[Edher]

Saludos,

      Why is it that the "Ion Product of Water" is deducted by multiplying the hydronium concentrarion times the hydroxide concentration?

Kw = [H30+] [OH-] = 1.0 x 10^-14 (1.0E-14)

Wouldn't it make more sense to ADD the concentrarions to determine the amount of ions present?

Thank You,
Edher

[Edher]

Maybe I should give a different example,

In the reaction

CO(g) + 2H2 <--> CH30H(g)

[CH3OH] = 14.5
[CO][H2]^2

Why are the concentrations of the reactants multiplied rather than added to find the rate of the reaction?

To me, adding would make more sense because one would actually be adding the number of ions of a certain molecule with those of another molecule thus, that would yield the net number of ions of reactants.

Edher

[Donaldson Tan]

for an aqueous system in equilibrium,
A  <-> C + D
k = [C][D]/[A]

given that any changes in value of [A] is neligible, then [A] can be regarded as a constant, hence [C][D] = k[A] = constant

For aqueous system, A refers to water, C refers to H+ and D refers to OH-. The concentration of water molecules hardly changes in an aqueus system, hence k[A] refers to the ionic product of water.

[dexangeles]

I'm trying to remember my General Chemistry here but I believe the equation you are using is for the reaction quotient (Q)

 aA + cC <----> dD  eE

Q= [D]^d[E]^e
     [A]^a[C]^c

and when the reaction is in equilibrium, Q = k (equilibrium constant)

the rate of reaction deals with change in concentration per unit time

[Donaldson Tan]

my point is:
if the denominator is a constant, then there's no point expressing the reaction quotient as a fraction, and we can simplify the fraction expression to a product of two variables: [H+] & [OH-]

[Borek]

Why is it that the "Ion Product of Water" is deducted by multiplying the hydronium concentrarion times the hydroxide concentration?

Kw = [H30+] [OH-] = 1.0 x 10^-14 (1.0E-14)

Wouldn't it make more sense to ADD the concentrarions to determine the amount of ions present?

If you want to calculate total concentration of ions in water, just add their concentrations. But that's not the point with ion product of water - as the product [H30+] [OH-] is constant for a given temperature it allows you to calculate concentration of other ions if you know concentration of the first one. Sum [H30+] +[OH-] has value of 1M fo pH = 0, 2*10^-7M for pH = 7 and 1M again for pH = 14.

[Retro]

This is nothing but an equilibrium case:
2H₂O <-> HO^- + H₃O⁺
Now the ionic produc of water  is the equilibrium constante at the system temperature, therefore:
 kw = [HO-][H3O+]
The [H2O] isn't in the constant expression because it is in liquid state, so it doesn't affect the equilibrium.