[Rutherford]

The activation energy of uncatalyzed decomposition of NH3 is 335kJ/mol and for the catalyzed decomposition Ea=165kJ/mol. The change of entalphy in this reactions is 46kJ/mol. Calculate the Ea of the catalyzed and uncatalyzed synthesis of NH3 from elements.
Never done before similar problems. I got the right answer by subtracting 46 from 335 and 165, but I don't know why. Is there a formula that I should know?

[Borek]

Try to draw a potential energy diagram for the reaction.

[Rutherford]

Do you mean the Born-Haber cyrcle?

[fledarmus]

No, more like this:

http://en.wikipedia.org/wiki/File:Activation_energy.svg

[Rutherford]

Thanks.

[Rutherford]

No, more like this:

http://en.wikipedia.org/wiki/File:Activation_energy.svg

Is this diagram usable if dH<0? If not, how would it look like?

[XGen]

It would be similar, only the right side would be lower than the left side and the difference in distance downward would be change in enthalpy.

http://www.kentchemistry.com/images/links/Kinetics/aim4.h9.jpg

Similar to this.

[Rutherford]

How can I calculate the Ea of the direct reaction if Ea of the reverse reaction and dH are given (dH<0)?

[sjb]

How can I calculate the Ea of the direct reaction if Ea of the reverse reaction and dH are given (dH<0)?

What is the definition of Ea, and of dH? This seems to be a precise example of Hess' law, no?

[Rutherford]

Never done such examples before. Can you explain it more clearly?