What is an activity?

[OwenSumter_2F]

I am reading through textbook topic 5G and I am confused on what an activity is. Is it just a broader term for the concentrations of certain pieces of an equilibrium or is it something else?

[Kelly Singh]

I think its just a way to generalize the K value so that it has no units. Since K can be taken from concentration or partial pressure values, we call the input activities.

[Kelly Singh]

but in all honesty I'm a little confused by it also haha

[Gillian Gabrielsen2F]

From my understanding, activity is a way to compensate the differences between ideal vs. real behavior in molecules. Real reactions don't occur as idealized due to the additional molecular interactions that occur outside of the specific reaction, so the real pressure/concentration doesn't match exactly with the idealized one. Activity takes into account intermolecular interactions that occur with the particular molecule, while molar concentration doesn't. When dealing with low concentrations, this difference doesn't matter since there's a lower number of molecules in proportion to the environment, so less interactions occur. But when there's a higher concentration, more intermolecular reactions occur outside of that "main" reaction, so the idealized value of K differs from the actual value. For finding the equilibrium constant, the difference between the activity and the concentration/partialpressure is small enough that it doesn't matter, which is why activity isn't used for this. (I might be wrong, and if I am please correct me)

[alette1a]

I had this same question too! I take it as meaning a species in a K expression. I don't think it really matters whether or not we know the actual thermodynamical definition because the textbook says that we learn more in-depth about it in a more advanced course. And, I haven't seen Dr. Lavelle use it.

[Sukhkiran Kaur 3I]

I was confused by this too but i think it might just be a generalization of the K value?

[Ritika Prasad 1A]

This was a bit confusing to me too but I think Dr. Lavelle mentions it briefly in one of the Audio/Visual Videos. I think it has to do with the measure of the substance being used in the K expression. So when we have the concentrations in the K ratio: [P]/[R], this can also be expressed as [activity of P]/[activity of R].

[Lilly Catarozoli 1B]

This also confused me! I think it is just referring to K though.

[805295074]

I believe he wrote the equilibrium constant equal to the activity in the first lecture.

[Akriti Ratti 1H]

I think activity just refers to the equilibrium constant, K.