How Does Water Concentration Impact Chemical Reactions?

[Aya]

my answer would be yes because, a concentration is the measure of how much of a given substance there is mixed with another substance, so aslong as the water is mixed with something elce you can measure the concentation of water right?

 

[dav2008]

Concentration is a measure of the amount of a substance per volume.

For water you can do a bit of conversions to determine the number of moles per volume:

$$\frac{1000\ \texterm{g}}{1\ \texterm{L}} \times \frac{1\ \texterm{mol}}{18\ g} = 55.56 \frac{\texterm{mo}l}{\texterm{L}}$$

 

[Aya]

^ why do you mulitply by 1mol/18g

 

[dav2008]

Because 1 mole of water has a mass of 18 grams.

(2 hydrogens and 1 oxygen)

Edit: Of course the above calculation is just calculating the number of water molecules per volume of pure water. If you had water mixed with another substance then you would have to adjust your volume accordingly.

Now that I think about it I guess concentration implies that something is dissolved in something else, but there's nothing stopping you from calculating the number of water molecules per volume of just water. I don't know if you would call it "concentration" though.

 

[Aya]

^Thank you

 

[Borek]

Side effect of water concentration is that sometimes you will find pKw (water ionic product) given as 14 and sometimes as 15.7. First number is based on assumption that water concentration is constant and doesn't change, second one takes care of water concentrations adding it to reaction quotient.

 

[Mattara]

Just as a correction to the above:

pKw is not technically the water ionic product per definition, that would be Kw. pKw is just another way of saying it.

pKw = - log Kw

 

[mcato_O]

Side effect of water concentration is that sometimes you will find pKw (water ionic product) given as 14 and sometimes as 15.7. First number is based on assumption that water concentration is constant and doesn't change, second one takes care of water concentrations adding it to reaction quotient.


Borek
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can you show me how do you get 15.7? Why do we just ignore the concentration of solid or solution solvent and replace it as one M? I always thought the reason is because the concentration of solid or water solvent does not change much so in practice we just move it to the Kc side” such as Kw = Kc[H2O]=[H][OH]? Now I am not so sure

 

[Borek]

Water dissociation:

H₂O -> H⁺ + OH^-

Kw is equilibrium constant for this reaction, so it should look like

Kw = [H⁺ ][OH^- ]/[H₂O ]

It is usually assumed that [H₂O ] doesn't change, so we use simplified formula

Kw = [H⁺ ][OH^- ]

Both concentrations can be measured which allows determination of Kw constant, Kw = 10^-14.

But if you decide to not ignore possible changes of water concentration you have to use full reaction quotient (you may use determined value of water ionic product in the numerator):

Kw = [H⁺ ][OH^- ]/[H₂O ] = 10^-14/[H₂O ]

[H₂O ] = 1000/18/1L = 55.5M
(where 1000 - mas of water in 1L, 18 - molar mass)

If so
Kw = 10^-14/[H₂O ] = 10^-14/55.5 = 1.8*10^-16

pKw = -log(1.8*10^-16) = 15.7

For most practical applications assumption that water concentration doesn't change is good enough - we rarely use dissociation constants determined with better precision than 2 significant digits, and with precision of 2 SD water concentration doesn't change even for 1M solutions - where thermodynamic effects are so strong that results of equilibrium calculations are already dubious (see ionic strength and activity coefficients lecture at my site). But in case of very precise potentiometric measurements (and Kw is determined for different temperatures with at least 4 SD accuracy) changes in water concentration should be easily visible in the results.

 

[GCT]

We've had a related discussion about this subject a while back. When working with concentration, it's usually in reference to water as a solvent. That is water has solvated the molecules, which are themselves the agents of any reaction dynamics that might incur. The usefulness of the concentration concept is in its incorporation into equations which depend on them, which relates to the molecules that are solvated in water. Water is the particular medium, and we're really concerned about the spatial density of the solvated agents.