Air is approximately 78% nitrogen, however the strong N_2 triple bond (BE = 941

Question

Air is approximately 78% nitrogen, however the strong N_2 triple bond (BE = 941 kJ mole^-1) makes it not readily available to biological organisms. As a result nitrogen "fixing" is necessary to make it bioavailable. In 1913 BASF began production of using the iron catalyzed Haber-Bosch process. This ammonia was vital to the German war effort, however the Haber-Bosch process has also had a tremendous humanitarian impact; today it provides 454 million tonnes of nitrogen fertilizer per year. Generally, the Haber-Bosch process affords ammonia by combining nitrogen from air and hydrogen derived from natural gas (i.e., methane). Write a balanced equation for the reaction of nitrogen with hydrogen. (It is not necessary to include the catalyst used in the Haber-Bosch process.) Calculate the heat of reaction for the production of ammonia from the combination of H_2 and N_2. (Given: H_2 BE = 436 kJ mole^-1)

Explanation / Answer

b.

N2 = 2 N                        ( 1 triple bond N:N is broken)

H2 = 2 H    | x3                     ( 3 H-H bonds is broken)

2N + 6H = 2NH3            ( 6 N-H bonds are formed)

Bond energy H2     = 436 kJ/mol

Bond energy N2     = 942 kJ/mol

Bond energy N-H   = 386 kJ/mol

You have to provide for this reaction

942 + 3x436 – 6x386 = - 66 kJ   (exothermic)

This is the heat of reaction ( the experimental value is -92kJ/mol)

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