Different Values for K and Kc?

Juliet Stephenson 4E
Posts: 100
Joined: Wed Sep 18, 2019 12:21 am

Different Values for K and Kc?

In Table 5G.2, a list of equilibrium constants for various reactions are provided. For many reactions, K and Kc have the same value, which I would expect based on the fact that these two constants are calculated in the same way. However, for the bottom half of the table, K and Kc have different values, which I don't quite understand. It seems to be that these reactions have especially strong reactants, but even so, I don't understand how they could have different values because they have the same formula. What am I missing? Why are the values different?

Example: For the rxn F2(g) <-> 2 F(g) at 500 degrees, K is 3.0 * 10-11 and Kc is 7.3 * 10-13
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Posts: 180
Joined: Wed Sep 18, 2019 12:20 am

Re: Different Values for K and Kc?

that is strange. I thought that K and Kc were the same. The only thing I can think of is if K were calculated in Pressure and than converted to Kc but I didn't do the math to test that. I also don't know why it would be calculated differently.

Deepika Reddy 1A
Posts: 125
Joined: Thu Jul 11, 2019 12:15 am

Re: Different Values for K and Kc?

The table is using K to represent Kp. For the reactions that the Kc and K values are the same, the moles of gas on both sides of the reaction is probably equal. This would mean that Kc=Kp, so therefore in the table, K is equal to Kc for those reactions.

Alex Chen 2L
Posts: 86
Joined: Wed Sep 18, 2019 12:21 am

Re: Different Values for K and Kc?

Deepika Reddy 1J wrote:The table is using K to represent Kp. For the reactions that the Kc and K values are the same, the moles of gas on both sides of the reaction is probably equal. This would mean that Kc=Kp, so therefore in the table, K is equal to Kc for those reactions.

This is correct. Temperature changes can change a reaction's equilibrium constant depending on if the reaction is endothermic or exothermic. However, temperature also affects pressure, which does not affect concentration. Therefore with reactions that have unequal numbers of moles of gas on both sides have disproportionate pressure changes from temperature, affecting the equilibrium constant further.