What really happens?

[thedarkshade]

We were discussing about magnesium last class and its reactions with water. And what I found interesting is that when the water is cold, it forms magnesium oxide, but when the water is hot (near boiling temperature) then it forms magnesium hydroxide. These are the reactions:

 

Mg + H20 --> MgO +H2 (cold water)

Mg + 2H2O --> Mg(OH)2 + H2 (hot water)

 

Interesting! WHY DOES THIS HAPPEN? WHAT CAUSES IT?

[psynapse]

temperature affects pH so what are the formation constants for each and check if the pK's are near that of different pH's. Thats where I would start.

[thedarkshade]

When we learned about pH, we mentioned that water can function as base and also as acid ([ce]H2O ->[/ce][math]H^{+} + OH^{-}[/math]) and we also mentioned that the the product of these two represents the ionic product for water :

 

[math]Kw=[H^{+}][OH{-}]=10^{-7}\frac{mol}{dm^3}\times 10^{-7}\frac{mol}{dm^3}=1\times 10^{-14}\frac{mol^2}{dm^6}[/math]

 

But this product is when the temperature is round 25C (or a bit lower), because this product changes with temperature. For example, in 50C the ionic product for water is 5 times greater than in 25C, and in 100C the product is 50 times greater.

 

So could this be the reason why Mg + H2O forms different substances in different temperature?

[ce]Mg + H2O -> MgO + H2[/ce] (cold water)

[ce]Mg + 2H2O -> Mg(OH)2 + H2 [/ce] (hot water)

So is the answer "the ionic product for water"?

[ChemSiddiqui]

Well as the temperature increases and looking at the equillibrium that is to say H20 -------> H+ + OH- . The process is endothermic. So according to Le Chateleir's Principal (don't mind the spellings), as we increase the temperature the forward reaction is favoured and so does the production of H+ ions. So ph does drop and the reaction will take place completely and more reactively.

[thedarkshade]

Well as the temperature increases and looking at the equillibrium that is to say H20 -------> H+ + OH- . The process is endothermic. So according to Le Chateleir's Principal (don't mind the spellings), as we increase the temperature the forward reaction is favoured and so does the production of H+ ions. So ph does drop and the reaction will take place completely and more reactively.

Yeah, you're right! It just has to maintain the balance somehow! Thanks for the info;) !

[ChemSiddiqui]

No problem!