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Topic: Why isn't H2O part of reaction with solutions?  (Read 4661 times)

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Offline Bel-p

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Why isn't H2O part of reaction with solutions?
« on: April 02, 2006, 06:51:38 PM »
This is something I really don't understand, and I'm not even sure how to ask the question - but I'll try.  Why isn't water part of the reaction in reactions between solutions?  

EG:
sodium hydroxide (aq) reacts with hydrochloric acid (aq) to make sodium chloride and water.
NaOH + HCl --> NaCl + H20

As both of the reactants contain water, why doesn't the first part of the equation contain H2O?  And how do I know that the H2O that is apparently produced in the reaction isn't just the H2O that was in the two reactants in the firsts place?

Am I making any sense at all?  This question has been baffling me since I started (Sept 05).  

Offline constant thinker

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Re:Why isn't H2O part of reaction with solutions?
« Reply #1 on: April 02, 2006, 08:32:04 PM »
If your looking for a really simple reason then...

Think of the water that the HCl and NaOH are in as being only a medium that the reaction takes place in. It's only holds the 2 compounds and the subsequent reaction.

If your looking for a more specific reason with more chemistry terms, then you'll just have to hope one of the many professional chemists here will respond to your question.
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Offline mike

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Re:Why isn't H2O part of reaction with solutions?
« Reply #2 on: April 02, 2006, 10:22:55 PM »
You can represent water as a subscript (aq) to show that the species is dissolved in water.

NaOH(aq) + HCl(aq) ---> NaCl(aq) + H2O(l)

You know this is the water molecule produced in this reaction simply because that is why you have written the reaction, to show the reaction between hydroxide and hydrogen ions:

OH- + H+ ---> H2O

If you did this experimentally you would have a hard time showing which molecule was produced from the reaction and which was already in teh bulk solution. You could possibly use deuterium (although I am not sure how exactly).

Water does not appear on the reactant side of the equation because it is a product of the reaction. It just also happens to be the medium the reaction is done in.
« Last Edit: April 02, 2006, 10:24:39 PM by mike »
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Re:Why isn't H2O part of reaction with solutions?
« Reply #3 on: April 03, 2006, 05:08:37 AM »
This is something I really don't understand, and I'm not even sure how to ask the question - but I'll try.  Why isn't water part of the reaction in reactions between solutions?  

EG:
sodium hydroxide (aq) reacts with hydrochloric acid (aq) to make sodium chloride and water.
NaOH + HCl --> NaCl + H20

As both of the reactants contain water, why doesn't the first part of the equation contain H2O?  And how do I know that the H2O that is apparently produced in the reaction isn't just the H2O that was in the two reactants in the firsts place?

Am I making any sense at all?  This question has been baffling me since I started (Sept 05).  

Don't mind the quote, did this to refresh memory.

This is my own theory which I thought about it :P

Firslty, NaOH in water

NAOH --> NA+(aq)  +  OH- (aq)
Not forgetting the equilibrium in water( H2O molecules ), there would be more OH- than H+ in water, making the solution alkaline. Same goes to HCL in water, which makes the solution acidic.

Mixing both of them only gives you

Na+(aq), CL-(aq), and H2O!?!? WHat happen to the H+ and OH-?

Simple, the only reaction taking place is only H+ and Oh-.( This can be confirm using standard enthapy of neutralisaion) Due to the abundance in the 2 solution, they reacted together to form H2O molecules maintaing the equlibrium in water.


Some may wonder why does only the H+ and OH- react? My only possible guess is the strong electrostatic attraction between these 2 molecules wheras NA+ and Cl- are still strongly bonded to H2O molecules via ion dipole attraction.

Note in some neutralisation, a SOLID salt( For eg silver chloride) is produced? Why? The direct and foremost reason is Kip salt > Ksp. However what does they explain? Someone might want to answer me that  ;D My only guess is the energy involed using some formula

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