Endothermic vs Exothermic Reactions in Ammonia Production

[Spacec0wboy]

Homework Statement

Is the reaction for the production of ammonia an endothermic reaction or an exothermic reaction? Explain.

Homework Equations

n/a

The Attempt at a Solution

I know that the production of ammonia in the Haber process is indeed exothermic and that the backwards reaction (NH3 -> N2 + H2) is endothermic, because I saw this when I was reading through Le Chatelier's principle, but I still am not sure why it is exothermic. Is it because the bonding between hydrogen and nitrogen releases energy? But even if that was true it's not detailed enough to answer the question, is it?

 

[MisterX]

Is it because the bonding between hydrogen and nitrogen releases energy? But even if that was true it's not detailed enough to answer the question, is it?

No, that doesn't answer the question. A better explanation is based on the idea that the reaction is exothermic if the energy required to break all the bonds in the reactants is less than the energy required to break all bonds in the products. In other words, the potential energy due to bonding is more negative for the products than the reactants in an exothermic reaction.

Using the chemical formula N₂ + 3 H₂ -> 2 NH₃ and a table with all the relevant bond dissociation energies, one might estimate that the Haber processes is exothermic.

Note that for N₂ and H₂, the bond dissociation energy will be the same as the bond energy, which may be found in a separate table.

Using "www.nist.gov/data/nsrds/NSRDS-NBS31.pdf"[/URL] (see pages 38 and 39) and a table of bond energies, one might predict the energy taken by the reaction to be -88 kJ per mol reactions, while the value given by Wikipedia for the Haber process is -92.22 kJ/mol reactions.

 

[pmsrw3]

It may be that your prof is only looking for an explanation of what it means to say a reaction is exothermic, and how that applies to the Haber process. Or he/she may be expecting you to deduce it from some other information, such as what happens to the equilibrium when the temperature is changed. (This seems particularly likely if you were just reading about Le Chatelier's Principle.)