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In the expression of equilibrium constant, each concentration term is raised to the power of their stoichiometric coefficient. But this could affect the equilibrium constant.

For example, we can write the chemical equation of ammonia in these two ways $$\ce{N_2(g) + 3H_2(g)<=>2NH_3(g)}$$ and $$\ce{\frac{1}{2}N_2(g) + \frac{3}{2}H_2(g)<=>NH_3(g)}$$ This affects the equilibrium constant. How do I decide which chemical equation's coefficients to use? I think maybe the stoichiometric coefficients used in the equation should be in the simplest ratio of natural numbers. Is that so?

Equilibrium constant should not depend upon stoichiometric coefficients because they just tell about how many molecules are striking each other to form a certain product. It shouldn't matter what the coefficients are because they do not give information about the actual number of moles reacting. Then why does the equilibrium constant (if it does) depend upon them?

I think if you could tell me how they ended up in the equilibrium constant expression in the first place, it would make things easier to understand.

Eyy boss
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  • You can rephrase "Equilibrium constant should not depend upon stoichiometric coefficients because they just tell about how many molecules are striking each other to form a certain product. It shouldn't matter what the coefficients are because they do not give information about the actual number of moles reacting." putting "does" instead of "should" and that would be an answer. Only coefficients describing how exactly rds looks like have bearing on kinetics. – Mithoron Oct 14 '20 at 16:10
  • https://chemistry.stackexchange.com/questions/20140/where-does-this-equation-for-the-activity-in-the-gas-phase-equilibrium-between-n – Mithoron Oct 14 '20 at 16:13
  • or https://chemistry.stackexchange.com/questions/42953/when-is-it-permissible-to-use-the-stoichiometry-of-a-reaction-to-determine-rate – Mithoron Oct 14 '20 at 16:15
  • So how can one decide which set of stoichiometric coefficients to use. There can be infinite such sets like $(1,3,2), (2,6,4), (3,9,6), (\frac{1}{2},\frac{3}{2},1)....$ When asked about equilibrium constant for, say formation of ammonia, which set of coefficients would one choose? – Eyy boss Oct 14 '20 at 16:57
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    In general afaik, if we're talking specifically about the formation of one product (here ammonia), we would choose the equation in which one mole of ammonia is formed as the product; this is similar to the case of when we talk about the heat of formation, we take the equation in which one mole is formed. Do note that there is no rigorous rule here, so it's always better if an explicit equation is given. If not, I would pick the one where one mole of product is formed, but this can obviously lead to a wrong answer if the question was designed with another equation in mind. – Box Box Box Box Oct 14 '20 at 17:39
  • So basically, they always mention the equation for which the constant is asked. – Eyy boss Oct 14 '20 at 18:02

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