Adding 2 equilibrium reactions

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Adding 2 equilibrium reactions

Postby Jose_Arambulo_2I » Mon Nov 21, 2016 6:59 pm

When you're adding two equilibrium reactions, such as
2BrCl=Br2+Cl K=377
H2+Cl2=2HCl K=4E31

Why do you multiply their K's rather than just adding them?

Rochelle Ellison 2H
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Joined: Wed Sep 21, 2016 3:00 pm

Re: Adding 2 equilibrium reactions

Postby Rochelle Ellison 2H » Mon Nov 21, 2016 10:30 pm

If you add the two equations together and write the equilibrium constant you get 2BrCl + H2 = Br2 + 2HCl with K=([Br2][HCl]^2)/([BrCl]^2[H2]). You can then compare this to the equilibrium constant expressions for the original 2 reactions
K=[BrCl]^2/[Br2][Cl2] and K=[H2][Cl2]/[HCl]^2. When you look at these 2 expressions you can tell that if you were to add them together, the Cl2 concentration would not cancel out (and it should because it does not appear in the overall reaction). If you multiply the 2 Ks you will notice that you do get the same equilibrium constant expression as if you had written the expression using the overall reaction (since the Cl2 concentrations will cancel).

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