[tragram]

Hello, I've got this problem here - how much heat is realeased in the following reaction?
4NH3 + 5O2 --> 4NO + 6H2O

From what I know, it's an exothermic reaction, i hope.
I'm not sure, if that's high school chemistry, so I apologise, if this is wrong section.
Thanks for you answers.

[sjb]

Hello, I've got this problem here - how much heat is realeased in the following reaction?
4NH₃ + 5O₂  4NO + 6H₂O

From what I know, it's an exothermic reaction, i hope.
I'm not sure, if that's high school chemistry, so I apologise, if this is wrong section.
Thanks for you answers.

Any ideas? Could you perhaps construct a Hess' law diagram? Or consider the bonds broken / made? Or measure it directly?

[tragram]

Sorry, but quite frankly, I'm yet on elementary school and I'm not a native speaker either, so it's hard for me to get information about it on the internet. I have no clue whatsoever.

[Corribus]

If you have not had a formal chemistry class then you are probably not yet ready to answer this question.  You need to look up enthalpies of formation for the products and reactants, and subtract the latter from the former.  This will give you the enthalpy change, per unit mole, of the reaction.  Under the condition that the process is done at a constant pressure, the enthalpy change is equivalent to the heat flow.

[tragram]

Here are the enthalpies:
NH₃: -46.0kJ/mol
O₂: 0kJ/mol
NO: 90.29kJ/mol
H₂O: −241.818kJ/mol

Therefore:
4(-46.0)+5(0)-(4(90.29)+6(-241.818))=905.748kJ
Is that right? So if I do this reaction, around 0.9MJ of energy is released? Or I have to put that energy into it?
Thanks for your help.

[Corribus]

Your sign is not quite right.  Products minus reactants.  Using your values

4(90.29) + 6(-241.818) - 4(-46) - 5(0) = -905.75 kJ / mol

Therefore 905.75 kJ of heat are released per mole, assuming a complete reaction occurs, and assuming constant pressure.  A negative delta H is the sign of an exothermic reaction, or one where heat is liberated.

However keep in mind -

Reactions are actually equilibria and do not go to completion. (That is, there is an equivalent reverse reaction at the same time; the relative concentrations of products and reactants after equilibrium is released depends on the Gibb energy change, which is related to enthalpy AND entropy AND temperature.)

Thermodynamics and kinetics are two different things.  Just because heat would be liberated in the ideal case of the reaction going to completion does not ensure that the reaction will go at all, or how fast it will happen.

Typically you have to put energy in to get energy out.  Simply mixing gasoline and oxygen does not cause combustion - you have to apply energy (in a car, this is the job of the spark plug).  This goes along with the concept of kinetics.

[tragram]

Yeah, I know about that - from what I´ve read, you have to heat up the air around to 600-700°C for the reaction to occur.