Activity in reactions

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Deepika Pugalenthi 1A
Posts: 60
Joined: Fri Sep 28, 2018 12:24 am

Activity in reactions

Postby Deepika Pugalenthi 1A » Fri Jan 11, 2019 12:04 pm

In lecture and discussion, it was mentioned that there was activity in reactions but aren't required to be incorporated in calculating the equilibrium constant. Can someone explain to me what exactly activity is?

Dayna Pham 1I
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Joined: Fri Sep 28, 2018 12:16 am
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Re: Activity in reactions

Postby Dayna Pham 1I » Fri Jan 11, 2019 12:17 pm

Pure substances, which are solids and liquids, are omitted from equilibrium constant calculations. This is because the molar concentration of pure substances does not change in a reaction.

This does make the calculation for K an approximation, as there may actually be a reaction going on with the solids and liquids. However, it is so small that we ignore it.

For example, H2O (l) is omitted from the calculation for K because the change in solvent concentration is insignificant.

Michael Novelo 4G
Posts: 64
Joined: Fri Sep 28, 2018 12:28 am

Re: Activity in reactions

Postby Michael Novelo 4G » Fri Jan 11, 2019 12:19 pm

I believe activity is everything that is occurring during an equilibrium reaction. If you look to page 400 on the textbook (7th edition) there is a table that shows activity and then the simplified version. In the activity for an ideal gas substance it is written as Pj/P^o the P^o in this case are the pure substances (liquids and gases) we do not include in our expression so that is where the simplified form Pj comes from. Similarly the activity in the solute in a dilute solution's activity is the concentration [J]/c^o. The c^o as explained in the reading is simply equal to 1 mol/L. Furthermore, [J] actual units are mol/L but since c^o is at the bottom those units cancel out just leaving the simplified version of [J] without units.

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