[Ankesh]

Sorry to post this again,but i couldnt undertstand it last time.So,i thought this would be a better place    .   

                Electric arc
N2   +   02         2NO   This is an endothermic reaction.
                  3500 C

Now, its reverse,

2NO         N2 +02  This is also an endothermic reaction.
         900 C

How can a reaction and its reverse both be endothermic? ?

[BluRay]

Sorry to post this again,but i couldnt undertstand it last time.So,i thought this would be a better place    .   

                Electric arc
N2   +   02         2NO   This is an endothermic reaction.
                  3500 C

Ok.

Now, its reverse,

2NO         N2 +02  This is also an endothermic reaction.
         900 C

This is not endothermic, it's exothermic.

[Ankesh]

Now, its reverse,

2NO         N2 +02  This is also an endothermic reaction.
         900 C
This is not endothermic, it's exothermic.

HOW??
we are heating it at 900c ,still it is endothermic???

[BluRay]

The fact you work at 900°C doesn't mean it's endothermic. Endothermic means that standard enthalpy of reaction is positive. If you work at constant pressure (in the case of this reaction pressure doesn't vary a lot even at constant volume, if temperature is kept constant during reaction) then enthalpy variation equals the heat you give to the system. So, in this case, endothermic = you give heat to the system, exothermic = the system gives off heat.
The fact you need to work at T > 25° mens that the reaction needs activation energy.
Think about H₂ + (1/2)O₂ --> H₂O
This reaction is not endothermic, it's exothermic (a lot! Blowtorch!) but it doesn't happen at room T, you need a spark, or heating the reagents at high T.