Kw constant
Moderators: Chem_Mod, Chem_Admin
-
- Posts: 120
- Joined: Wed Sep 30, 2020 9:43 pm
- Been upvoted: 1 time
-
- Posts: 143
- Joined: Wed Sep 30, 2020 9:37 pm
- Been upvoted: 2 times
Re: Kw constant
I was curious about this question too. But I looked at my lecture notes, and since [H2O] is in large excess, we consider the concentration to be unchanged and therefore, we don't include it in the equation.
-
- Posts: 100
- Joined: Wed Sep 30, 2020 9:40 pm
- Been upvoted: 1 time
Re: Kw constant
I agree with the post above. The autoprotolysis of water is really small so it's almost negligible, and the H2O molecules remain in excess.
-
- Posts: 100
- Joined: Wed Sep 30, 2020 9:33 pm
Re: Kw constant
We leave out H2O out of the calculations because the effect that it has is almost zero.
-
- Posts: 97
- Joined: Wed Sep 30, 2020 9:45 pm
- Been upvoted: 1 time
Re: Kw constant
we leave out h2o because there is an excess of it. It is both in the products and reactants so they cancel out.
-
- Posts: 101
- Joined: Wed Sep 30, 2020 10:01 pm
- Been upvoted: 1 time
Re: Kw constant
We leave out H2O because its concentration does not change significantly enough to where it needs to be considered in the calculation of the equilibrium constant.
-
- Posts: 121
- Joined: Wed Sep 30, 2020 9:56 pm
Re: Kw constant
Since water is in large excess, we assume its concentration to be relatively unchanged. That is, we aren't able to detect the change in water since it's probably something in the decimal units so it is essentially undetectable.
-
- Posts: 102
- Joined: Wed Sep 30, 2020 10:06 pm
-
- Posts: 101
- Joined: Wed Sep 30, 2020 9:40 pm
- Been upvoted: 1 time
Re: Kw constant
H2O is in excess so the change in concentration would be negligible. if we were to add it into Kw it would be in the numerator and denominator, and therefore would cancel out.
-
- Posts: 104
- Joined: Wed Sep 30, 2020 9:52 pm
Re: Kw constant
You leave out H2O because it serves as a solvent. Because of this, the concentration is in excess and you don't need to include it in the equilibrium constant expression. The same thing would happen if you had any other substance as a solvent, such as ethanol in some reactions
-
- Posts: 111
- Joined: Wed Sep 30, 2020 9:59 pm
- Been upvoted: 1 time
Re: Kw constant
[H2O] is left out if water is the solvent. As Dr. Lavelle mentioned in yesterday's lecture, any other solvent such as ethanol would be left out of the equilibrium constant as well.
-
- Posts: 106
- Joined: Mon Oct 05, 2020 12:16 am
Re: Kw constant
[H2O] will be left out due to the large excess which is why it can be disregarded due to its presence in both the reactant and products as stated in the lecture.
-
- Posts: 50
- Joined: Mon Jun 17, 2019 7:25 am
Re: Kw constant
H2O is in large excess and is present in both the reactant and products, meaning there is a negligible change in concentration of water. As such, water is not included when calculating the equilibrium constant.
-
- Posts: 103
- Joined: Wed Sep 30, 2020 9:42 pm
Re: Kw constant
Hi! H2O is left out of the calculations because it is present in excess and therefore the change in concentration is not significant.
-
- Posts: 96
- Joined: Wed Sep 30, 2020 9:54 pm
-
- Posts: 105
- Joined: Wed Sep 30, 2020 9:39 pm
Re: Kw constant
If you don't want to understand why: just remember we always leave out solids and liquids, molecules with the (s) and (l) subscript. If you want to know why, its because the reaction is happening in a solution of water, that means there are TREMENDOUS amounts of water compared to the other molecules so if any water ends up being used up or created in a reaction it really will make no difference. It's like adding adding or removing a droplet of water in a pool.
-
- Posts: 103
- Joined: Wed Sep 30, 2020 10:09 pm
Re: Kw constant
[H2O] is left out because it is found in large amounts on both the product and reactant side of the equation. This means, when calculating for Kw, the amounts of [H20] would just cancel each other out.
-
- Posts: 96
- Joined: Wed Sep 30, 2020 9:36 pm
Re: Kw constant
H2O is in excess amounts on both sides of the equation and can therefore be neglected as its minimal changes would not have any considerable effects
-
- Posts: 94
- Joined: Wed Sep 30, 2020 9:28 pm
Re: Kw constant
I think it is because H20 is present in excess and it’s change in concentration would be insignificant.
-
- Posts: 104
- Joined: Wed Sep 30, 2020 9:56 pm
Re: Kw constant
When calculating Kw, we leave out [H2O] because it is always present in excess in these reactions. As a result, any change would be negligible. Because of this, we leave it out when doing Kw.
-
- Posts: 51
- Joined: Thu Dec 17, 2020 12:17 am
Re: Kw constant
Yes we always leave out H2O from the calculations since it is in excess, so the change in concentration is not significant.
-
- Posts: 16
- Joined: Wed Nov 20, 2019 12:20 am
Re: Kw constant
The amount of H2O is high. As such, any change in concentration is negligible and does not need to be considered in calculations.
-
- Posts: 53
- Joined: Thu Dec 17, 2020 12:19 am
Re: Kw constant
[H2O] is in excess. Because there is a lot of H2O a change in concentration would have minimal effect, and thus it is negligible.
Re: Kw constant
Since such a little amount of H2O is ionized, it's concentration will pretty much stay the same making it insignificant.
Re: Kw constant
A solvent like H2O is usually in such a large excess that it's affect on the system is so minimal it's a waste of time to include it.
-
- Posts: 102
- Joined: Wed Sep 30, 2020 9:39 pm
Re: Kw constant
Like everyone has stated, it is because H20 is the solvent and thus in excess. This means that it will be on both sides of the reaction resulting in it canceling out during the calculations. However, water is not always left out only when it is the solvent.
Return to “Equilibrium Constants & Calculating Concentrations”
Who is online
Users browsing this forum: No registered users and 28 guests