not include any solid/liquid reactants or products in the formula?

Many thanks.

[Lear]

I believe you don’t include both reactants and products that are solid/liquid as they have a concentration of one and therefore do not affect Kc.

[TheBigC]

Hi everyone,
In the Kc formula, do you not include any solid/liquid reactants in the formula or you do not include any solid/liquid reactants or products in the formula?

Many thanks.

It is BOTH reactants and products that are solids or liquids as they have unchanging concentrations. Consequently, they are omitted from the equilibrium expression and are said to be 'absorbed' into the equilibrium constant (Kc).

I believe you don’t include both reactants and products that are solid/liquid as they have a concentration of one and therefore do not affect Kc.

I would like to clarify this. They do not necessarily have a concentration of 1M. For example,
Let there be 100mL of water. In 100mL of water, we have:



To find concentration,



As can be seen, this is clearly more than 1 M. The same applies for solids.

Note that the value of Kc has no real meaning* other than providing qualitative information regarding extent of reaction (in relation to Qc), hence the omission of a constant (as would be produced from the addition of liquids and solids into the expression) will not affect its interpretation.

*(At a VCE Level: I am pretty sure that in uni, it is used in calculating values for Gibbs free energy)

[MAGGOT]

Also, someone told me that water is supposed to omitted in a way  as the concentration is always 1, is that right?

[Lear]

Oh crap looks like I had a misconception then. Interestingly enough my SAC on AOS2 had an equilibrium question where water was in the (ORGANIC) state (Which I still don’t know the meaning of) and therefore had to be included in the K constant.

[TheBigC]

Oh crap looks like I had a misconception then. Interestingly enough my SAC on AOS2 had an equilibrium question where water was in the (ORGANIC) state (Which I still don’t know the meaning of) and therefore had to be included in the K constant.

Not your fault Lear. I am pretty sure that the Heinemann VCE Chemistry Units 3&4 text says that it is assigned as 1. It is not actually 1 M, just 'assigned' as 1 M. This pedantic statement is extremely deceiving.

Also, water with an ORGANIC state? what the hell. I have never seen that before.

[Lear]

It was water and some other substances dissolved in an organic solvent apparently. I thankfully included water in my K calculations. I’m still confused as to how we can dissolve a polar substance such as water into an organic solvent which is usually non polar

[TheBigC]

It was water and some other substances dissolved in an organic solvent apparently. I thankfully included water in my K calculations. I’m still confused as to how we can dissolve a polar substance such as water into an organic solvent which is usually non polar

Organic solvents are merely carbon-containing. They can be either polar or non-polar. For example, butanol solvent could dissolve certain amounts of water due to the presence of hydroxyl groups.