[taregg (OP)]

If we say ....k  flame temperature has 771c
 and Mg flame temperature has 3100c
But in series reactive table in air reaction.......potassium will be the first element on the top of the chart for the most burning element ...can you explain.
Here is an example

[taregg (OP)]

http://universe-review.ca/I12-17-reactive.jpge

[taregg (OP)]

http://www.docbrown.info/page12/gifs/ReactivitySeriesList.gif

[Bored chemist]

I think the flame temperature of potassium is a lot higher than 771 K

[chiralSPO]

As we have noted in previous threads (http://www.thenakedscientists.com/forum/index.php?topic=49202.msg435285#msg435285), activity (how easily something reacts) is not the same as how much heat is released. The heat of formation of MgO is much greater than for K₂O (–601 kJ/mol vs –361 kJ/mol) when we adjust for heat released per mass of metal burned, the difference is even more dramatic (–25 kJ/g for Mg vs –4.6 kJ/g for K).

[lightarrow]

As we have noted in previous strings (http://www.thenakedscientists.com/forum/index.php?topic=49202.msg435285#msg435285), activity (how easily something reacts) is not the same as how much heat is released. The heat of formation of MgO is much greater than for K₂O (–601 kJ/mol vs –361 kJ/mol) when we adjust for heat released per mass of metal burned, the difference is even more dramatic (–25 kJ/g for Mg vs –4.6 kJ/g for K).

There is even other that you have to consider: MgO immediately formed in the combustion doesn't vaporize, K₂O does and this absorbs heat subtracting it to the flame, I suppose.

--
lightarrow

[chiralSPO]

I would expect the vaporization of the metal to increase the rate of reaction, thereby increasing the rate at which heat is released, and increasing the apparent temperature of the flame. Though the heat of vaporization of potassium is about 89 kJ/mol (and two equivalents required to compare to heat of formation of K₂O₂, which is –465 kJ/mol), the product of reaction is still solid K₂O₂ (I don't think K₂O₂ has a significant vapor pressure, even at 1000 °C), so even though there is a higher energy intermediate (K_(g)), the excess energy is released upon turning into K₂O_2(s).

Note: I initially assumed that K₂O would be the product of reaction, but after looking into it further, it appears that K₂O₂ is the major product...


I still think the main reason the Mg flame is hotter is because of the difference in heat of reaction per gram of material.

[Bored chemist]

Are we trying to explain something that does not exist.
Can anyone confirm the low flume temperature for potassium?