Question

Nitrogen-fixing bacteria are essential for survival. A bioengineer is seeking to estimate the (standard state) change in enthalpy of reaction of this process. (Strictly speaking in biological systems, it doesn’t happen in the gas phase, but the order of magnitude is correct):

N₂ (g) + # H₂ -> ## NH₃ (g)

Determine the coefficients # and ##. Using the average bond enthalpy table, estimate ΔH_r°. Is this reaction exothermic?

Answer 1

for finding cofficients of H2 and NH3
balance the number of each atom on both sides
we will get cofficient of H2 = 3
and cofficient of NH3 = 2
so, complete reaction is
N2 + 3H2 -- > 2NH3

delta Hr = (bond dissociation enthalpy of N2)+3(bond dissociation enthalpy of N2)-2*(bond dissociation enthalpy of NH)
bond dissociation enthalpy of N2 = 945.33 KJ
bond dissociation enthalpy of H2 = 436 KJ
bond dissociation enthalpy of NH of NH3 = 435 KJ

delta Hr = 945.33 + (2*436) - (3*435)
= 945.33 + 872 - 1305
= 512.33 KJ