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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: vaseygirl on November 19, 2007, 08:41:49 PM

Title: Solid sodium hydroxide's reaction with water.
Post by: vaseygirl on November 19, 2007, 08:41:49 PM
    Why does sodium hydroxide emit heat when it goes from a solid into a solution when exposed to water? All it's doing is dissolving, so why is there a change in heat?
    Also, why does solid sodium hydroxide react differently with hydrochloric acid than sodium hydroxide in a 1M solution does?
Title: Re: Solid sodium hydroxide's reaction with water.
Post by: agrobert on November 19, 2007, 08:57:08 PM
Adding sodium hydroxide to water is a solvolysis reaction but it is exothermic because it is an acid-base reaction.  As sodium hydroxide is added to water there are proton exchanges back and forth between water and hydroxide which is exothermic unti equilibrium is reached (complete solvolysis).
Title: Re: Solid sodium hydroxide's reaction with water.
Post by: Borek on November 20, 2007, 02:46:29 AM
Quote from: agrobert on November 19, 2007, 08:57:08 PM
Adding sodium hydroxide to water is a solvolysis reaction but it is exothermic because it is an acid-base reaction.  As sodium hydroxide is added to water there are proton exchanges back and forth between water and hydroxide which is exothermic unti equilibrium is reached (complete solvolysis).

No reaction here. Back and forth exchanges can't produce more energy (forth) than they consume (back) so such process is thermally neutral.

http://en.wikipedia.org/wiki/Solvation
Title: Re: Solid sodium hydroxide's reaction with water.
Post by: djay on November 24, 2007, 10:19:09 AM
its the separation between Na and OH that is exothermic
Title: Re: Solid sodium hydroxide's reaction with water.
Post by: Borek on November 24, 2007, 10:34:34 AM
Quote from: djay on November 24, 2007, 10:19:09 AM
its the separation between Na and OH that is exothermic

Quite the opposite - separation is endothermic, you have to add energy to separate charges.
Title: Re: Solid sodium hydroxide's reaction with water.
Post by: djay on November 24, 2007, 11:27:07 AM
true but don't forget that some reactions after absorbing a little bit of heat releases a great amount of heat after. I remember one of my teachers said "some reactions are like burning a house with a lighter". It's true that all separations needs heat but with bonds high in energy, they will release even more after breaking up.
Exothermic reaction if I recall it corectly.
Title: Re: Solid sodium hydroxide's reaction with water.
Post by: enahs on November 24, 2007, 11:44:00 AM
Quote
It's true that all separations needs heat but with bonds high in energy, they will release even more after breaking up.
Exothermic reaction if I recall it corectly.


It is true that it requires energy to break bonds. But just because you break bonds that are high in energy does not cause a release in energy. The release in energy comes when those higher energy reactive species create new bonds of lower energy state; thus having excess energy to give off.


Title: Re: Solid sodium hydroxide's reaction with water.
Post by: Borek on November 24, 2007, 12:51:50 PM
Quote from: djay on November 24, 2007, 11:27:07 AM
don't forget that some reactions after absorbing a little bit of heat releases a great amount of heat after

I remember about it - it was you who forgot :)

Separation of Na+ and OH- is endothermic. But process of dissolution doesn't stop here - there is next stage that is strongly exothermic and is responsible for the NaOH dissolution thermal effect.