# How do we derive the equilibrium constant formula?

1. Dec 9, 2004

### Cheman

How do we derive the equilibrium constant formula?

ie: Kc= [C]^c*[D]^d/ [A]^a*^b

Thanks in advance.

Last edited by a moderator: Feb 25, 2016
2. Dec 9, 2004

### chem_tr

To start with basics: the product of concentrations of ionic species belonging to right side (products) over the product of concentrations of ionic species belonging to the left side (reagents)

Now let me give a real example:

$$5Fe^{2+}+MnO_4^-+8H^+\leftrightharpoons 5Fe^{3+}+Mn^{2+}+4H_2O$$

The equilibrium constant may be written like this:

$$K_{redox}=\frac {[Fe^{3+}]^5\times [Mn^{2+}]}{[Fe^{2+}]^5\times [MnO_4^-] \times [H^+]^8}$$

Last edited: Dec 9, 2004
3. Dec 9, 2004

### gravenewworld

The equilibrium constant Kc=[Products]^coeffcient/[Reactants]^coefficients

In words: The equilibrium constant is the concentration of the products raised to the power of the coefficient in front of them of the balanced equation divided by the concentration of all the reactants raised to their coefficients. This is of course the "easy" explanation of an equilibrium consant, you probably don't want to see the P-chem explanation.

4. Dec 9, 2004

### chem_tr

Gravenewworld, my P-chem really sucks, would you please show it, so that I can learn how to do it by using P-chem?

5. Dec 9, 2004

### gravenewworld

http://www.sdsc.edu/chemdyn/classnotes/equil.html

I have no idea how to use Latex, so it would almost be impossible to write down how it is derived. That website shows how to derive the equilibrium constant for gases which will give you an idea of how it is derived in general. You have to work with chemical potentials etc. to derive the equilibrium constant.

6. Dec 9, 2004

7. Dec 9, 2004

### chem_tr

Wow, they are very good, even too good for me to nearly impossible to understand them easily However I will devote my energy to learn the basics in it. Thank you for your interest.

8. Dec 9, 2004

### GCT

Your chemistry text should explain this in simple terms, the easiest way to understand equilibrium constant is in terms of the rate constants. The derivation from the forward and reverse rate constants should be given in your text. It is quite important that you understand the relationship between rate constants and equilibrium constatn.

It seems that they have explained the rest. although it is a bit more complex . You should study this section more in depth.

Chem tr. where did you get the redox example (your first post)?

Last edited: Dec 9, 2004
9. Dec 10, 2004

### chem_tr

I have totally made up this one, but it doesn't seem to be wrong...

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