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For instance, I have a chemical reaction like : $$\ce{Fe^{2+} + $0.5$O2 + H^{+} ->Fe^{3+} + $0.5$ H2O}$$

And I would like to use Action-Mass law, but I need to have equilibrium constant for this reaction. And this for many other reaction. That's why I'm wondering if there exist a nice database with this information ?

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  • $\begingroup$ There are lots of reaction kinetics databases. A cursory search on the NIST databases for reaction kinetics didn't give me anything for this reaction but feel free to take a look. (kinetics.nist.gov) $\endgroup$ – rch Jul 16 '14 at 17:41
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The information you are looking for is out there, but sometimes you might have to work a little for it.

The example reaction you post is a redox reaction. If we split the redox reaction into its two half-reactions, we can look up their reduction potentials (easier to find sometimes than equilibrium constants) and use the Nernst equation to calculate the equilibrium constant. I use Wikipedia for my list of standard potentials, but only because the values are identical to those in textbooks that I own.

Oxidation half-reaction

$$\ce{Fe^{2+} -> Fe^{3+} + e-} \ \ \ E^\circ_{ox}= -0.77 \ \text{V}$$

Reduction half-reaction

$$\ce{O2 + 4H+ + 4e- -> 2H2O} \ \ \ E^\circ_{red} = +1.229 \ \text{V}$$

Full balanced redox reaction

$$\ce{4Fe^{2+} + O2 +4H+ -> 4Fe^{3+} + 2H2O}$$

Cell potential

$$E^\circ_{cell}=E^\circ_{ox} + E^\circ_{red} = -0.77 \ \text{V} + 1.229 \ \text{V} = 0.459 \ \text{V}$$

Nernst Equation

$$E^\circ_{cell}=\frac{RT}{zF}\ln{K_{eq}}$$

$$k_{eq}=e^{\dfrac{E^\circ_{cell}zF}{RT}}$$

In this equation, $z$ is the number of electrons transferred, $T$ is the temperature in kelvins, $R$ is the ideal gas constant, and $F$ is the Faraday constant. At 0 Celsius (273.15 K):

$$K_{eq}=e^{\dfrac{0.459 \ \text{V} \cdot 4\text{ mol}\cdot 96.49\times 10^3 \frac{\text{C}}{\text{mol}}}{ 8.314\ \frac{\text{J}}{\text{K}\cdot \text{mol}}\cdot 273.15\ \text{K} }}=7.565\times 10^{33}$$

If you can look up the enthalpy change or free energy change for the reaction (many of which are available at webbook.nist.gov/chemistry, then you can use a variant of the van 't Hoff equation

$$\Delta_r G^\circ = -RT\ln{K_{eq}}$$ $$K_{eq}=e^{\dfrac{-\Delta_R G^\circ}{RT}}$$

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