Bacteria are not limited to organic matter as a source of electrons or energy. A
ID: 717841 • Letter: B
Question
Bacteria are not limited to organic matter as a source of electrons or energy. Autotrophic denitrification occurs when NO3- is used as an electron acceptor and converted to N2 gas and H2 gas is used a source of electrons.
Below are the two half reactions we will be working with.
2NO3- + 10e- -> N2
2H+ + 2e- -> H2
Which reaction needs to be reversed? Enter the letter A or the letter B
Balance the half reaction for the conversion of nitrate to dinitrogen gas. Input the reaction co-efficient for each component of the half reaction in the blank provided. The co-efficient might be 1 in some cases.
2NO3- + e- + H+ -> N2 + H2O
Balance the half reaction for the conversion of hydrogen gas to hydrogen ions. Questions 12-13 are asking for the reaction co-efficients for each piece of the reaction. The co-efficient might be 1 in some cases.
H2 + -> 2e- + H+
Combine and balance both equations.
NO3- + H+ + H2 -> N2 + H2O
How many electrons were transferred?
What is the redox potential of this reactions in volts? E0cell = ?
Using the Nernst Equation, calculate the Gibbs Free Energy to deduce how many kilojoules are available to do work based on the stoichiometry you determined? G = -nFE, where F = 96.48 kJ (mol e-) (Report your answer to 3 decimal places)
Explanation / Answer
H2 gas is used a source of electrons.
H2 gas produces electrons
The two half cell reactions are
Reduction reaction
2NO3- + 10e- - - - - > N2
Ered = 0.74 V
Oxidation reaction (The reverse reaction)
H2 - - - - - > 2H+ + 2e-
Eox = 0.42 V
The balanced reaction
unbalanced equation
NO3- + H+ + H2 = N2 + H2O
oxidation numbers for each atom within brackets
N(+5)O(-2)3- + H(+1)+ + H(0)2 = N(0)2 + H(+1)2O(-2)
The two half cell reactions are
Oxidation reaction
H2 = H2O
H is being oxidized
Reduction reaction
NO3- = N2
N is bring oxidized
Balance all atoms except H and O.
Oxidation reaction
H2 = H2O
Reduction reaction
2NO3- = N2
Balance the oxygen atoms by adding water molecules.
O:
H2 + H2O = H2O
R:
2NO3- = N2 + 6H2O
Balance the hydrogen atoms by adding protons (H+).
O:
H2 + H2O = H2O + 2H+
R:
2NO3- + 12H+ = N2 + 6H2O
Balance the charge.
O:
H2 + H2O = H2O + 2H+ + 2e-
R:
2NO3- + 12H+ + 10e- = N2 + 6H2O
Make electron balance
O:
H2 + H2O = H2O + 2H+ + 2e-
Multiply by 5
R:
2NO3- + 12H+ + 10e- = N2 + 6H2O
Multiply by 1
O:
5H2 + 5H2O = 5H2O + 10H+ + 10e-
R:
2NO3- + 12H+ + 10e- = N2 + 6H2O
Add the half-reactions together.
5H2 + 2NO3- + 5H2O + 12H+ + 10e- N2 + 10H+ + 11H2O + 10e-
Simplify
5H2 + 2NO3- + 2H+ = 1N2 + 6H2O
10 electrons were transfered
E°cell = Eox + Ered
= 0.74 + 0.42
= 0.98 V
Free Energy change
G = -nFE
= 10 x 96480 Coulomb x 0.980 V
= 945504 x 1kJ/1000 J
= 945.504 kJ
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