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Water chemistry

markosheehan

Member
Jun 6, 2016
136
why does the water become more acidic as temperature goes up. H2O <-> (H^+) + (OH^-) . why does this dissociate into the ions more at higher temperatures. also even if it does dissociate more it should not become more acidic as for every H^+ formed a OH^- is formed.
 

Klaas van Aarsen

MHB Seeker
Staff member
Mar 5, 2012
9,491
why does the water become more acidic as temperature goes up. H2O <-> (H^+) + (OH^-) . why does this dissociate into the ions more at higher temperatures. also even if it does dissociate more it should not become more acidic as for every H^+ formed a OH^- is formed.
It takes energy to break the bonds in $\ce{H2O}$.
And any change in status quo prompts an opposing reaction in the responding system (Le Chatelier's principle).
So when heat is applied to the system, and temperature goes up, the equilibrium shifts to compensate, meaning $\ce{H2O}$ absorbs some of the heat and dissociates.

As for acidity, that is defined as the activity of $\ce{H+}$ ions (as $\ce{pH}$).
When temperature goes up, that activity goes up as well, meaning the solution gets a lower $\ce{pH}$ than $7$.
Note that the water is still neutral though.
It's just that a neutral solution only has a $\ce{pH}$ of $7$ at standard temperature ($25 ^\circ C$).
For the record, at $0 ^\circ C$, the $\ce{pH}$ of pure water is 7.47. At $25 ^\circ C$, it's 7.00, and at $100 ^\circ C$ it's 6.14.
 

markosheehan

Member
Jun 6, 2016
136
i understand that the more acidic something is the more the H^+ ions concentration there will be. but the measure of how basic something is measured by the OH^-1 concentration. would the hydroxide ions and hydrogen ions not cancel out no matter what the temperature is.

the pOH=-log to the base 10 of the hydroxide concentration. this will go up if water dissociates.
 
Last edited:

Klaas van Aarsen

MHB Seeker
Staff member
Mar 5, 2012
9,491
i understand that the more acidic something is the more the H^+ ions concentration there will be. but the measure of how basic something is measured by the OH^-1 concentration. would the hydroxide ions and hydrogen ions not cancel out no matter what the temperature is.

the pOH=-log to the base 10 of the hydroxide concentration. this will go up if water dissociates.
Yep. So a solution can be simultaneously more acidic and more basic by applying heat.
And they would cancel out.
We'd just have both more acidic reactions and more basic reactions.
That is, at a higher temperature we have more reactivity of all types.
 

markosheehan

Member
Jun 6, 2016
136
Yep. So a solution can be simultaneously more acidic and more basic by applying heat.
And they would cancel out.
We'd just have both more acidic reactions and more basic reactions.
That is, at a higher temperature we have more reactivity of all types.
so its not more acidic or basic no matter what the temperature
 

Klaas van Aarsen

MHB Seeker
Staff member
Mar 5, 2012
9,491