Nitrogen is a vital component of proteins and nucleic acids, and thus is necessa

Question

Nitrogen is a vital component of proteins and nucleic acids, and thus is necessary for life. The atmosphere is composed of roughly 80% N2, but most organisms cannot directly utilize N2 for biosynthesis. Bacteria capable of “fixing” nitrogen (i.e., converting N2 to a chemical form, such as NH3, which can be utilized in the biosynthesis of proteins and nucleic acids) are called diazotrophs. The ability of some plants like legumes to fix nitrogen is due to a symbiotic relationship between the plant and nitrogen-fixing bacteria that live in the plant’s roots. Assume that the hypothetical reaction for fixing nitrogen biologically is

N2(g) + 3H2O(l) -------> 2NH3(aq) + 3/2O2(g)

b. In some bacteria, glycine is produced from ammonia by the reaction

NH3(g) + 2CH4(g) + 5/2O2(g) ------> NH2CH2COOH(s) + H2O(l)

c. Calculate the standard enthalpy change for the synthesis of glycine from nitrogen, oxygen, and methane.

Explanation / Answer

a)

for the enthalpy change of N2:

HRxn = Hproducts- Hreactants

HRxn = 2*H-NH3 + 3/2*H-O2) - (H-N2 + 3*H-H2O(l))

HRxn = (2*(-80.3 ) + 3/2*0) - (0 + 3*-285.8) = 696.8 kJ/mol

b)

for glycine, find HRxn

HRxn = (NH2CH2COOH + H2O(l) ) - (NH3 + 2*CH4 + 5/2*O2)

HRxn = (-537.2 + -285.8) - (-45.6 + 2*-73.4 + 5/2*0) = -630.6kJ/mol

c. Calculate the standard enthalpy change for the synthesis of glycine from nitrogen, oxygen, and methane.

the reaction

CH4 + N2 + O2 = NH2CH2COOH

balance --> NH2CH2COOH --> NC2O2H5

2 N2 + 8 CH4 + 7 O2 = 4 NH2CH2COOH + 6 H2O

HRxn = Hprod - Hreact = 4*(-537.2 ) + 6*(-285.8) - (2*0 + 8*-73.4 + 7*2) = -3290.4kJ/mol

this is per 4 mol of glycine --> per 1 mol --> 1/4 = 1/4*3290.4 = - 822.6 kJ/mol

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