Hello everyone,
I know perhaps this in not the right forum to ask this question but all the users are using GWB software and would be well versed with thermodynamics. So though this queries are nor related to GWB software, i would appreciate a help.
I am Mustafa Vohra, working on a project involving CO2 sequestration and methane production from coal. My works includes understanding microbial syntrophy in my consortia and production of methane from coal. I am microbiologist by nature and have some calculations on calculation of free energy for my reactions. After going through literature, I have finally calculated the free energy for my reactions but I need to validate it. So I am requesting you to go through my calculations and check for its correctness. Please don’t mind for some naïve questions since I am not an expert in it. Any help, suggestion, reference, guidance appreciated.
1) Calculation of partial pressure and mmoles of gases.
I do analysis of gas (CH4, H2 and CO2) with the help of GC. From the area, I calculate % of gas.
Eg.
• After 30 days of analysis, CH4 = 2.042% = % 0.02042
• The gauge pressure measured from needle manometer, after 30 days in bottle = 0.5 psi
Now 1 atm pressure =101.3 KPa =760mm Hg = 14.7 psi
Therefore, 0.5 psi = 0.5 x 101.3 = 3.44 KPa
14.7
• Absolute total pressure (PT) = Gauge pressure, KPa + Atmospheric pressure, KPa
= 3.44 KPa + 101.3 KPa
= 104.74 KPa
• CH4 gas partial pressure (PCH4)
PCH4 = % CH4 x PT
= 0.02042 x 104.74
= 2.138 KPa
My question is, whether in the calculations of partial pressure, should I include total pressure or guage pressure?
Thermodynamic calculations
Consider the following reaction
CH3COO- + H+ + 4H2O = 2HCO3+ + 2H+ + 4H2O
Calculation of ΔG
ΔG = ΔGo + RT ln{ [HCO3+]2 [H+]2 [pH2]4
/[CH3COO-] [H+] }
Query 1: Should I use ΔGo or ΔGoI :
ΔGo is free energy under standard conditions of temp= 25oC, pressure =1 atm, conc= 1 M and pH=0. While ΔGoI is free energy under physiological condition of pH=7.
Now, the pH of my medium for this reaction is suppose 5.9 and I want to take this in to consideration, than I think I should use ΔGo and take account of the concentration of H+ ions [H+].
I think [H+] can be calculated by [H+] = - log pH
If not, is it necessary to consider pH of reaction i.e should I use ΔGoI for my equation. If yes than should I consider H+ in my equation?
Query 2: Temperature effect:
ΔGo or ΔGoI are calculated at 25oC but my reaction goes at 60oC. Below is the calculation done to correct my ΔGo for 60oC
ΔGoT = ΔHo – RBT
Where, ΔGoT = ΔGo at corrected temperature
ΔHo = change in enthalpy, R= ideal gas constant,
B = integration constant
Also RB = S (entropy)
The integration constant B can be calculated from above equation using ΔGo value at 25oC and T= 298oK
ΔHo is not dependent of temp up to 0 to 100oC and so value of Hf at 25oC can be used.
I calculated, ΔHo by taking Hf value of all the component involved in my reaction. Then I calculated B. From these values, I calculated ΔGoT
Is this the correct way?
Query 3: [pCO2] or [HCO3+]
In most of the references, I have seen people use [HCO3+] for free energy calculations but in some papers I have seen people using [pCO2]. Which one should I use?
If I have to use [HCO3+] than I dint measure it and had to calculate it from pCO2 value.
Now [H+] X [HCO3+] = [H2CO3] = [CO2] X [H2O]
Taken from aquatic chemistry by stuns
[H2CO3] = KH pCO2
Where KH = henry coefficient
Putting value & Calculating further we get, log [H2CO3] = -1.5 + log PCO2
Further deriving I get final equation,
Log [HCO3+] = - 6.4 + pH + log [H2CO3] 1
In some reference from net, I got another equation
pH = 6.4 + log {[HCO3+]
/0.03 pCO2 } 2
Here I think they consider [H+] X [HCO3+] = [CO2] X [H2O] without taking [H2CO3] in to consideration. So according to me eq 1 should be used for this conversion. What do you think?
Query 4: Calculate ΔGoT or critical pH2
I want to understand syntrophy between my acetogen and methanogens. To prove syntrophy, I will need to do thermodynamics calculation. There are two ways it is done
1) Calculate ΔGoT from above equation and tell whether reaction is exergonic or endergonic. Or
2) Calculate critical pH2 by taking ΔGoT = 0. Usually syntrophy depends on interspecies H2 or formate transfer and most of the people have used critical pH2 value for their discussion.
What do think I sould calculate? Any reason?
Query 5: Software?
I have lots of such free any calculations to do. Is there any software I can use? Can GWB software be used? I have heard and downloaded SUPCRAT92 and PHREEQC. Are they useful for my work? How should I use them?
