Equilibrium Constant Units

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Wilson 2E
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Joined: Sat Aug 17, 2019 12:18 am

Equilibrium Constant Units

Postby Wilson 2E » Sat Jan 18, 2020 6:41 pm

I know that Kc is a value without units, but can someone explain why that is? The units don't strictly cancel out all of the time.

Prasanna Padmanabham 4I
Posts: 111
Joined: Thu Jul 25, 2019 12:17 am

Re: Equilibrium Constant Units

Postby Prasanna Padmanabham 4I » Sat Jan 18, 2020 9:57 pm

Dr. Lavelle, in the lecture, talked about how Kc should technically be measured by how much "activity" is occurring in the equation. So how much activity that is produced by the forward reaction and how much activity is produced by the reverse reaction. Since it is a bit hard to measure this, chemists, for the most part, use concentrations. The concentration of reactants or products is supposed to be representative of how much activity is occurring on either side. At equilibrium, since the forward and reverse reactions are occurring at the same rate (activity both ways is the same), the concentrations of products and reactants should be the same.

Kavya Immadisetty 2B
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Joined: Sat Aug 24, 2019 12:17 am
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Re: Equilibrium Constant Units

Postby Kavya Immadisetty 2B » Sat Jan 18, 2020 10:05 pm

The reason why is because when you say you're using concentrations of partial pressures, you're actually using activities of each species. I think when you calculate the activity of each species, the units cancel out.

Andrew F 2L
Posts: 103
Joined: Sat Aug 17, 2019 12:17 am

Re: Equilibrium Constant Units

Postby Andrew F 2L » Sat Jan 18, 2020 10:16 pm

I think Kc is unit less because it is simply a ratio and the units don't exactly mean anything and only the number or ratio really matters. Also I think there was something said about how we are calculating activities of the reactants and products so those themselves don't have units

Alexandra Bartolome 2H
Posts: 102
Joined: Sat Jul 20, 2019 12:17 am

Re: Equilibrium Constant Units

Postby Alexandra Bartolome 2H » Sat Jan 18, 2020 11:13 pm

It is unitless because in the calculation of the equilibrium constant, you're using the activities of the reactants and products, as someone else said.

Section 5G.2 in the textbook pretty much explains why it's unitless: "Thus, for a substance J that forms an ideal solution, the partial pressure in the expression for K is replaced by the molar concentration [J] relative to its standard value, c° = 1 mol/L. Although K should be written in terms of the dimensionless ratio [J]/c°, it is common practice to write K in terms of [J] alone and to interpret each [J] as the molar concentration with the units “mol/L” struck out."

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