Lecture 8 – Chapter 14, Sections 5-6Thermodynamics – Applied• Nitrogenase

• Phase changes

• Bioenergetics

Nitrogen

• All organisms need nitrogen• BUT triple bond in N2 is very hard to break• Most organisms can only use NO3

– or NH3

• Why? Let’s look in terms of ΔG• What if we want to make a simple amino acid – glycine?

– Start with something very similar – acetic acid – as reactant 4CH3CO2H + 2N2 + 2H2O 4H2NCH2CO2H + O2

ΔGf°(H2NCH2CO2H) = –367 kJ/mol ΔGf

°(O2) = 0 kJ/mol

ΔGf°(CH3CO2H) = –389 kJ/mol ΔGf

°(N2) = 0 kJ/mol

ΔGf°(H2O) = –237 kJ/mol

What is ΔGrxn for the formation of glycine from N2? (per mole of glycine)4CH3CO2H + 2N2 + 2H2O 4H2NCH2CO2H + O2

ΔGf°(H2NCH2CO2H) = –367 kJ/mol ΔGf

°(O2) = 0 kJ/molΔGf

°(CH3CO2H) = –389 kJ/mol ΔGf°(N2) = 0 kJ/mol

ΔGf°(H2O) = –237 kJ/mol

0%

0%

0%

0% 1. 64.8 kJ/ mol2. 141 kJ/mol3. 259 kJ/mol4. 562 kJ/mol

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Ammonia better than N2

• If we do the same thing, but use ammonia as our N-sourceCH3CO2H + NH3 + ½O2 H2NCH2CO2H + H2O

ΔGrxn = –198 kJ/mol

• So, ammonia is much better fertilizer than N2• Production of ammonia is big business – fertilizer

– 5 of the top 15 industrial chemicals (in amountproduced) are primarily used in fertilizer production

• Industrial ammonia is made from N2 and H2 gasesN2 + 3H2 2NH3 ΔGrxn = –32.8 kJ/mol

H2 gas is tough

• H2 gas is rare on earth – must be manufactured• Made from methane (‘natural gas’ mostly methane)

CH4 + H2O CO + 3H2

ΔΗ°rxn

= +205.9 kJ/mol

ΔS°rxn

= +214.6 J/mol·K

ΔGrxn = ΔH°

rxn – TΔS°

rxn = 205.9 kJ/mol – (298 K)(0.2146 kJ/mol·K)

ΔG°rxn

= +142 kJ/mol

Using ΔGrxn = ΔH°

rxn – TΔS°

rxnand ΔΗ°

rxn = +205.9 kJ/mol

ΔS°rxn

= +214.6 J/mol·KAt what temperature does the reaction become spontaneous?

0%

0%

0%

0% 1. 0.9595 K2. 8.7 K3. 959.5 K4. 1042 K

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Nitrogen Fixation

• So, NH3 production is expensive because you need lots of heatto produce H2

• Sources of ‘fixed’ nitrogen:– 1% is N2 converted to NO3

– by lightning– 30 % is man-made – N2 converted to NH3 for fertilizer– 70 % is N2 converted to NH3 by bacteria – nitrogenase

Phase Changes

• The spontaneous direction of a phase change depends onpressure

• This is obvious for the liquid-vapor equilbrium– Free energy (entropy) of vapor depends strongly on pressure

• For a given T, there is some P at which ΔGvap = 0.

Clausius-Clapeyron Equation

• Describes pressure-dependence of liquid-vapor phase line• For instance look at normal boiling of water: H2O(l) H2O(g)

ΔH°f H2O(l) = –285.83 kJ/mol ΔH°

f H2O(g) = –241.83 kJ/molΔS° H2O(l) = 69.95 J/mol·K ΔS° H2O(g) = 188.835 J/mol·K

ΔH°vap = (–241.83 kJ/mol) – (–285.83 kJ/mol) = 44.00 kJ/mol

ΔS°vap = (188.835 J/mol·K) – (69.95 J/mol·K) = 118.89 J/mol·K

( )( )bar 124.1

1167.0KJ/mol 314.8

KJ/mol 89.118

K 15.373KJ/mol 314.8

J/mol 000,44ln

=

=!

!+

!"=

vp

vp

We got 1.124 bar, but actual answer is1 atm = 1.013 bar. What’s the deal?

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0%

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0% 1. The ΔH°vap isn’t very accurate anymore at 373 K

2. The ΔS°vap isn’t very accurate anymore at 373 K

3. The ΔG°vap is not actually 0 when water is boiling at 373 K

4. The Clausius-Clapeyron equation isn’t valid for water

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ΔG is the currency for biological reactions

• Glucose oxidation gives off energy ΔG°rxn = –2870 kJ/mol

• Used to convert ADP into ATP ΔG°rxn = +30.6 kJ/mol

• ATP is easily transported and easily hydrolyzed back into ADP• Each mole of ATP provides 30 kJ of energy to do biological work

• Note that 1 mole of glucose (2870 kJ) should yield over 90 moles of ATP– In reality the yield is only about 36 moles of ATP – significant loss

Glucose comes from the sun, or maybe the air

Today• Seminar 4:00 Schaap 1000 (Conjugated Polymers)

• Read Chapter 15• Fall in love with kinetics• CAPA #5

– CAPA help! Sun night 7:00 Schaap 3031 Wed night 8:00 Schaap 3031

Tuesday• Special seminar 11:00 VDW 102

Monday