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For a generic reaction
- aA + bB < - > cC + dD
the
equilibrium
constant expression has the form
- K =
[C]c[D]d/[A]a[B]b
One thing
that you should note quickly is that the equilibrium constant expression depends
on how we write the chemical reaction.
For example, consider the following reaction
- N2O5(g) < - > 2NO2(g) +
1/2O2(g) K =
[NO2]2[O2]1/2/[N2O5]
However,
we can just as easily multiply the whole reaction by 2- perhaps we don't like
the stoichiometric coefficient of 1/2 in front of the oxygen. This gives
- 2N2O5(g) < - > 4NO2(g) +
O2(g) K' =
[NO2]4[O2]/[N2O5]2
K
depends on how you write the chemical reaction! If you multiply the chemical
equation by a given number (such as 2), it is the same as raising the
equilibrium constant expression to that power. (K' = K2 in
the above example.)
Another property of the equilibrium constant expression is that if you write
the reaction backwards, the Kreverse is just equal to
1/Kforward
- N2O5(g) < - > 2NO2(g) +
1/2O2(g) K =
[NO2]2[O2]1/2/[N2O5]
2NO2(g) + 1/2O2(g) < - >
N2O5(g) Kreverse =
[N2O5]/[NO2]2[O2]1/2
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