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I have only a chemical equation, like: $$\ce{N2 + 3H2 <=> 2NH3}$$ How can I calculate the value of equilibrium constant from only this equation. Provided temperature is $\pu{298K}$. I can use other standard data. I am really stumped on this question.

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  • $\begingroup$ You get the equilibrium constant by dividing the concentrations on the right side with the concentrations on the left side. Note that you should account for the coefficients by using them as powers in your equilibrium equation. $\endgroup$ – user21398 Oct 27 '16 at 9:29
  • $\begingroup$ Use $K= \frac{(NH_3)^2 }{ (N_2) (H_2)^3}$ where the brackets indicate concentration. $\endgroup$ – user14857 Oct 27 '16 at 9:32
  • $\begingroup$ there is no concentration given $\endgroup$ – Vidyanshu Mishra Oct 27 '16 at 18:42
  • $\begingroup$ Note that the standard Gibbs free energy of formation for everything on the left side is zero. The total change in free energy in this reaction is just double whatever the standard Gibbs free energy of formation of ammonia is. $\endgroup$ – Zhe Nov 2 '16 at 19:29
  • $\begingroup$ @THELONEWOLF. The concentration is not given in the question. $\endgroup$ – Shoubhik Raj Maiti Nov 7 '16 at 15:44
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By definition, the standard equilibrium constant is:

$K = e^{-\Delta_rG/RT}$

Where $\Delta_rG$ is the difference between the Gibbs Energies of the products and reactants.

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  • $\begingroup$ I need the value of equilibrium constant. $\endgroup$ – Shoubhik Raj Maiti Nov 7 '16 at 15:46
  • $\begingroup$ Don't you want to calculate it from standard Gibbs energy tables? $\endgroup$ – DavePhD Nov 7 '16 at 16:27
  • $\begingroup$ Maybe I can use standard Gibbs energy table. I was looking for an direct method. But now it seems that there is no way of getting the value of $K_c$ without knowing concentrations or Gibbs energy change. $\endgroup$ – Shoubhik Raj Maiti Nov 20 '16 at 4:16

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